gogeshop's Blog

NASA's manned Apollo program was developed in parallel; after a series of unmanned and manned tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar flight, in 1968 Apollo 8 made the first crewed mission to lunar orbit. Panasonic LUMIX DMC-TZ4S Battery The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race. Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969.Sony NP-FH100 Battery The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) returned 382 kg of lunar rock and soil in 2,196 separate samples. The American Moon landing and return was enabled by considerable technological advances in the early 1960s, Toshiba Equium M50-192 Battery in domains such as ablation chemistry, software engineering and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking. Scientific instrument packages were installed on the lunar surface during all the Apollo missions. Apple MacBook Pro Battery Long-lived instrument stations, including heat flow probes,seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 due to budgetary considerations, Apple PowerBook G4 Battery but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.Acer BATBL50L6 Battery

Current era: 1990-present

Post-Apollo and Luna, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission.Canon EOS 450D Battery In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. This was followed in 1998 by theLunar Prospector mission,Acer Travelmate 2400 Battery whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface.Dell Inspiron 1525 Adapter China has expressed ambitious plans for exploring the Moon, and successfully orbited its first spacecraft, Chang'e-1, from 5 November 2007 until its controlled lunar impact on 1 March 2008. In its sixteen-month mission, it obtained a full image map of the Moon.Compaq Presario 2100 Adapter Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's Kaguya (Selene) mission, a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.Acer Aspire 1362 Battery India's first lunar mission, Chandrayaan I, orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil.Toshiba Equium M50-192 Battery The Indian Space Research Organisation plans to launch Chandrayaan II in 2013, which is slated to include a Russian robotic lunar rover. The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor and follow-up observation orbiter on 18 June 2009;Dell XPS M1330 Battery LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, while LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. Other upcoming lunar missions include Russia's Luna-Glob: an unmanned lander, set of seismometers, and an orbiter based on its Martian Phobos-Gruntmission, which is slated to launch in 2012.Acer Travelmate 2200 Adapter Privately funded lunar exploration has been promoted by the Google Lunar X Prize, announced 13 September 2007, which offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria. NASA began to plan to resume manned missions following the call by U.S. President George W. HP Compaq 6735b Adapter Bush on 14 January 2004 for a manned mission to the Moon by 2019 and the construction of a lunar base by 2024.The Constellation program was funded and construction and testing begun on a manned spacecraft and launch vehicle, and design studies for a lunar base.Dell Vostro 1000 Battery However, that program has been cancelled in favour of a manned asteroid landing by 2025 and a manned Mars orbit by 2035. India has also expressed its hope to send a manned mission to the Moon by 2020.

Astronomy from the Moon

For many years, the Moon has been recognized as an excellent site for telescopes.HP Compaq nx7300 Adapter It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth.Asus f82 Battery The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies in the construction of mirrors up to 50 meters in diameter. A lunar zenith telescope can be made cheaply with ionic liquid. Dell XPS M1330 Battery

Legal status

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation currently claims ownership of any part of the Moon's surface. Russia and the U.S. are party to the 1967 Outer Space Treaty,which defines the Moon and all outer space as the "province of all mankind".HP Compaq Business 6910p Battery This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations.Compaq PRESARIO R3000 Battery While several individuals have made claims to the Moon in whole or in part, none of these are considered credible.

In culture

The Moon's regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars.HP Compaq Business NX6110 Battery Tally sticks, notched bones dating as far back as 20-30,000 years ago, are believed by some to mark the phases of the Moon. The ~30-day month is an approximation of the lunar cycle. The English noun month and its cognates in other Germanic languages stem from Proto-Germanic *m?n?th-, Dell Latitude D610 Adapter which is connected to the above mentioned Proto-Germanic *m?n?n, indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar. The same Indo-European root as moon led, via Latin, to measure and menstrual, Asus Eee PC 701 Adapter words which echo the Moon's importance to many ancient cultures in measuring time (see Latin mensis and Ancient Greek ????? (m?nas), meaning "month"). The Moon has been the subject of many works of art and literature and the inspiration for countless others. HP Compaq Business NX6325 Battery It is a motif in the visual arts, the performing arts, poetry, prose and music. A 5,000-year-old rock carving at Knowth, Ireland, may represent the Moon, which would be the earliest depiction discovered. The contrast between the brighter highlands and darker maria create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. Dell Studio XPS 16 Battery In many prehistoric and ancient cultures, the Moon was personified as a deity or other supernaturalphenomenon, and astrological views of the Moon continue to be propagated today. The Moon has a long association with insanity and irrationality; the words lunacy and loony are derived from the Latin name for the Moon, Luna. Acer Aspire 5520 Battery Philosophers such as Aristotle and Pliny the Elderargued that the full Moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person.Acer TravelMate 5720 Adapter Even today, people insist that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full Moon, although there is no scientific evidence to support such claims. The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is quite different from that of the Earth. HP Compaq 6735s Adapter The Moon lacks a significant atmosphere and any bodies of water, which eliminates erosion due to weather; it does not possess any form of plate tectonics, it has a lower gravity, and because of its small size, it cools more rapidly. The complex geomorphology of the lunar surface has been formed by a combination of processes, chief among which are impact cratering and volcanism. Dell XPS M1330 Battery The Moon is a differentiated body, possessing a crust, mantle and core. Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data.HP Compaq nc6230 Adapter A few locations were sampled directly during the Apollo missions in the late 1960s and early 1970s, which returned approximately 385kilograms of lunar rock and soil to Earth, as well as several missions of the Soviet Luna programme. The Moon is the only extraterrestrial body for which we possess samples with a known geologic context. Dell T117C Battery A handful of lunar meteorites have been recognized on Earth, though their source craters on the Moon are unknown. A substantial portion of the lunar surface has not been explored, and a number of geological questions remain unanswered.Asus x5dij-sx039c Battery

Elemental composition

Elements known to be present on the lunar surface include, among others, oxygen (O), silicon (Si), iron (Fe),magnesium (Mg), calcium (Ca), aluminium (Al), manganese (Mn) and titanium (Ti). Among the more abundant are oxygen, iron and silicon. The oxygen content is estimated at 45%. Dell Inspiron 1464 Battery Carbon (C) and nitrogen (N) appear to be present only in trace quantities from deposition by solar wind. Neutron spectrometry data from the Lunar Prospector indicate the presence of hydrogen (H) concentrated at thepoles.HP Compaq 6720s Adapter

Formation

For a long time, the fundamental question regarding the history of the Moon was of its origin. Early hypotheses included fission from the Earth, capture, and co-accretion. Today, the giant impact hypothesis is widely accepted by the scientific community.Dell Latitude D630 Adapter

Fission hypothesis

The idea that the early Earth, with an accelerated rotation, expelled a piece of its mass was proposed by George Darwin (son of the famous biologist Charles Darwin). It was commonly assumed that the Pacific Ocean represented the scar of this event. HP Pavilion dm4 Battery However, today it is known that the oceanic crust that makes up this ocean basin is relatively young, about 200 million years old or less, whereas the Moon is much older. This hypothesis cannot account for the angular momentum of the Earth-Moon system.Sony NP-FS11 Battery

Lunar capture

This hypothesis states that the Moon was captured, completely formed, by the gravitational field of the Earth. This is unlikely, since a close encounter with the Earth would have produced either a collision or an alteration of the trajectory of the body in question, so if it had indeed happened, the Moon probably would never return to meet again with the Earth. Dell Inspiron 1520 Adapter For this hypothesis to function, there would have to be a large atmosphere extended around the primitive Earth, which would be able to slow the movement of the Moon before it could escape. This hypothesis is considered to explain the irregular satellite orbits ofJupiter and Saturn. Toshiba PA3399U-1BRS Battery In addition, this hypothesis has difficulty explaining the similar oxygen isotope ratio of the two worlds.

Co-accretion hypothesis

This hypothesis states that the Earth and the Moon formed together as a double system from the primordial accretion disk of the Solar System.Acer Aspire 5920G Adapter The problem with this hypothesis is that it does not explain the angular momentum of the Earth-Moon system, nor why the Moon has a relative small iron core compared to the Earth (25% of its radius compared to 50% for the Earth).

Giant impact theory

At present the best explanation for the origin of the Moon involves a collision of two protoplanetary bodies during the early accretional period of Solar System evolution. Acer Aspire 5920 Battery This "giant impact theory", which became popular in 1984 (although Reginald Aldworth Daly, a Canadian professor at Harvard college, originated it in the 1940s), satisfies the orbital conditions of the Earth and Moon and can account for the relatively small metallic core of the Moon.Nikon COOLPIX 4300 Battery Collisions between planetesimals are now recognized to lead to the growth of planetary bodies early in the evolution of the Solar System, and in this framework it is inevitable that large impacts will sometimes occur when the planets are nearly formed.IBM Thinkpad T40 Adapter The theory requires a collision between a body about 90% the present size of the Earth, and another the diameter of Mars (half of the terrestrial radius and a tenth of its mass). The colliding body has sometimes been referred to as Theia, the mother of Selene, the Moon goddess in Greek mythology. Apple MB771 Battery This size ratio is needed in order for the resulting system to possess sufficient angular momentum to match the current orbital configuration. Such an impact would have put enough material into orbit about the Earth to have eventually accumulated to form the Moon.Sony DSC-P5 Battery Computer simulations of this event appear to show that the collision must occur with a somewhat glancing blow. This will cause a small portion of the colliding body to form a long arm of material that will then shear off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. Dell Inspiron 6400 Adapter The energy involved in this collision is impressive: trillions of tons of material would have been vaporized and melted. In parts of the Earth the temperature would have risen to 10,000 C (18,000 F). This formation theory helps explain why the Moon possesses only a small iron core (roughly 25% of its radius, in comparison to about 50% for the Earth).Toshiba PA3534U-1BRS Battery Most of the iron core from the impacting body is predicted to have accreted to the core of the Earth. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccreation of material in orbit about the Earth would have been sufficient to melt a large portion of Moon, leading to the generation of a magma ocean.Sony NP-FP50 Battery The newly formed moon orbited at about one-tenth the distance that it does today, and became tidally-locked with the Earth, where one side continually faces toward the Earth. The geology of the Moon has since been independent of the Earth. While this theory explains many aspects of the Earth-Moon system,HP Compaq nx7300 Adapter there are still a few unresolved problems facing this theory, such as the Moon's volatileelements not being as depleted as expected from such an energetic impact.

Geologic history

The geological history of the Moon has been defined into six major epochs, called the lunar geologic timescale. Dell Latitude D820 Adapter Starting about 4.5 billion years ago, the newly formed Moon was in a molten state and was orbiting much closer to the Earth. The resulting tidal forces deformed the molten body into an ellipsoid, with the major axis pointed towards Earth. The first important event in the geologic evolution of the Moon was the crystallization of the near global magma ocean. Toshiba PA3399U-1BRS Battery It is not known with certainty what its depth was, but several studies imply a depth of about 500 km or greater. The first minerals to form in this ocean were the iron and magnesium silicates olivineand pyroxene. Because these minerals were denser than the molten material around them, they sank. Dell Studio XPS 1640 Battery After crystallization was about 75% complete, less dense anorthositic plagioclase feldspar crystallized and floated, forming an anorthositic crust about 50 km in thickness. The majority of the magma ocean crystallized quickly (within about 100 million years or less), though the final remaining KREEP-rich magmas,^{Dell Latitude D830 Battery} which are highly enriched in incompatible and heat producing elements, could have remained partially molten for several hundred million (or perhaps 1 billion) years. It appears that the final KREEP-rich magmas of the magma ocean eventually became concentrated within the region of Oceanus Procellarum and the Imbrium basin,^{Dell Inspiron E1505 Adapter} a unique geologic province that is now known as the Procellarum KREEP Terrane. Quickly after the lunar crust formed, or even as it was forming, different types of magmas that would give rise to the Mg-suite norites and troctolites began to form, although the exact depths at which this occurred are not known precisely. ^{Asus Eee PC 1201PN Battery} Recent theories suggest that Mg-suite plutonism was largely confined to the region of the Procellarum KREEP Terrane, and that these magmas are genetically related to KREEP in some manner, though their origin is still highly debated in the scientific community. ^{Asus Eee PC 701 Adapter} The oldest of the Mg-suite rocks have crystallization ages of about 3.85 Ga. However, the last large impact that could have excavated deep into the crust (the Imbrium basin) also occurred at 3.85 Ga before present. Thus, it seems probable that Mg-suite plutonic activity continued for a much longer time, and that younger plutonic rocks exist deep below the surface.^{Dell XPS M1330 Battery} Analysis of the lunar samples seem to imply that a significant percentage of the lunar impact basins formed within a very short period of time between about 4 and 3.85 Ga ago. This hypothesis is referred to as the lunar cataclysm or late heavy bombardment. ^{fujifilm np95 Battery} However, it is now recognized that ejecta from the Imbrium impact basin (one of the youngest large impact basins on the Moon) should be found at all of the Apollo landing sites. It is thus possible that ages for some impact basins (in particular Mare Nectaris) could have been mistakenly assigned the same age as Imbrium.^{Asus Eee PC 900 Adapter} The lunar maria represent ancient flood basaltic eruptions. In comparison to terrestrial lavas, these contain higher iron abundances, have low viscosities, and some contain highly elevated abundances of the titanium-rich mineral ilmenite. The majority of basaltic eruptions occurred between about 3 and 3.5 Ga ago, ^{Sony VAIO PCG-713 Battery} though some mare samples have ages as old as 4.2 Ga, and the youngest (based on the method of crater counting) are believed to have erupted only 1 billion years ago. Along with mare volcanism came pyroclastic eruptions, which launched molten basaltic materials hundreds of kilometres away from the volcano.^{Dell Latitude D830 Adapter} A large portion of the mare formed, or flowed into, the low elevations associated with the nearside impact basins. However, Oceanus Procellarum does not correspond to any known impact structure, and the lowest elevations of the Moon within the farside South Pole-Aitken basin are only modestly covered by mare (see lunar mare for a more detailed discussion).Dell Studio XPS 1640 Battery Impacts by meteorites and comets are the only abrupt geologic force acting on the Moon today, though the variation of Earth tides on the scale of the Lunar anomalistic month causes small variations in stresses. Some of the most important craters used in lunar stratigraphy formed in this recent epoch. ^{Panasonic LUMIX DMC-TZ4S Battery} For example, the crater Copernicus, which has a depth of 3.76 km and a radius of 93 km, is believed to have formed about 900 million years ago (though this is debatable). The Apollo 17 mission landed in an area in which the material coming from the crater Tycho might have been sampled. ^{HP Compaq Business NX6320 Battery} The study of these rocks seem to indicate that this crater could have formed 100 million years ago, though this is debatable as well. The surface has also experienced space weathering due to high energy particles, solar wind implantation, and micrometeorite impacts. ^{Canon POWERSHOT G2 Battery} This process causes the ray systems associated with young craters to darken until it matches the albedo of the surrounding surface. However, if the composition of the ray is different from the underlying crustal materials (as might occur when a "highland" ray is emplaced on the mare), the ray could be visible for much longer times.Dell Latitude E6500 Battery After resumption of Lunar exploration in the 1990s, it was discovered there are scarps across the globe which are caused by the contraction due to cooling of the Moon.

Lunar landscape

The lunar landscape is characterized by impact craters, their ejecta, a few volcanoes, hills, lava flows and depressions filled by magma.^{Dell LATITUDE D630 Battery}

The highlands

The most distinctive aspect of the Moon is the contrast between its light and dark zones. Lighter surfaces are the lunar highlands, which receive the name ofterrae (singular terra, from the Latin for Earth), and the darker plains are called maria (singular mare, from the Latin for sea), after Johannes Kepler who introduced the name in the 17th century.Asus R33030 Adapter The highlands are anorthositic in composition, whereas the maria are basaltic. The maria often coincide with the "lowlands," but it is important to note that the lowlands (such as within the South Pole-Aitken basin) are not always covered by maria. The highlands are older than the visible maria, and hence are more heavily cratered.Dell Mini 1011 Battery

The maria

The major products of volcanic processes on the Moon are evident to the Earth-bound observer in the form of the lunar maria. These are large flows ofbasaltic lava that correspond to low-albedo surfaces covering nearly a third of the near side. toshiba pa3332u-1brs Battery Only a few percent of the farside has been affected by mare volcanism. Even before the Apollo missions confirmed it, most scientists believed that the maria were lava-filled plains, since they possessed lava flowpatterns and collapses attributed to lava tubes.HP Compaq 6710b Adapter The ages of the mare basalts have been determined both by direct radiometric dating and by the technique of crater counting. The oldest radiometric ages are about 4.2 Ga, whereas the youngest ages determined from crater counting are about 1 Ga (1 Ga = 1 billion years). ^{HP Compaq nx6325 Adapter} Volumetrically, most of the mare formed between about 3 and 3.5 Ga before present. The youngest lavas erupted within Oceanus Procellarum, whereas some of the oldest appear to be located on the farside. The maria are clearly younger than the surrounding highlands given their lower density of impact craters.Acer Aspire 5720 Adapter A large portion of maria erupted within, or flowed into, the low-lying impact basins on the lunar nearside. Nevertheless, it is unlikely that a causal relationship exists between the impact event and mare volcanism because the impact basins are much older (by about 500 million years) than the mare fill. ^{Toshiba Equium A110-252 Battery} Furthermore, Oceanus Procellarum, which is the largest expanse of mare volcanism on the Moon, does not correspond to any known impact basin. It is commonly suggested that the reason the mare only erupted on the nearside is that the nearside crust is thinner than the farside. ^{Acer Aspire 5052ANWXMi Battery} While crustal thickness variations might act to modulate the amount of magma that ultimately reaches the surface, this hypothesis does not explain why the farside South Pole-Aitken basin, whose crust is thinner than Oceanus Procellarum, was only modestly filled by volcanic products.Dell Mini 1011v Battery Finally, it should be noted that the Earth's gravity played no preferential role in causing mare volcanism to occur on the near side, as the Earth's gravitational attraction is exactly balanced by the centrifugal acceleration resulting from the Moon's rotation. Another type of deposit associated with the maria, although it also covers the highland areas, are the "dark mantle" deposits. ^{HP Compaq nx7300 Adapter} These deposits cannot be seen with the naked eye, but they can be seen in images taken from telescopes or orbiting spacecraft. Before the Apollo missions, scientists believed that they were deposits produced by pyroclastic eruptions. Some deposits appear to be associated with dark elongated ash cones, reinforcing the idea of pyroclasts. ^{Dell Vostro 1510 Battery} The existence of pyroclastic eruptions was later confirmed by the discovery of glass spherules similar to those found in pyroclastic eruptions here on Earth. Many of the lunar basalts contain small holes called vesicles, which were formed by gas bubbles exsolving from the magma at the vacuum conditions encountered at the surface. ^{Toshiba PA3534U-1BRS Battery} It is not known with certainty which gases escaped these rocks, but carbon monoxide is one candidate. The samples of pyroclastic glasses are of green, yellow, and red tints. The difference in color indicates the concentration of titanium that the rock possesses, with the green particles having the lowest concentrations (about 1%), and red particles having the highest concentrations (up to 14%, much more than the basalts with the highest concentrations).^{Asus M50 Battery}

Rilles

Rilles on the Moon sometimes resulted from the formation of localized lava channels. These generally fall into three categories, consisting of sinuous, arcuate, or linear shapes. By following these meandering rilles back to their source, they often lead to an old volcanic vent. ^{Asus Eee PC 901 Battery} One of the most notable sinuous rilles is the Vallis Schr teri feature, located in the Aristarchus plateau along the eastern edge of Oceanus Procellarum. An example of a sinuous rille exists at the Apollo 15 landing site, Rima Hadley, located on the rim of the Imbrium Basin. Dell Inspiron 6400 Battery Based on observations from the mission, it is generally believed that this rille was formed by volcanic processes, a topic long debated before the mission took place.

Domes

A variety of shield volcanoes can be found in selected locations on the lunar surface, such as on Mons R mker. HP Compaq Business NC6220 Battery These are believed to be formed by relatively viscous, possibly silica-rich lava, erupting from localized vents. The resulting lunar domes are wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the mid-point. They are typically 8-12 km in diameter, but can be up to 20 km across. Some of the domes contain a small pit at their peak.Sony VGP-BPS9 Battery

Wrinkle ridges

Wrinkle ridges are features created by compressive tectonic forces within the maria. These features represent buckling of the surface and form long ridges across parts of the maria. Some of these ridges may outline buried craters or other features beneath the maria. A prime example of such an outlined feature is the crater Letronne.Sony VGP-BPS13 Battery

Grabens

Grabens are tectonic features that form under extension stresses. Structurally, they are composed of two normal faults, with a down-dropped block between them. Most grabens are found within the lunar maria near the edges of large impact basins.HP Compaq nx7300 Adapter

Impact craters

It may be surprising to learn that the origin of the Moon's craters as impact features became widely accepted only in the 1940s. This realization allowed the impact history of the Moon to be gradually worked out by means of the geologic principle of superposition. That is, if a crater (or its ejecta) overlaid another, it must be the younger. HP Compaq 2510p Adapter The amount of erosion experienced by a crater was another clue to its age, though this is more subjective. Adopting this approach in the late 1950s, Gene Shoemaker took the systematic study of the Moon away from the astronomers and placed it firmly in the hands of the lunar geologists.Dell Studio PP39L Battery Impact cratering is the most notable geological process on the Moon. The craters are formed when a solid body, such as an asteroid or comet, collides with the surface at a high velocity (mean impact velocities for the Moon are about 17 km per second). The kinetic energy of the impact creates a compression shock wave that radiates away from the point of entry. Nikon COOLPIX 5700 Battery This is succeeded by a rarefaction wave, which is responsible for propelling most of the ejecta out of the crater. Finally there is a hydrodynamic rebound of the floor that can create a central peak. These craters appear in a continuum of diameters across the surface of the Moon, ranging in size from tiny pits to the immense South Pole-Aitken Basinwith a diameter of nearly 2,500 km and a depth of 13 km.^{Dell Studio XPS 16 Battery} In a very general sense, the lunar history of impact cratering follows a trend of decreasing crater size with time. In particular, the largest impact basins were formed during the early periods, and these were successively overlaid by smaller craters. ^{HP Compaq nx7400 Adapter} Thesize frequency distribution (SFD) of crater diameters on a given surface (that is, the number of craters as a function of diameter) approximately follows apower law with increasing number of craters with decreasing crater size. The vertical position of this curve can be used to estimate the age of the surface.HP Pavilion DV6500 Adapter The most recent impacts are distinguished by well-defined features, including a sharp-edged rim. Small craters tend to form a bowl shape, while larger impacts can have a central peak with flat floors. Larger craters generally display slumping features along the inner walls that can form terraces and ledges. Dell T117C Battery The largest impact basins, the multiring basins, can even have secondary concentric rings of raised material. The impact process excavates high albedo materials that initially gives the crater, ejecta, and ray system a bright appearance. The process of space weathering gradually decreases the albedo of this material such that the rays fade with time. Acer PA-1650-02 Adapter Gradually the crater and its ejecta undergo impact erosion from micrometeorites and smaller impacts. This erosional process softens and rounds the features of the crater. The crater can also be covered in ejecta from other impacts, which can submerge features and even bury the central peak.Acer Aspire 5920G Adapter The ejecta from large impacts can include larges blocks of material that reimpact the surface to form secondary impact craters. These craters are sometimes formed in clearly discernible radial patterns, and generally have shallower depths than primary craters of the same size. ^{Asus S1 Adapter} In some cases an entire line of these blocks can impact to form a valley. These are distinguished from catena, or crater chains, which are linear strings of craters that are formed when the impact body breaks up prior to impact. Generally speaking, a lunar crater is roughly circular in form. HP Pavilion dv6000 Battery Laboratory experiments at NASA's Ames Research Center have demonstrated that even very low-angle impacts tend to produce circular craters, and that elliptical craters start forming at impact angles below five degrees. However, a low angle impact can produce a central peak that is offset from the mid-point of the crater. Ibm Thinkpad T30 Battery Additionally, the ejecta from oblique impacts show distinctive patterns at different impact angles: asymmetry starting around 60? and a wedge-shaped "zone of avoidance" free of ejecta in the direction the projectile came from starting around 45?. Dell Studio PP39L Battery Dark-halo craters are formed when an impact excavates lower albedo material from beneath the surface, then deposits this darker ejecta around the main crater. This can occur when an area of darker basaltic material, such as that found on the maria, is later covered by lighter ejecta derived from more distant impacts in the highlands. ^{HP Pavilion dv5 Adapter} This covering conceals the darker material below, which is later excavated by subsequent craters. The largest impacts produced melt sheets of molten rock that covered portions of the surface which could be as thick as a kilometer. Examples of such impact melt can be seen in the northeastern part of the Mare Orientale impact basin.HP Compaq Business NC6400 Battery

Regolith

The surface of the Moon has been subject to billions of years of collisions with both small and large asteroidal and cometary materials. Over time, these impact processes have pulverized and "gardened" the surface materials, forming a fine grained layer termed "regolith".HP Compaq 490306-001 Battery The thickness of the regolith varies between 2 meters beneath the younger maria, to up to 20 meters beneath the oldest surfaces of the lunar highlands. The regolith is predominantly composed of materials found in the region, but also contains traces of materials ejected by distant impact craters. ^{Compaq Presario CQ35-100 Battery} The term "mega-regolith" is often used to describe the heavily fractured bedrock directly beneath the near-surface regolith layer. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts. In most of the lunar regolith, half of the particles are made of mineral fragments fused by the glassy particles; ^{HP Compaq HSTNN-LB51 Battery} these objects are called agglutinates. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in aluminium and silica, just as the rocks in those regions. The regolith in the maria is rich in iron and magnesium and is silica-poor, as the basaltic rocks from which it is formed.^{HP Pavilion ZE4500 Battery} The lunar regolith is very important because it also stores information about the history of the Sun. The atoms that compose the solar wind - mostly helium,neon, carbon and nitrogen - hit the lunar surface and insert themselves into the mineral grains.^{HP G62-100EB Battery} Upon analyzing the composition of the regolith, particularly its isotopic composition, it is possible to determine if the activity of the Sun has changed with time. The gases of the solar wind could be useful for future lunar bases, since oxygen, hydrogen (water), carbon and nitrogen are not only essential to sustain life, but are also potentially very useful in the production of fuel. ^{HP Pavilion DV6-1211AX Battery} The composition of the lunar regolith can also be used to infer its source origin.

Lunar lava tubes

Lunar lava tubes form a potentially important location for constructing a future lunar base, which may be used for local exploration and development, or as a human outpost to serve exploration beyond the Moon. HP Pavilion DV6-1220SO Battery A lunar lava cave potential has long been suggested and discussed in literature and thesis. Any intact lava tube on the moon could serve as a shelter from the severe environment of the lunar surface, with its frequent meteorite impacts, high-energy ultraviolet radiation and energetic particles, and extreme diurnal temperature variations.HP EliteBook 6930P Battery Following the launch of the Lunar Reconnaissance Orbiter, many lunar lava tubes have been imaged. These lunar pits are found in several locations across the moon, including Marius Hills, Mare Ingenii and Mare Tranquillitatis.

The lunar magma ocean

The first rocks brought back by Apollo 11 were basalts. HP Pavilion DV6-1210SA Battery Although the mission landed on Mare Tranquillitatis, a few millimetric fragments of rocks coming from the highlands were picked up. These are composed mainly of plagioclase feldspar; some fragments were composed exclusively of anorthositic plagioclase. Compaq Presario CQ71 Battery The identification of these mineral fragments led to the bold hypothesis that a large portion of the Moon was once molten, and that the crust formed by fractional crystallization of this magma ocean. A natural outcome of the giant impact event is that the materials that reaccreted to form the Moon must have been hot.HP Pavilion DV7-1290EO Battery Current models predict that a large portion of the Moon would have been molten shortly after the Moon formed, with estimates for the depth of this magma ocean ranging from about 500 km to full moon melting. Crystallization of this magma ocean would have given rise to a differentiated body with a compositionally distinct crust and mantle and accounts for the major suites of lunar rocks.HP G6000 Battery As crystallization of the lunar magma ocean proceeded, minerals such as olivine and pyroxene would have precipitated and sank to form the lunar mantle. After crystallization was about three-quarters complete, anorthositic plagioclase would have begun to crystallize, and because of its low density, float, forming an anorthositic crust. HP HDX X18-1005EA Battery Importantly, elements that are incompatible (i.e., those that partition preferentially into the liquid phase) would have been progressively concentrated into the magma as crystallization progressed, forming a KREEP-rich magma that initially should have been sandwiched between the crust and mantle. ^{HP Pavilion DV6-3013SL Battery} Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials.

Surface materials

The Apollo program brought back 381.7 kg (841.5 lb) of lunar surface material, most of which is stored at the Lunar Receiving Laboratory in Houston, Texas. HP Compaq HSTNN-IB62 Battery These rocks have proved to be invaluable in deciphering the geologic evolution of the Moon. Lunar rocks are in large part made of the same common rock forming minerals as found on Earth, such as olivine, pyroxene, and plagioclase feldspar(anorthosite).^{HP Pavilion DV6100 Battery} Plagioclase feldspar is mostly found in the lunar crust, while pyroxene and olivine are typically seen in the lunar mantle. The mineral ilmenite is highly abundant in some mare basalts, and a new mineral named armalcolite (named for Armstrong, Aldrin, and Collins, the three members of the Apollo 11 crew) was first discovered in the lunar samples.^{HP EliteBook 2730P Battery} The maria are composed predominantly of basalt, whereas the highland regions are iron-poor and composed primarily of anorthosite, a rock composed primarily of calcium-rich plagioclase feldspar. Another significant component of the crust are the igneous Mg-suite rocks, such as the troctolites, norites, and KREEP-basalts. HP Pavilion DV9000 Battery These rocks are believed to be genetically related to the petrogenesis ofKREEP. Composite rocks on the lunar surface often appear in the form of breccias. Of these, the subcategories are called fragmental, granulitic, and impact-melt breccias, depending on how they were formed. HP Pavilion DV7 Battery The mafic impact melt breccias, which are typified by the low-K Fra Mauro composition, have a higher proportion of iron and magnesium than typical upper crust anorthositic rocks, as well as higher abundances of KREEP.

Composition of the maria

The main characteristics of the basaltic rocks with respect to the rocks of the lunar highlands is that the basalts contain higher abundances of olivine and pyroxene, and less plagioclase.^{HP Pavilion DV7-1290EO Battery} They are more rich in iron than terrestrial basalts, and also have lower viscosities. Some of them have high abundances of a ferro-titanic oxide called ilmenite. Since the first sampling of rocks contained a high content of ilmenite and other related minerals, they received the name of "high titanium" basalts. ^{HP Compaq Business 6735S Battery} The Apollo 12 mission returned to Earth with basalts of lower titanium concentrations, and these were dubbed "low titanium" basalts. Subsequent missions, including the Soviet unmanned probes, returned with basalts with even lower concentrations, now called "very low titanium" basalts. ^{HP 540 Battery} The Clementine space probe returned data showing that the mare basalts possess a continuum in titanium concentrations, with the highest concentration rocks being the least abundant.

Internal structure of the Moon

The current model of the interior of the Moon was derived using seismometers left behind during the manned Apollo program missions, as well as investigations of the Moon's gravity field and rotation.^{HP Compaq Business NW9440 Battery} The mass of the Moon is sufficient to eliminate any voids within the interior, so it is believed to be composed of solid rock throughout. Its low bulk density (~3346 kg m^?3) indicates a low metal abundance. Mass and moment of inertia constraints indicate that the Moon likely has an iron core that is less than about 450 km in radius.^{HP Compaq Business TC4400 Battery} Studies of the Moon's physical librations (small perturbations to its rotation) furthermore indicate that the core is still molten. Most planetary bodies and moons have iron cores that are about half the size of the body. The Moon is thus anomalous in possessing a core whose size is only about one quarter of its radius.HP Compaq 491278-001 Battery The crust of the Moon is on average about 50 km thick (though this is uncertain by about 15 km). It is widely believed that the far-side crust is on average thicker than the near side by about 15 km. Seismology has constrained the thickness of the crust only near the Apollo 12 and 14 landing sites. HP HSTNN-LB31 Battery While the initial Apollo-era analyses suggested a crustal thickness of about 60 km at this site, recent reanalyses of this data set suggest a thinner value, somewhere between about 30 and 45 km. Compared to that of Earth, the Moon has only a very weak external magnetic field.HP Pavilion DV3-4000 Battery Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating.HP Pavilion TX1000 Battery The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on airless bodies such as the Moon, transient magnetic fields could be generated during impact processes. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the largest impact basins. HP Pavilion DV6-1210SA Battery While the Moon does not possess a dipolar magnetic field like the Earth does, some of the returned rocks possess strong magnetizations. Furthermore measurements from orbit show that some portions of the lunar surface are associated with strong magnetic fields.HP 448007-001 Battery,HP Pavilion DM4 Battery

The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, a quarter the diameter of Earth and ¹?₈₁ its mass (Charon is proportionally larger in comparison toPluto, but Pluto has been reclassified as a dwarf planet). HP 586006-321 Battery The Moon is the second densest satellite after Io. It is in synchronous rotation with Earth, always showing the same face; the near side is marked with dark volcanic maria among the bright ancient crustal highlands and prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a similar reflectance to coal.HP 586006-361 Battery Its prominence in the sky and its regular cycle of phases have since ancient times made the Moon an important cultural influence on language, calendars, art andmythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day.HP 586028-341 Battery The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. The Moon is the only celestial body on which humans have landed. HP h588178-141 Battery While the Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959, the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972 the first being Apollo 11. HP 593553-001 Battery These missions returned over 380 kg of lunar rocks, which have been used to develop a detailed geological understanding of the Moon's origins (it is thought to have formed some 4.5 billion years ago in a giant impact event involving Earth), the formation of its internal structure, and its subsequent history.HP 593554-001 Battery After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft, notably by the final Soviet Lunokhod rover. Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters. HP HSTNN-CB0W Battery These spacecraft have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. Future manned missions to the Moon have been planned, including government as well as privately funded efforts. HP HSTNN-CB0X Battery The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.

Name and etymology

The English proper name for Earth's natural satellite is "the Moon". The noun moon derives from moone (around 1380), which developed frommone (1135), which derives from Old English m?na (dating from before 725), which, like all Germanic language cognates, ultimately stems fromProto-Germanic *m?n?n. HP HSTNN-I79C Battery The principal modern English adjective pertaining to the Moon is lunar, derived from the Latin Luna. Another less common adjective is selenic, derived from the Ancient Greek Selene (??????), from which the prefix "seleno-" (as in selenography) is derived.HP HSTNN-I81C Battery

Formation

Several mechanisms have been proposed for the Moon's formation 4.527 0.010 billion years ago,^{[nb 5]} some 30-50 million years after the origin of the Solar System. These include the fission of the Moon from the Earth's crust through centrifugal forces, which would require too great an initial spin of the Earth, HP HSTNN-IB0N Battery the gravitational capture of a pre-formed Moon, which would require an unfeasibly extended atmosphere of the Earth todissipate the energy of the passing Moon,^[13] and the co-formation of the Earth and the Moon together in the primordial accretion disk, which does not explain the depletion of metallic iron in the Moon.HP HSTNN-IB0X Battery These hypotheses also cannot account for the high angular momentum of the Earth-Moon system. The prevailing hypothesis today is that the Earth-Moon system formed as a result of a giant impact: a Mars-sized body hit the nearly formed proto-Earth, blasting material into orbit around the proto-Earth, which accreted to form the Moon.HP HSTNN-LB0W Battery Giant impacts are thought to have been common in the early Solar System. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth-Moon system, and the small size of the lunar core; they also show that most of the Moon came from the impactor, not from the proto-Earth.HP HSTNN-OB0X Battery However, meteorites show that other inner Solar System bodies such as Mars and Vesta have very different oxygen and tungsten isotopic compositions to the Earth, while the Earth and Moon have near-identical isotopic compositions. Post-impact mixing of the vaporized material between the forming Earth and Moon could have equalized their isotopic compositions, although this is debated.HP HSTNN-OB0Y Battery The large amount of energy released in the giant impact event and the subsequent reaccretion of material in Earth orbit would have melted the outer shell of the Earth, forming a magma ocean. The newly formed Moon would also have had its own lunar magma ocean; estimates for its depth range from about 500 km to the entire radius of the Moon.HP HSTNN-Q61C Battery Internal structure The Moon is a differentiated body: it has a geochemically distinct crust, mantle, and core. The moon has a solid iron-rich inner core with a radius of 240 kilometers and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometers. Around the core is a partially molten boundary layer with a radius of about 500 kilometers.HP HSTNN-Q62C Battery This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, HP HSTNN-YB0X Battery clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust on top. The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance ofincompatible and heat-producing elements.HP NBP6A174B1 Battery Consistent with this, geochemical mapping from orbit shows the crust is mostly anorthosite, and moon rocksamples of the flood lavas erupted on the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron rich than that of Earth. Geophysical techniques suggest that the crust is on average ~50 km thick.HP HSTNN-YB0X Battery The Moon is the second densest satellite in the Solar System after Io. However, the inner core of the Moon is small, with a radius of about 350 km or less; this is only ~20% the size of the Moon, in contrast to the ~50% of most other terrestrial bodies. HP NBP6A174 Battery Its composition is not well constrained, but it is probably metallic iron alloyed with a small amount of sulphur and nickel; analyses of the Moon's time-variable rotation indicate that it is at least partly molten.

Surface geology

The topography of the Moon has been measured with laser altimetry and stereo image analysis.HP NBP6A174B1 Battery The most visible topographic feature is the giant far side South Pole - Aitken basin, some 2,240 km in diameter, the largest crater on the Moon and the largest known crater in the Solar System. At 13 km deep, its floor is the lowest elevation on the Moon.HP NBP6A175 Battery The highest elevations are found just to its north-east, and it has been suggested that this area might have been thickened by the oblique formation impact of South Pole - Aitken. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims.HP NBP6A175B1 Battery The lunar far side is on average about 1.9 km higher than the near side.

Volcanic features

The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called maria (Latin for "seas"; singular mare), since they were believed by ancient astronomers to be filled with water.HP WD548AA Battery They are now known to be vast solidified pools of ancient basaltic lava. While similar to terrestrial basalts, the mare basalts have much higher abundances of iron and are completely lacking in minerals altered by water.The majority of these lavas erupted or flowed into the depressions associated with impact basins.HP WD549AA Battery Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side maria. Maria are found almost exclusively on the near side of the Moon, covering 31% of the surface on the near side, compared with a few scattered patches on the far side covering only 2%.HP Envy 17 Notebook PC Battery This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.HP Envy 17T Battery Most of the Moon's mare basalts erupted during the Imbrian period, 3.0-3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years, and the youngest eruptions, dated by crater counting, appear to have been only 1.2 billion years ago.HP HSTNN-Q61C Battery The lighter-coloured regions of the Moon are called terrae, or more commonly highlands, since they are higher than most maria. They have been radiometrically dated as forming 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean.HP GSTNN-Q62C Battery In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.

Impact craters

The other major geologic process that has affected the Moon's surface is impact cratering, with craters formed when asteroids and comets collide with the lunar surface. HP HSTNN-IB1E Battery There are estimated to be roughly 300,000 craters wider than 1 km on the Moon's near side alone. Some of these are named for scholars, scientists, artists and explorers. The lunar geologic timescale is based on the most prominent impact events, including Nectaris, HP HSTNN-IB0X Battery Imbrium, and Orientale, structures characterized by multiple rings of uplifted material, typically hundreds to thousands of kilometres in diameter and associated with a broad apron of ejecta deposits that form a regionalstratigraphic horizon. The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved. HP HSTNN-YB0X Battery While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Since impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.HP 586006-361 Battery The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment of impacts. Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. HP NBP6A174 Battery The finer regolith, the lunar soil of silicon dioxide glass, has a texture like snow and smell like spent gunpowder. The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10-20 m in the highlands and 3-5 m in the maria.HP NBP6A174B1 Battery Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometres thick.

Presence of water

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociationand is lost to space. HP 588178-141 Battery However since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly survive in cold, permanently shadowed craters at either pole on the Moon.HP Pavilion dv5 Adapter Computer simulations suggest that up to 14,000 km² of the surface may be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.Sony NP-F330 Battery In years since, signatures of water have been found to exist on the lunar surface. In 1994, the bistatic radar experiment located on the Clementinespacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters.Asus Eee PC 904HA Adapter In 1998, the neutron spectrometer located on the Lunar Prospector spacecraft, indicated that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. In 2008, an analysis of volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water to exist in the interior of the beads.Nikon D80 Battery The 2008, Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. Sony PCGA-AC19V9 Adapter The spacecraft showed that concentrations may possibly be as high as 1,000 ppm. In 2009, LCROSS sent a 2300 kg impactor into a permanently shadowed polar crater, and detected at least 100 kg of water in a plume of ejected material. Another examination of the LCROSS data showed the amount of detected water, to be closer to 155-kilograms ( 12 kg).Dell T555C Battery In May 2011, Erik Hauri et al. reported^[65] 615-1410 ppm water in melt inclusions in lunar sample 74220, the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17mission in 1972. The inclusions were formed during explosive eruptions on the moon approximately 3.7 billion years ago.HP Pavilion DV6-3013SL Battery This concentration is comparable with that of magma in Earth's upper mantle. While of considerable selenological interest, this announcement affords little comfort to would-be lunar colonists. The sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.HP Pavilion DV7-2085EL Battery

Gravity and magnetic fields

The gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins,HP Pavilion DV9700 Battery partly caused by the dense mare basaltic lava flows that fill these basins. These anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.HP Mini 702EA Battery The Moon has an external magnetic field of the order of one to a hundred nanoteslas, less than one-hundredth that of the Earth. It does not currently have a global dipolar magnetic field, as would be generated by a liquid metal core geodynamo, and only has crustal magnetization, probably acquired early in lunar history when a geodynamo was still operating.HP Pavilion DV7-1120EO Battery Alternatively, some of the remnant magnetization may be from transient magnetic fields generated during large impact events, through the expansion of an impact-generated plasma cloud in the presence of an ambient magnetic field this is supported by the apparent location of the largest crustal magnetizations near the antipodes of the giant impact basins.HP Compaq 491278-001 Battery

Atmosphere

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tons. The surface pressure of this small mass is around 3 10^?15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, the release of atoms from the bombardment of lunar soil by solar wind ions.HP Pavilion DV6-1030US Battery Elements that have been detected include sodiumand potassium, produced by sputtering, which are also found in the atmospheres of Mercury and Io; helium-4 from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle.HP Compaq 490306-001 Battery The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60-70 degrees; it is possibly generated from the sublimationof water ice in the regolith.HP Pavilion DV5-1211EM Battery These gases can either return into the regolith due to the Moon's gravity, or be lost to space: either through solar radiation pressure, or if they are ionised, by being swept away by the solar wind's magnetic field.

Seasons

The Moon's axial tilt is only 1.54 , much less than the 23.44 of the Earth. HP EliteBook 2530P Battery Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects.From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of Peary crater at the Moon's north pole remain illuminated for the entire lunar day, creating peaks of eternal light.HP Pavilion DV4385EA Battery No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters, and these dark craters are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (?238 C), and just 26 K close to the winter solstice in north polarHermite Crater. Compaq PP2100 Battery This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto.

Orbit

The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days^{[nb 6]} (its sidereal period). HP Compaq KU532AA Battery However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show the samephase to Earth, which is about 29.5 days^{[nb 7]} (its synodic period).^[38] Unlike most satellites of other planets, the Moon orbits nearer the ecliptic plane than to the planet's equatorial plane. HP Pavilion DV6-1270EQ Battery The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbital motion gradually rotates, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.HP Pavilion DV4385EA Battery

Relative size

The Moon is exceptionally large relative to the Earth: a quarter the diameter of the planet and 1/81 its mass.It is the largest moon orbiting aplanet in the solar system relative to the size of its planet (although Charon is larger relative to the dwarf planet Pluto, at slightly more than 1/9 (11.6%) of Pluto's mass.)HP Compaq Business NC4400 Battery However, the Earth and Moon are still considered a planet-satellite system, rather than a double-planet system, as their barycentre, the common centre of mass, is located 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.^{HP Pavilion ZD8227 Battery}

Appearance from Earth

The Moon is in synchronous rotation: it rotates about its axis in about the same time it takes to orbit the Earth. This results in it nearly always keeping the same face turned towards the Earth. Nikon D200 Battery The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and becametidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth. The side of the Moon that faces Earth is called the near side, and the opposite side the far side.Acer PA-1650-02 Adapter The far side is often called the "dark side," but in fact, it is illuminated as often as the near side: once per lunar day, during the new Moon phase we observe on Earth when the near side is dark. The Moon has an exceptionally low albedo, giving it a similar reflectance to coal. Despite this, it is the second brightest object in the sky after theSun.Sony CYBER-SHOT DSC-T10 Battery This is partly due to the brightness enhancement of the opposition effect; at quarter phase, the Moon is only one-tenth as bright, rather than half as bright, as at full Moon. Additionally,colour constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, HP Compaq Business 6715b Battery and since the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full Moon seem as bright as the centre, with no limb darkening, due to the reflective properties of lunar soil, which reflects more light back towards the Sun than in other directions.Kodak KLIC-7004 Battery The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC. The full Moon subtends an arc of about 0.52 (on average) in the sky, roughly the same apparent size as the Sun (see eclipses).Acer Aspire 5920G Adapter The highest altitude of the Moon in the sky varies: while it has nearly the same limit as the Sun, it alters with the lunar phase and with the season of the year, with the full Moon highest during winter. The 18.6-year nodes cycle also has an influence: when the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can go as far as 28 each month. Canon Digital IXUS 90 IS Battery This means the Moon can go overhead at latitudes up to 28 from the equator, instead of only 18 . The orientation of the Moon's crescent also depends on the latitude of the observation site: close to the equator, an observer can see a smile-shaped crescent Moon.HP 2133 Mini-Note PC Adapter The distance between the moon and the Earth varies from around 356,400 km to 406,700 km at the extreme perigees (closest) and apogees (farthest). On 19 March 2011, it was closer to the earth while at full phase than it has been since 1993. Reported as a "super moon", this closest point coincides within an hour of a full moon, and it thus appeared 30 percent brighter, and 14 percent larger than when at its greatest distance. Toshiba PA3536U-1BRS Battery There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. HP Compaq Business NX6110 Battery However, outgassing does occasionally occur, and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.HP Pavilion DV9000 Adapter The Moon's appearance, like that of the Sun, can be affected by Earth's atmosphere: common effects are a 22 halo ring formed when the Moon's light is refracted through the ice crystals of high cirrostratus cloud, and smallercoronal rings when the Moon is seen through thin clouds. Dell Studio 1537 Battery

Tidal effects

The tides on the Earth are mostly generated by the gradient in intensity of the Moon's gravitational pull from one side of the Earth to the other, the tidal forces. This forms two tidal bulges on the Earth, which are most clearly seen in elevated sea level as ocean tides.HP PPP009H Adapter Since the Earth spins about 27 times faster than the Moon moves around it, the bulges are dragged along with the Earth's surface faster than the Moon moves, rotating around the Earth once a day as it spins on its axis. The ocean tides are magnified by other effects: Sony VGP-BPS8 Battery frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The gravitational attraction of the Sun on the Earth's oceans is almost half that of the Moon, and their gravitational interplay is responsible for spring and neap tides.Dell PA-12 Adapter Gravitational coupling between the Moon and the bulge nearest the Moon acts as a torque on the Earth's rotation, draining angular momentum and rotationalkinetic energy from the Earth's spin. In turn, angular momentum is added to the Moon's orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period.Sony Cyber-shot DSC-T7/B Battery As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down. Measurements from lunar ranging experiments with laser reflectors left during the Apollo missions have found that the Moon's distance to the Earth increases by 38 mm per year (though this is only 0.10 ppb/year of the radius of the Moon's orbit).Dell LATITUDE M65 Battery Atomic clocks also show that the Earth's day lengthens by about 15 microseconds every year, slowly increasing the rate at which UTC is adjusted by leap seconds. This tidal drag will continue until the spin of the Earth has slowed to match the orbital period of the Moon; however, long before this could happen, the Sun will have become a red giant, engulfing the Earth. HP Compaq 6720s Adapter The lunar surface also experiences tides of amplitude ~10 cm over 27 days, with two components: a fixed one due to the Earth, as they are in synchronous rotation, and a varying component from the Sun. The Earth-induced component arises from libration, a result of the Moon's orbital eccentricity; Acer AS07B41 Battery if the Moon's orbit were perfectly circular, there would only be solar tides. Libration also changes the angle from which the Moon is seen, allowing about 59% of its surface to be seen from the Earth (but only half at any instant). The cumulative effects of stress built up by these tidal forces produces moonquakes. Dell Vostro 1310 Battery Moonquakes are much less common and weaker than earthquakes, although they can last for up to an hour - a significantly longer time than terrestrial earthquakes - because of the absence of water to damp out the seismic vibrations. The existence of moonquakes was an unexpected discovery fromseismometers placed on the Moon by Apollo astronauts from 1969 through 1972.Sony VGP-BPL13 Battery

Eclipses

Eclipses can only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur near a new Moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur near a full Moon, when the Earth is between the Sun and Moon. Sony VGP-BPS13 Battery The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the precise vastly greater distance that coincidentally gives it the same apparent size as the much closer and much smaller Moon from the perspective of the Earth. Sony VGP-BPS13/B Battery The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular(with the Moon appearing smaller than the Sun) solar eclipses.Sony VGP-BPS13/S Battery In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slowly increasing over time, the angular diameter of the Moon is decreasing.Sony VGP-BPS13A/B Battery This means that hundreds of millions of years ago the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, about 600 million years from now (if the angular diameter of the Sun does not change), the Moon will no longer cover the Sun completely, and only annular eclipses will occur.Sony VGP-BPS13A/S Battery Because the Moon's orbit around the Earth is inclined by about 5 to the orbit of the Earth around the Sun, eclipses do not occur at every full and new Moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. The periodicity and recurrence of eclipses of the Sun by the Moon,Sony VGP-BPS13AS Battery and of the Moon by the Earth, is described by the saros cycle, which has a period of approximately 18 years. As the Moon is continuously blocking our view of a half-degree-wide circular area of the sky, the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. Sony VGP-BPS13B/B Battery In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to the Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.Sony VGP-BPS13B/Q Battery

Early studies

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, and Indian astronomers had described the Moon's monthly elongation.Sony VGP-BPS13B/S Battery The Chinese astronomer Shi Shen (fl. 4th century BC)gave instructions for predicting solar and lunar eclipses. Later, the physical form of the Moon and the cause of moonlight became understood. The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.Sony VGP-BPS13Q Battery Although the Chinese of the Han Dynasty believed the Moon to be energy equated to qi, their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78-37 BC) noted the sphericity of the Moon.Sony VGP-BPS13S Battery In 499 AD, the Indian astronomerAryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. The astronomer and physicist Alhazen (965-1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions.Sony VGP-BPL21 Battery Shen Kuo (1031-1095) of the Song Dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. Sony VGP-BPS21 Battery In Aristotle's (384-322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries.Sony VGP-BPS21/S Battery However, in the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the Earth radius for the distance.Sony VGP-BPS21A Battery These figures were greatly improved by Ptolemy (90-168 AD): his values of a mean distance of 59 times the Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively. Archimedes (287-212 BC) invented a planetarium calculating motions of the Moon and the known planets.Sony VGP-BPS21A/B Battery During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth". In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters. Sony VGP-BPS21B Battery Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834-6 Mappa Selenographica of Wilhelm Beer and Johann Heinrich M dler, and their associated 1837 book Der Mond,Sony VGP-BPL8 Battery the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions.Sony VGP-BPL8A Battery This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology. Sony VGP-BPL8B Battery

Soviet missions

The Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon. Once launchers had the necessary capabilities, these nations sent unmanned probes on both flyby and impact/lander missions. Sony VGP-BPS8 Battery Spacecraft from the Soviet Union's Luna programwere the first to accomplish a number of goals: following three unnamed, failed missions in 1958, the first man-made object to escape Earth's gravity and pass near the Moon was Luna 1; the first man-made object to impact the lunar surface was Luna 2,Sony VGP-BPS8A Battery and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966.Sony VGP-BPS8B Battery Rock and soil samples were brought back to Earth by three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total. Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme.Sony VAIO VGN-AW41JF Battery

United States missions

American lunar exploration began with robotic missions aimed at developing understanding of the lunar surface for an eventual manned landing: the Jet Propulsion Laboratory's Surveyor program landed its first spacecraft four months after Luna 9. HP Compaq Business 6910p Battery

the asteroid 433 Eros in 2001 (NEAR Shoemaker), and Saturn's moon Titan (Huygens) and the comet Tempel 1 (Deep Impact) in 2005. The Galileo orbiter also dropped a probe into Jupiter's atmosphere in 1995; since Jupiter has no physical surface, it was destroyed by increasing temperature and pressure as it descended.Toshiba PA3534U-1BRS Battery To date, only two worlds in the Solar System, the Moon and Mars, have been visited by mobile rovers. The first rover to visit another celestial body was the Soviet Lunokhod 1, which landed on the Moon in 1970. The first to visit another planet was Sojourner, which travelled 500 metres across the surface of Mars in 1997. Sony NP-FP50 Battery The only manned rover to visit another world was NASA's Lunar rover, which travelled with Apollos 15, 16 and 17 between 1971 and 1972.

Manned exploration

Manned exploration of the Solar System is currently confined to Earth's immediate environs. HP Compaq nx7300 Adapter The first human being to reach space (defined as an altitude of over 100 km) and to orbit the Earth was Yuri Gagarin, a Soviet cosmonaut who was launched in Vostok 1 on April 12, 1961. The first man to walk on the surface of another Solar System body was Neil Armstrong, who stepped onto the Moon on July 21, 1969 during the Apollo 11 mission;Dell Latitude D820 Adapter five more Moon landings occurred through 1972. The United States' Space Shuttle, which debuted in 1981, is the only reusable spacecraft to successfully make multiple orbital flights. The five shuttles that have been built have flown a total of 121 missions, with two of the craft destroyed in accidents. Toshiba PA3399U-1BRS Battery The first orbital space station to host more than one crew was NASA's Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999.Dell Studio XPS 1640 Battery It was decommissioned in 2001, and its successor, the International Space Station, has maintained a continuous human presence in space since then. In 2004,SpaceShipOne became the first privately funded vehicle to reach space on a suborbital flight. That year, U.S. President George W. ^{Dell Latitude D830 Battery} Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars. An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in a heliocentric model. ^{Dell Inspiron E1505 Adapter} Though the Greeks had working planetaria, the first orrery was a planetarium of the modern era was produced in 1704, and one was presented to the Earl of Orrery whence the name came. They are typically driven by a clockwork mechanism with a globe representing the Sun at the centre, and with a planet at the end of each of the arms.^{Asus Eee PC 1201PN Battery}

History

The first modern orrery was built circa 1704 by George Graham and Thomas Tompion. Graham gave the first model (or its design) to the celebrated instrument maker John Rowley of London to make a copy for Prince Eugene of Savoy. Rowley was commissioned to make another copy for his patronCharles Boyle, 4th Earl of Orrery, from which the device took its name.^{Asus Eee PC 701 Adapter} This model was presented to Charles' son John, later the 5th Earl. Its importance was partially in that mechanical models of the universe, correctly named planeteriums gained the name Orrery, There were planetary machines before 1704, indeed according to Cicero, the Greek philosopher who was writing in the first century BC, Posidoniusconstructed an planetary model.^{Dell XPS M1330 Battery} The Antikythera mechanism, discovered in 1904 in 42m of water off the Greek island of Antikythera and extensively studied, exhibited the diurnal motions of the sun, moon, and the five known planets. The Antikythera mechanism is now considered one of the first orreries but for many decades was ignored as it thought to be far too complex to be genuine,^{fujifilm np95 Battery} and was not mentioned in the 1967 Science Museum booklet. It was heliocentric and used as a mechanical calculator designed to calculate astronomical positions. Copernicus in De Revolutionibus Orbium Coelestium published in Nuremberg in 1543 challenged the Western religious teaching of a geocentric universe where the sun rotated around the earth. ^{Asus Eee PC 900 Adapter} He found that some Greek philosophers had proposed a heliocentric universe. This simplified the apparent epicyclic motions of the planets, making it feasible to represent the planets path as a simple circle. This could be modelled by the use of gears. ^{Sony VAIO PCG-713 Battery} Tycho Brahe'simproved instruments made precise observations of the skys (1576-1601), and from these Johannes Kepler (1621) deduced that planets orbited the sun in ellipses. In 1687 Isaac Newton explained the cause of this motion in his theory of gravitation.^{Dell Latitude D830 Adapter} Christian Huygens published details of a heliocentric planetary machine in 1703, which he built while resident in Paris between 1665 and 1681. He calculated the gear trains that were need to represent a year length of 365.242 days, and using that to produce the cycles of the principal planets. As lated as 1650, P. Dell Studio XPS 1640 Battery Schirleus has produced a Geocentric planetarium showing the sun as a planet with Mercury and Venus moving around it as moons. Joseph Wright's picture "A Philosopher giving a Lecture on the Orrery in which a lamp is put in place of the Sun" (ca. 1766) which hangs in Derby Museum and Art Gallery,^{Panasonic LUMIX DMC-TZ4S Battery} features a group (three men, three children, and a lone woman) listening to a lecture by a 'natural philosopher' the only light in the otherwise darkened room is from the "sun" in the brass orrery, which, in this case, has rings that cause it to appear to be similar to an armillary sphere. The demonstration was thereby able to cover eclipses.^{HP Compaq Business NX6320 Battery} To put this in context, it was in 1762 that John Harrison's Chronometer allowed longitude to be accurately measured and also in 1766 that the astronomerTitius first demonstrated that the mean distance of the planets could be represented by the progression.^{Canon POWERSHOT G2 Battery} That is, 0.4, 0.7, 1.0, 1.6, 2.8, 5.2 ... The numbers refer to astronomical units, that is 1.496 x 10? km (93 x 10? miles). The Derby Orrery does not show mean distance, but demonstrated the relative planetary movements: all part of a process understanding contemporary cutting edge scientific thinking.Dell Latitude E6500 Battery Eisinga's "Planetarium" was built from 1774 to 1781 by Eise Eisinga in his home in Franeker, in the Netherlands. It displays the planets across the width of a room's ceiling, and has been in operation almost continually since it was created. This orrery, was a planetarium in both senses of the word- firstly a complex planetary machine, ^{Dell LATITUDE D630 Battery} and secondly it was displayed in a special room, that was a sort of theatre for the observers. Eisinga house was bought by the Dutch Royal family who gave him a pension. In 1764, Benjamin Martin devised a new type of planetary model, where the planets were carried on brass arms leading from a series of concentric or coaxial tubes. Asus R33030 Adapter With this construction it was difficult to make the planets revolve, and to get the moons to turn around the planets. Martin suggested that the conventional orrery should be consist of three parts: The planetarium where the planets revolved around the sum, the tellurian which showed the inclined axis of the earth and how it revolved around the sun,Dell Mini 1011 Battery and the lunarium which showed the eccentric rotations of the moon around the earth. In one orrery, these three motions could be mounted on a common table, separately using the central spindle as a prime mover.

Explanation

All orreries are planetariums or planetaria (alternative plural). The term orrery has only existed since 1714. toshiba pa3332u-1brs Battery A grand orrery is one that includes the outer planets known at the time of its construction. The word planetarium, (plural always planetariums) has been captured and now usually refers to hemispherical theatres in which images of the night sky are projected onto an overhead surface. HP Compaq 6710b Adapter Planetariums (orreries) can range widely in size from hand-held to room-sized. An orrery is used to demonstrate the motion of the planets, while a mechanical device used to predict eclipses and transits is called an Astrarium. An orrery should properly include the sun, earth and the (earth's) moon (plus optionally other planets.) ^{HP Compaq nx6325 Adapter} A model that only includes the earth, its moon and the sun is called a tellurion, and one which only includes the earth and moon a lunarium. A jovilabe is a model of Jupiter and its moons. A planetarium will show the orbital period of each planet and the rotation rate, as shown in the table above. Acer Aspire 5720 Adapter A tellurian will show the earth with the moon revolving around the sun. It will use the angle ofinclination of the equator from the table above to show how it rotates around its own axis. It was show the earths moon, rotating around the earth. A lunarium is designed to show the complex motions of the moon as it revolves around the earth.^{Toshiba Equium A110-252 Battery} Orreries are usually not built to scale. Some fixed solar system scale models have been built and are often many kilometres in size. A normal mechanical clock could be used to produce an extremely simple orrery with the Sun in the centre, Earth on the minute hand and Jupiter on the hour hand; ^{Acer Aspire 5052ANWXMi Battery} Earth would make 12 revolutions around the Sun for every 1 revolution of Jupiter. Note however that Jupiter's actual year is 11.86 Earth years long, so this particular example would lose accuracy rapidly. A real orrery would be more accurate and include more planets, and would perhaps make the planets rotate as well.Dell Mini 1011v Battery

Projection orreries

Many planetariums (buildings) have a projection orrery, which projects onto the dome of the planetarium a Sun with either dots or small images of the planets. These usually are limited to the planets from Mercury to Saturn, although some include Uranus. ^{HP Compaq nx7300 Adapter} The light sources for the planets are projected onto mirrors which are geared to a motor which drives the images on the dome. Typically the Earth will circle the Sun in one minute, while the other planets will complete an orbit in time periods proportional to their actual motion. ^{Dell Vostro 1510 Battery} Thus Venus, which takes 224.7 days to orbit the Sun, will take 42 seconds to complete an orbit on an orrery, and Jupiter will take 11.86 minutes. Some planetariums have taken advantage of this to use orreries to simulate planets and their moons. ^{Toshiba PA3534U-1BRS Battery} Thus Mercury orbits the Sun in 0.24 of an Earth year, while Phobos and Deimos orbit Mars in a similar 4:1 time ratio. Planetarium operators wishing to show this have placed a red cap on the Sun (to make it resemble Mars) and turned off all the planets but Mercury and Earth. Similar tricks can be used to show Pluto and its three moons.^{Asus M50 Battery}

Notable orreries

Shoemaker John Fulton of Fenwick, Ayrshire, built three between 1823 and 1833 - the last is in Glasgow's Kelvingrove Art Gallery and Museum. The Franeker Planetarium built by a wool carder called Eise Eisinger in his own living room, in the small city of Franeker in Friesland, is in fact an orrery. ^{Asus Eee PC 901 Battery} It was constructed between 1774 to 1781. The "face" of the model looks down from the ceiling of a room, with most of the mechanical works in the space above the ceiling. It is driven by a pendulum clock, which has 9 weights or ponds. The planets move around the model in real time.Dell Inspiron 6400 Battery An innovative concept is to have people play the role of the moving planets and other Solar system objects. Such a model, called a human orrery, has been laid out with precision at the Armagh Observatory.HP Compaq Business NC6220 Battery

Meccano Planetaria

The first Meccano Orrery was described in June 1918 Meccano Manual, though it is the last quarter of the 20th Century that Alan Partridge, John Nuttall, Pat Briggs and Michael Whiting have experimented with the limitations and possibilty of this medium. There are six methods of building orreries:Sony VGP-BPS9 Battery

• Telescopic concentric tubing, though the tubing is not a Meccano part. For a full 9 planet system, 18 concentric tubes and 19 geared motions are needed
• Nested hollow turntables with internal gearing. A 1984 Jovilabe showing Jupiter and the 14 then known moons required 14 tuntables and 324 gears; it is accurate to within 0.01% .Sony VGP-BPS13 Battery
• Fixed central rod, with top drive and sun and planet gearing. In this method, the sun drives the first planet and this then drives the second and so on. The errors are thus cummulative, but non Meccano gearing can keep this with in 0.02%
• Fixed central tube, with bottom drive and sun and planet gearing. This removes the need for an overhead gantry- improving the appearance.HP Compaq nx7300 Adapter
• Two component epicyclic gearing.
• Fixed Central Rod, Epicyclic gearing, top drive by 'by-pass gearing' from below. HP Compaq 2510p Adapter

Solar System models, especially mechanical models, called orreries, that illustrate the relative positions and motions of the planets and moons in the Solar System have been built for centuries. While they often showed relative sizes, these models were usually not built to scale. Dell Studio PP39L Battery The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the Solar System a challenging task. As one example of the difficulty, the distance between the Earth and the Sun is almost 12 000 times the diameter of the Earth.Nikon COOLPIX 5700 Battery If the smaller planets are to be easily visible to the naked eye, large outdoor spaces are generally necessary, as is some means for highlighting objects that might otherwise not be noticed from a distance. The objects in such models do not move. Traditional orreries often did move and some used clockworks to make the relative speeds of objects accurate.^{Dell Studio XPS 16 Battery} These can be thought of as being correctly scaled in time instead of distance. One 1:6 300 000 000 scale model, designed to be easily replicated, is called The Thousand-Yard Model and spans about a kilometre. In it, the Earth is represented by a peppercorn. ^{HP Compaq nx7400 Adapter} A school class building this model might tape the peppercorn to an index card to make it more visible. Another scale model is the 1:10 000 000 000 model, in which 100 000 km is represented by 1 cm. In this model, the Sun is 600m from the Kuiper belt and dwarf planet Pluto. The largest scale model in the world is the Sweden Solar System.HP Pavilion DV6500 Adapter In July 2005 the Austrian art group monochrom placed the planets true to scale (sun, 4 meters in diameter at Machine Gallery, Alvarado Street, near Echo Park) throughout the Los Angeles cityscape. Then they conducted an 'illegal space car race' through the Solar System.Dell T117C Battery In regional Australia, in the area surrounding Coonabarabran (Australia's Astronomy Capital) there is a model Solar System that reaches from the Observatory (The Sun) to surrounding towns and localities. In the US, a precedent-setting model Solar System project the Voyage National Program began with installation of Voyage on the National Mall in Washington, DC, in front of the Smithsonian museums.Acer PA-1650-02 Adapter It is a 1:10 000 000 000 scale model approved by the US Commission of Fine Arts and National Capital Planning Commission. It was designed for replication and permanent installation in communities across the nation. It was also designed as a center-piece for sustained community-wide science education, Acer Aspire 5920G Adapter embracing a "Learning Community Model" of programming for students, teachers, families, and the public. Grade K-12 lessons available for download enhance the exhibition experience. Voyage is now permanently installed in Kansas City, Houston, and Corpus Christi.^{Asus S1 Adapter} It is approved for installation in Des Moines, Orlando, and Baltimore, and a dozen other sites are exploring it as a site feature in their community. For the International Year of Astronomy 2009, Voyage has now become available worldwide as "the Voyage International Program" with storyboard translation into any language. HP Pavilion dv6000 Battery The Voyage National Program also has a Facebook page with photoalbums for all the Voyage communities. In 2009, to celebrate International Year of Astronomy, the Scottish Solar System project created an approximate scale model of the Solar System, with correctly scaled objects representing the major planets sited temporarily, Ibm Thinkpad T30 Battery for Autumn Moonwatch week (Oct 24th - Nov 1st 2009) at the locations of a number of Scottish amateur astronomy groups and societies. In this model the Sun was represented by the main building of Glasgow Science Centre. The Scottish Solar System project was coordinated by Dr Martin Hendry, astronomer at the University of Glasgow,Dell Studio PP39L Battery and Glasgow Science Centre. Although the model did not include Pluto (which on the same scale would be located roughly in the Faroe Islands), its scale of 1:8 200 000 was smaller than any other model listed in the table below. On the same scale the nearest star beyond the Solar System, Proxima Centauri, would still lie nearly 5 million km from Glasgow Science Centre.^{HP Pavilion dv5 Adapter}

A model based on a classroom globe

Relating the size of the Solar System to familiar objects can make it easier for students to grasp the relative distances. Most classroom globes are 41 cm (16 inches) in diameter. If the Earth were reduced to this size, the Moon would be a 10 cm (4 in) baseball floating 12 metres (40 feet) away.HP Compaq Business NC6400 Battery The Sun would be abeach ball 14 stories tall (somewhat smaller than the Spaceship Earth ride at Epcot) floating 5 kilometres (3 miles) away. While a complete model to this scale has never been built, a Solar System built centered in Washington DC, London, or Sydney, to that scale (approximately 1:31 000 000) would look like this:HP Compaq 490306-001 Battery If the scale of the above model is increased to 1:310 000 000, i.e. all distances and sizes reduced by a factor of 10, then the Earth and Venus can be modeled by ping pong balls, the Moon and smaller planets by various size marbles or lumps of modeling clay, the gas giants by balloons or larger playing balls, ^{Compaq Presario CQ35-100 Battery} and a circle the diameter of the Sun can be drawn on the floor of most classrooms. The scale distance to ? Centauri would be 1/3 of the way to the Moon.

A model based on a sports field

Relating the size of the Solar System to familiar objects can make it easier for students to grasp the relative distances.^{HP Compaq HSTNN-LB51 Battery} Most American schools have a football field associated with the high school (100 yards or 92 meters long). Other schools may have a soccer field nearby (90 to 120 m long). If the Sun was reduced to slightly less than one inch (21 mm), Pluto would be a 0.002 inch (0.05 mm) speck floating 100 yards (91.4 meters) away.^{HP Pavilion ZE4500 Battery} Jupiter would be less than three-thirty-secondths of an inch (2.38 mm) in diameter and would sit on the 13 yd (11.88 m) line. Uranus would be less than one-thirty-secondth of an inch (0.79 mm) sitting nearly on the 50 yd (45.72 m) line. At that scale, the speed of light would be about 1 inch every 5 seconds (5 mm per second). ^{HP G62-100EB Battery} Light takes about 5.5 hours to go from the Sun to Pluto. Here is what the Solar System built to that scale would look like. This complete model would be simple to make with scale planets taped to wood stakes or metal rods. The scale is approximately 1:64 700 000 000.^{HP Pavilion DV6-1211AX Battery}

A model for primary school children

Relating the size of objects to the planets can be difficult for children, particularly if the objects are so small that they cannot be seen. The Scale below is 10 x (times) the scale above, which is a convenient size, virtually all of the objects can be seen. At this scale the distance from the sun to Pluto is just under 1 km (metric units only). HP Pavilion DV6-1220SO Battery At this scale, the speed of light would be about 50 mm per second. Light takes about 5.5 hours to go from the Sun to Pluto. The scale is approximately 1:6 470 000 000. At this scale the Sun could be represented by a children's soccer ball, and ? Centauri A is about the size of an adult soccer ball. HP EliteBook 6930P Battery The smaller planets are about the size of "ice cream sprinkles" (which vary in size from less than 1mm to close to 2mm). Even at this scale ? Centauri A is on another continent or at the centre of the earth, a distance of about 6,300km.HP Pavilion DV6-1210SA Battery Children can have some fun with this model, for example Mercury could be a small red sprinkle while Earth might be a larger blue sprinkle. Venus might be smaller. Children can look for two small red spirnkles for Mercury and Mars. To maintain relativities Mercury should be slightly smaller than Mars etc. Compaq Presario CQ71 Battery The objects can be assembled in the classroom and then taken outside to a large park.

Misleading models

The models sketched here are an eye opener to many people interested in, but not knowing much about astronomy. HP Pavilion DV7-1290EO Battery They are far cry from the drawings of the Solar System one usually sees in books or on the internet, such as the one to the right. Even if it is stated in the text that the layout is not to scale, it is often difficult for the reader to fully comprehend how discrepant the scale of the distances in the image is compared to the sizes of the objects depicted.HP G6000 Battery,HP HDX X18-1005EA Battery

The Solar System consists of the Sun and the astronomical objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the many objects that orbit the Sun, most of themass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. HP 586006-321 Battery The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn,HP 586006-361 Battery are composed mainly of hydrogen and helium; the two outermost planets,Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". The Solar System is also home to two regions populated by smaller objects. HP 586028-341 Battery The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these two regions, five individual objects, Ceres, Pluto, Haumea, HP h588178-141 Battery Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.^[e] In addition to thousands of small bodies^[e] in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions.HP 593553-001 Battery The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.HP 593554-001 Battery Six of the planets and three of the dwarf planets are orbited by natural satellites,^[b] usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.

Discovery and exploration

For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. HP HSTNN-CB0W Battery People believed the Earth to be stationary at the center of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos,^[1]Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.HP HSTNN-CB0X Battery His 17th-century successors, Galileo Galilei, Johannes Kepler and Isaac Newton, developed an understanding of physics that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. HP HSTNN-I79C Battery In more recent times, improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena such as mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.HP HSTNN-I81C Battery

Structure

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86 percent of the system's known mass and dominates it gravitationally. The Sun's four largest orbiting bodies, the gas giants, account for 99 percent of the remaining mass, with Jupiter and Saturn together comprising more than 90 percent.HP HSTNN-IB0N Battery Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are frequently at significantly greater angles to it. All the planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above the Sun's north pole).HP HSTNN-IB0X Battery For exceptions, see retrograde motion.The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the outer Kuiper belt of icy objects.HP HSTNN-LB0W Battery Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets. Since the discovery of the Kuiper belt, HP HSTNN-OB0X Battery the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Kepler's laws of planetary motion describe the orbits of objects about the Sun. According to Kepler's laws, each object travels along an ellipse with the Sun at one focus. HP HSTNN-OB0Y Battery Objects closer to the Sun (with smaller semi-major axes) travel more quickly, as they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion.HP HSTNN-Q61C Battery The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits. Due to the vast distances involved, many representations of the Solar System show orbits the same distance apart.HP HSTNN-Q62C Battery In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 astronomical units (AU) farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. HP HSTNN-YB0X Battery Attempts have been made to determine a relationship between these orbital distances (for example, the Titius-Bode law), but no such theory has been accepted. Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which are larger than the planetMercury),HP NBP6A174B1 Battery or, in the case of the four gas giants, by planetary rings; thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. The objects of the inner Solar System are composed mostly of rock,HP HSTNN-YB0X Battery the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula. Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapor pressure such asmolecular hydrogen,HP NBP6A174 Battery helium, and neon, which were always in the gaseous phase in the nebula. ces, like water, methane, ammonia, hydrogen sulfide and carbon dioxide, have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature.HP NBP6A174B1 Battery They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.HP NBP6A175 Battery Together, gases and ices are referred to as volatiles.

Sun

The Sun is the Solar System's star, and by far its chief component. Its large mass (332,900 Earth masses) produces temperatures and densities in its core great enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation, peaking in the 400-700 nm band we call visible light.HP NBP6A175B1 Battery The Sun is classified as a type G2 yellow dwarf, but this name is misleading as, compared to the majority of stars in our galaxy, the Sun is rather large and bright. Stars are classified by the Hertzsprung-Russell diagram, a graph that plots the brightness of stars with their surface temperatures.HP WD548AA Battery Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarfs, are common, making up 85 percent of the stars in the galaxy. HP WD549AA Battery It is believed that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.HP Envy 17 Notebook PC Battery The Sun is a population I star; it was born in the later stages of the universe's evolution, and thus contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars. Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, HP Envy 17T Battery so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a planetary system, because planets form from accretion of "metals".HP HSTNN-Q61C Battery

Interplanetary medium

Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (seeheliopause).HP GSTNN-Q62C Battery This is known as the interplanetary medium. Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturb the heliosphere, creating space weather and causing geomagnetic storms. The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.HP HSTNN-IB1E Battery Earth's magnetic field stops its atmosphere from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space.Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. HP HSTNN-YB0X Battery The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into the Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles. Cosmic rays originate outside the Solar System. HP 586006-361 Battery The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.HP NBP6A174 Battery The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.HP NBP6A174B1 Battery The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.

Inner Solar System

The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids.HP 588178-141 Battery Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.

Inner planets

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. HP Pavilion dv5 Adapter They are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather;Sony NP-F330 Battery all have impact craters and tectonic surface features such as rift valleys andvolcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).Asus Eee PC 904HA Adapter

Mercury

Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history.Nikon D80 Battery

Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.Sony PCGA-AC19V9 Adapter

Venus

Venus (0.7 AU from the Sun) is close in size to Earth (0.815 Earth masses), and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. Dell T555C Battery

It is the hottest planet, with surface temperatures over 400 C, most likely due to the amount of greenhouse gases in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, HP Pavilion DV6-3013SL Battery

which suggests that its atmosphere is regularly replenished by volcanic eruptions.

Earth

Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the universe where life is known to exist.HP Pavilion DV7-2085EL Battery

Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.HP Pavilion DV9700 Battery

It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars

Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It possesses an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6 percent that of the Earth's).HP Mini 702EA Battery

Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxide (rust) in its soil.Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.HP Pavilion DV7-1120EO Battery

Asteroid belt

Asteroids are mostly small Solar System bodies^[e] composed mainly of refractory rocky and metallic minerals. The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.HP Compaq 491278-001 Battery Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta and Hygiea may be reclassed as dwarf planets if they are shown to have achievedhydrostatic equilibrium.HP Pavilion DV6-1030US Battery The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth. The main belt is very sparsely populated; spacecraft routinely pass through without incident.HP Compaq 490306-001 Battery Asteroids with diameters between 10 and 10^?4 m are called meteoroids.

Ceres

Ceres (2.77 AU) is the largest body in the asteroid belt and is classified as a dwarf planet. It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. HP Pavilion DV5-1211EM Battery Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids. It was again reclassified in 2006 as a dwarf planet.

Asteroid groups

Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics.HP EliteBook 2530P Battery Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets, which may have been the source of Earth's water.HP Pavilion DV4385EA Battery Trojan asteroids are located in either of Jupiter's L₄ or L₅ points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.Compaq PP2100 Battery The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.

Outer Solar System

The outer region of the Solar System is home to the gas giants and their large moons. Many short period comets, including the centaurs, also orbit in this region. HP Compaq KU532AA Battery Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia and methane, than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid.HP Pavilion DV6-1270EQ Battery

Outer planets

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are each many tens of times the mass of the Earth and consist overwhelmingly of hydrogen and helium;HP Pavilion DV4385EA Battery Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. For these reasons, some astronomers suggest they belong in their own category, "ice giants." All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. HP Compaq Business NC4400 Battery The term outer planet should not be confused withsuperior planet, which designates planets outside Earth's orbit and thus includes both the outer planets and Mars.

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. ^{HP Pavilion ZD8227 Battery}

It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot.

Jupiter has 63 known satellites. Nikon D200 Battery

The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, is larger than Mercury.

Saturn

Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Acer PA-1650-02 Adapter

Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Solar System. The rings of Saturn are made up of small ice and rock particles.

Saturn has 62 confirmed satellites; Sony CYBER-SHOT DSC-T10 Battery

two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice.^[56] Titan, the second largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.HP Compaq Business 6715b Battery

Uranus

Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants, and radiates very little heat into space.Kodak KLIC-7004 Battery

Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn.Acer Aspire 5920G Adapter

Neptune has 13 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen. Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune Trojans, that are in 1:1 resonance with it.Canon Digital IXUS 90 IS Battery

Comets

Comets are small Solar System bodies, typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. HP 2133 Mini-Note PC Adapter When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye. Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years.Toshiba PA3536U-1BRS Battery Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent. Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.HP Compaq Business NX6110 Battery Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.

Centaurs

The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter's (5.5 AU) and less than Neptune's (30 AU). HP Pavilion DV9000 Adapter The largest known centaur, 10199 Chariklo, has a diameter of about 250 km.The first centaur discovered, 2060 Chiron, has also been classified as comet (95P) since it develops a coma just as comets do when they approach the Sun.

Trans-Neptunian region

The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. Dell Studio 1537 Battery It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.HP PPP009H Adapter

Kuiper belt

The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from the Sun. Though it contains at least three dwarf planets, it is composed mainly of small Solar System bodies.Sony VGP-BPS8 Battery However, many of the largest Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth.Dell PA-12 Adapter Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic. The Kuiper belt can be roughly divided into the "classical" belt and the resonances. Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two).Sony Cyber-shot DSC-T7/B Battery The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB₁, and are still in near primordial, low-eccentricity orbits.Dell LATITUDE M65 Battery

Pluto and Charon

Pluto (39 AU average), a dwarf planet, is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. HP Compaq 6720s Adapter

Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.

Charon, Pluto's largest moon, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycenter of gravity above their surfaces (i.e., they appear to "orbit each other"). Acer AS07B41 Battery

Beyond Charon, two much smaller moons,Nix and Hydra, orbit within the system.

Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.Dell Vostro 1310 Battery

Haumea and Makemake

Haumea (43.34 AU average), and Makemake (45.79 AU average), while smaller than Pluto, are the largest known objects in the classical Kuiper belt (that is, they are not in a confirmed resonance with Neptune). Haumea is an egg-shaped object with two moons.Sony VGP-BPL13 Battery

Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated 2003 EL₆₁ and 2005 FY₉ respectively, they were given names and designated dwarf planets in 2008.^[72] Their orbits are far more inclined than Pluto's, at 28 and 29 .Sony VGP-BPS13 Battery

Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends much further outwards, is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration.Sony VGP-BPS13/B Battery Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."Sony VGP-BPS13/S Battery Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.

Eris

Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, Sony VGP-BPS13A/B Battery since it is 25% more massive than Pluto and about the same diameter. It is the most massive of the known dwarf planets. It has one moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.Sony VGP-BPS13A/S Battery

Farthest regions

The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun;Sony VGP-BPS13AS Battery this heliopause is considered the beginning of the interstellar medium. However, the Sun's Roche sphere, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.

Heliopause

The heliosphere is divided into two separate regions. Sony VGP-BPS13B/B Battery The solar wind travels at roughly 400 km/s until it collides with the interstellar wind; the flow of plasma in the interstellar medium. The collision occurs at the termination shock, which is roughly 80-100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.Sony VGP-BPS13B/Q Battery Here the wind slows dramatically, condenses and becomes more turbulent,forming a great oval structure known as the heliosheath. This structure is believed to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; Sony VGP-BPS13B/S Battery but evidence from the Cassini andInterstellar Boundary Explorer spacecraft has suggested that it is in fact forced into a bubble shape by the constraining action of the interstellar magnetic field. Both Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively.Sony VGP-BPS13Q Battery The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space. The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south,Sony VGP-BPS13S Battery e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through theMilky Way. No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space.Sony VGP-BPL21 Battery It is expected that NASA's Voyager spacecraft will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth. How well the heliosphere shields the Solar System from cosmic rays is poorly understood. Sony VGP-BPS21 Battery A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.

Oort cloud

The hypothetical Oort cloud is a spherical cloud of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (LY)), Sony VGP-BPS21/S Battery and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, Sony VGP-BPS21A Battery the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.

Sedna

90377 Sedna (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Sony VGP-BPS21A/B Battery Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object 2000 CR₁₀₅,Sony VGP-BPS21B Battery which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years. Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun. Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.Sony VGP-BPL8 Battery

Boundaries

Much of our Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.Sony VGP-BPL8A Battery Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.^[91] Objects may yet be discovered in the Solar System's uncharted regions.Sony VGP-BPL8B Battery

Galactic context

The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars. Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur.Sony VGP-BPS8 Battery The Sun lies between 25,000 and 28,000 light years from the Galactic Centre, and its speed within the galaxy is about 220 kilometres per second, so that it completes one revolution every 225-250 million years. This revolution is known as the Solar System's galactic year.Sony VGP-BPS8A Battery The solar apex, the direction of the Sun's path through interstellar space, is near the constellation of Hercules in the direction of the current location of the bright star Vega. The plane of the ecliptic lies at an angle of about 60 to thegalactic plane. The Solar System's location in the galaxy is very likely a factor in the evolution of life on Earth.Sony VGP-BPS8B Battery Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous supernovae, this has given Earth long periods of interstellar stability for life to evolve.HP Compaq Business 6910p Battery The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. Panasonic LUMIX DMC-TZ4S Battery The intense radiation of the galactic centre could also interfere with the development of complex life. Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun as radioactive dust grains and larger, comet-like bodies.Sony NP-FH100 Battery

Neighbourhood

The immediate galactic neighbourhood of the Solar System is known as the Local Interstellar Cloud or Local Fluff, an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light years across. Toshiba Equium M50-192 Battery The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae. There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light years away. Apple MacBook Pro Battery Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 light years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). Apple PowerBook G4 Battery The largest star within ten light years is Sirius, a bright main sequence star roughly twice the Sun's mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system Luyten 726-8 (8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).Acer BATBL50L6 Battery Our closest solitary sun-like star is Tau Ceti, which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity. The closest known extrasolar planet to the Sun lies around the star Epsilon Eridani, a star slightly dimmer and redder than the Sun, which lies 10.5 light years away. Acer Travelmate 2400 Battery Its one confirmed planet, Epsilon Eridani b, is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years.

Formation and evolution

The Solar System formed from the gravitational collapse of a giant molecular cloud 4.568 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.Dell Inspiron 1525 Adapter As the region that would become the Solar System, known as the pre-solar nebula,collapsed, conservation of angular momentum made it rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.Compaq Presario 2100 Adapter As the contracting nebula rotated, it began to flatten into a spinning protoplanetary disc with a diameter of roughly 200 AU^[104] and a hot, dense protostar at the centre. At this point in its evolution, the Sun is believed to have been a T Tauri star. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001-0.1 solar masses,Acer Aspire 1362 Battery with the vast majority of the mass of the nebula in the star itself. The planets formed by accretion from this disk. Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.Toshiba Equium M50-192 Battery The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged main sequencestar.Dell XPS M1330 Battery The Solar System as we know it today will last until the Sun begins its evolution off of the main sequence of the Hertzsprung-Russell diagram. As the Sun burns through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself.Acer Travelmate 2200 Adapter This increase in pressure heats the core, so it burns even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years. Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main sequence phase.HP Compaq 6735b Adapter As the hydrogen reactions shut down, the core will contract further, increasing pressure and temperature, causing fusion to commence via the helium process. Helium in the core burns at a much hotter temperature, and the energy output will be much greater than during the hydrogen process.Dell Vostro 1000 Battery At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).HP Compaq nx7300 Adapter Eventually, helium in the core will exhaust itself at a much faster rate than the hydrogen, and the Sun's helium burning phase will be but a fraction of the time compared to the hydrogen burning phase. The Sun is not massive enough to commence fusion of heavier elements, and nuclear reactions in the core will dwindle. Asus f82 Battery Its outer layers will fall away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth.^[114] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.Dell XPS M1330 Battery For many thousands of years, civilizations, with a few notable exceptions, did not recognize the existence of the Solar System. It was believed the Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky.HP Compaq Business 6910p Battery While the Indian mathematician-astronomer Aryabhata and the Greek philosopherAristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus first developed a mathematically predictive heliocentric system. Compaq PRESARIO R3000 Battery His 17th-century successors Galileo Galilei,Johannes Kepler, and Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth. HP Compaq Business NX6110 Battery In more recent times, this led to the investigation of geological phenomena such as mountains and craters and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets. The first exploration of the Solar System was conducted by telescope, when astronomers first began to map those objects too faint to be seen with the naked eye.Dell Latitude D610 Adapter Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System. He discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn.Asus Eee PC 701 Adapter Giovanni Domenico Cassini later discovered four more moons of Saturn, the Cassini division in Saturn's rings.^[3] Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning regularly once every 75-76 years. This was the first evidence that anything other than the planets orbited the Sun. HP Compaq Business NX6325 Battery Around this time (1704), the term "Solar System" first appeared in English. In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. Its orbit revealed that it was a new planet, Uranus, the first ever discovered.Dell Studio XPS 16 Battery Giuseppe Piazzi discovered Ceres in 1801, a small world between Mars and Jupiter that initially was considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual reclassification as asteroids.Acer Aspire 5520 Battery By 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out. Urbain Le Verrier's calculations eventually led to the discovery of Neptune.^[8] The excess perihelion precession of Mercury's orbit led Le Verrier to postulate the intra-Mercurian planet Vulcan in 1859, but that would turn out to be an irrelevant thesis.Acer TravelMate 5720 Adapter While it is debatable when the Solar System was truly "discovered," three 19th century observations determined its nature and place in the universe beyond reasonable doubt. First, in 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by the Earth's motion around the Sun.HP Compaq 6735s Adapter This was not only the first direct, experimental proof of heliocentrism, but also revealed, for the first time, the vast distance between our Solar System and the stars. Then, in 1859, Robert Bunsen and Gustav Kirchhoff, using the newly invented spectroscope,Dell XPS M1330 Battery examined the spectral signature of the Sun and discovered that it was composed of the same elements as existed on Earth, establishing for the first time a physical link between the Earth and the heavens.^[9] Then, Father Angelo Secchi compared the spectral signature of the Sun with those of other stars, and found them virtually identical. HP Compaq nc6230 Adapter The realisation that the Sun was a star led to the hypothesis that other stars could have systems of their own, though this was not to be proven for nearly 140 years. Further apparent discrepancies in the orbits of the outer planets led Percival Lowell to conclude that yet another planet, "Planet X", must lie beyond Neptune. Dell T117C Battery After his death, his Lowell Observatoryconducted a search which ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, and its discovery was therefore coincidental.Asus x5dij-sx039c Battery Like Ceres, it was initially considered to be a planet, but after the discovery of many other similarly sized objects in its vicinity it was reclassified in 2006 as a dwarf planet by the IAU. In 1992, the first evidence of a planetary system other than our own was discovered, orbiting the pulsar PSR B1257+12. Dell Inspiron 1464 Battery Three years later, 51 Pegasi b, the first extrasolar planet around a Sunlike star, was discovered. As of 2008, 221 extrasolar systems have been found. Also in 1992, astronomers David C. Jewitt of the University of Hawaii and Jane Luu of the Massachusetts Institute of Technology discovered (15760) 1992 QB₁. HP Compaq 6720s Adapter This object proved to be the first of a new population, which became known as the Kuiper belt; an icy analogue to the asteroid belt of which such objects as Pluto and Charon were deemed a part. Mike Brown, Chad Trujillo and David Rabinowitz announced the discovery of Eris in 2005, a scattered disc object larger than Pluto and the largest object discovered in orbit round the Sun since Neptune.Dell Latitude D630 Adapter

Observations by spacecraft

Since the start of the Space Age, a great deal of exploration has been performed by robotic spacecraft missions that have been organized and executed by various space agencies. All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. HP Pavilion dm4 Battery Through these unmanned missions, humans have been able to get close-up photographs of all the planets and, in the case of landers, perform tests of the soils and atmospheres of some. The first artificial object sent into space was the Soviet satellite Sputnik 1, launched in 1957, which successfully orbited the Earth until January 4 the following year. Sony NP-FS11 Battery The American probe Explorer 6, launched in 1959, was the first satellite to image the Earth from space.

Flybys

The first successful probe to fly by another Solar System body was Luna 1, which sped past the Moon in 1959. Originally meant to impact with the Moon, it instead missed its target and became the first artificial object to orbit the Sun.Dell Inspiron 1520 Adapter Mariner 2 was the first probe to fly by another planet, Venus, in 1962. The first successful flyby of Mars was made by Mariner 4 in 1965. Mariner 10 first passed Mercury in 1974. The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. Toshiba PA3399U-1BRS Battery The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980 - 1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989.Acer Aspire 5920G Adapter The Voyager probes are now far beyond Neptune's orbit, and are on course to find and study the termination shock, heliosheath, and heliopause. According to NASA, bothVoyager probes have encountered the termination shock at a distance of approximately 93 AU from the Sun.Acer Aspire 5920 Battery The first flyby of a comet occurred in 1985, when the International Cometary Explorer (ICE) passed by the cometGiacobini-Zinner, while the first flybys of asteroids were conducted by the Galileo space probe, which imaged both 951 Gaspra (in 1991) and 243 Ida (in 1993) on its way to Jupiter.IBM Thinkpad T40 Adapter No Kuiper belt object has yet been visited by a spacecraft. Launched on January 19, 2006, the New Horizons probe is currently en route to becoming the first man-made spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in July 2015. Nikon COOLPIX 4300 Battery Should it prove feasible, the mission will then be extended to observe a number of other Kuiper belt objects.

Orbiters, rovers and landers

In 1966, the Moon became the first Solar System body beyond Earth to be orbited by an artificial satellite (Luna 10), followed by Mars in 1971 (Mariner 9),Apple MB771 Battery Venus in 1975 (Venera 9), Jupiter in 1995 (Galileo), the asteroid 433 Eros in 2000 (NEAR Shoemaker), Saturn in 2004 (Cassini-Huygens), and Mercury in 2011 (MESSENGER). The Dawn spacecraft is currently set to orbit the asteroid Vesta in 2011 and the dwarf planet Ceres in 2015.Sony DSC-P5 Battery The first probe to land on another Solar System body was the Soviet Luna 2 probe, which impacted the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on or impacting the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3, although a fully successful landing didn't occur until Viking 1 in 1976),Dell Inspiron 6400 Adapter

implying either 4,080,000 stadia (755,000 km) or 804,000,000 stadia (148 to 153 million kilometers or 0.99 to 1.02 AU); the latter value is correct to within a few percent. In the 1st century CE, Ptolemy estimated the distance as 1,210 times the Earth radius, approximately 7.71 million kilometers (0.0515 AU).Dell XPS M1330 Battery The theory that the Sun is the center around which the planets move was first proposed by the ancient Greek Aristarchus of Samos in the 3rd century BCE, and later adopted by Seleucus of Seleucia (see Heliocentrism). This largely philosophical view was developed into fully predictive mathematical model of a heliocentric system in the 16th century by Nicolaus Copernicus.HP Compaq Business 6910p Battery In the early 17th century, the invention of the telescope permitted detailed observations ofsunspots by Thomas Harriot, Galileo Galilei and other astronomers. Galileo made some of the first known telescopic observations of sunspots and posited that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun.Compaq PRESARIO R3000 Battery Sunspots were also observed since the Han Dynasty (206 BCE - 220 CE) by Chinese astronomers who maintained records of these observations for centuries. Averroes also provided a description of sunspots in the 12th century. Arabic astronomical contributions include Albatenius discovering that the direction of the Sun's eccentric is changing,HP Compaq Business NX6110 Battery and Ibn Yunus observing more than 10,000 entries for the Sun's position for many years using a large astrolabe.The transit of Venus was first observed in 1032 by Avicenna, who concluded that Venus is closer to the Earth than the Sun, while one of the first observations of the transit of Mercury was conducted by Ibn Bajjah in the 12th century.Dell Latitude D610 Adapter In 1672 Giovanni Cassini and Jean Richer determined the distance to Mars and were thereby able to calculate the distance to the Sun. Isaac Newton observed the Sun's light using a prism, and showed that it was made up of light of many colors, while in 1800 William Herschel discovered infrared radiation beyond the red part of the solar spectrum.Asus Eee PC 701 Adapter The 19th century saw advancement in spectroscopic studies of the Sun; Joseph von Fraunhofer recorded more than 600 absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines. In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle.HP Compaq Business NX6325 Battery Lord Kelvin suggested that the Sun was a gradually cooling liquid body that was radiating an internal store of heat. Kelvin and Hermann von Helmholtz then proposed a gravitational contraction mechanism to explain the energy output. Unfortunately the resulting age estimate was only 20 million years, Dell Studio XPS 16 Battery well short of the time span of at least 300 million years suggested by some geological discoveries of that time. In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun. Acer Aspire 5520 Battery Not until 1904 was a documented solution offered. Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggested radioactive decay as the source. However, it would be Albert Einstein who would provide the essential clue to the source of the Sun's energy output with his mass-energy equivalence relation E = mc².Acer TravelMate 5720 Adapter In 1920, Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass. The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne. HP Compaq 6735s Adapter The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun. Finally, a seminal paper was published in 1957 by Margaret Burbidge, entitled "Synthesis of the Elements in Stars".Dell XPS M1330 Battery The paper demonstrated convincingly that most of the elements in the universe had been synthesized by nuclear reactions inside stars, some like our Sun.

Solar space missions

The first satellites designed to observe the Sun were NASA's Pioneers 5, 6, 7, 8 and 9, which were launched between 1959 and 1968.HP Compaq nc6230 Adapter These probes orbited the Sun at a distance similar to that of the Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long time, transmitting data until May 1983. In the 1970s, two Helios spacecraft and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona.Dell T117C Battery The Helios 1 and 2 probes were U.S.-German collaborations that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit atperihelion.^[150] The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station.Asus x5dij-sx039c Battery Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona.^[58] Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.Dell Inspiron 1464 Battery In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV radiation from solar flares during a time of high solar activity and solar luminosity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state.HP Compaq 6720s Adapter In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before re-enteringthe Earth's atmosphere in June 1989.Dell Latitude D630 Adapter Launched in 1991, Japan's Yohkoh (Sunbeam) satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. HP Pavilion dm4 Battery Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005. One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on 2 December 1995.Sony NP-FS11 Battery Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009. It has proven so useful that a follow-on mission, the Solar Dynamics Observatory, was launched in February 2010. Situated at the Lagrangian point between the Earth and the Sun (at which the gravitational pull from both is equal), Dell Inspiron 1520 Adapter SOHO has provided a constant view of the Sun at many wavelengths since its launch.^[58] Besides its direct solar observation, SOHO has enabled the discovery of a large number of comets, mostly very tiny sungrazing comets which incinerate as they pass the Sun.Toshiba PA3399U-1BRS Battery All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to study the Sun's polar regions. It first travelled to Jupiter, to "slingshot" past the planet into an orbit which would take it far above the plane of the ecliptic.Acer Aspire 5920G Adapter Serendipitously, it was well-placed to observe the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s which was slower than expected, Acer Aspire 5920 Battery and that there were large magnetic waves emerging from high latitudes which scattered galactic cosmic rays. Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood.IBM Thinkpad T40 Adapter A solar wind sample return mission,Genesis, was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its parachute failed to deploy on re-entry into Earth's atmosphere. Nikon COOLPIX 4300 Battery Despite severe damage, some usable samples have been recovered from the spacecraft's sample return module and are undergoing analysis. The Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006. Apple MB771 Battery Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind the Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.

Observation and effects

Sunlight is very bright, and looking directly at the Sun with the naked eye for brief periods can be painful, but is not particularly hazardous for normal, non-dilated eyes.Sony DSC-P5 Battery Looking directly at the Sun causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness.Dell Inspiron 6400 Adapter UV exposure gradually yellows the lens of the eye over a period of years and is thought to contribute to the formation of cataracts, but this depends on general exposure to solar UV, not on whether one looks directly at the Sun. Long-duration viewing of the direct Sun with the naked eye can begin to cause UV-induced,Toshiba PA3534U-1BRS Battery sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused;^[165][166]conditions are worsened by young eyes or new lens implants (which admit more UV than aging natural eyes), Sun angles near the zenith, and observing locations at high altitude.Sony NP-FP50 Battery Viewing the Sun through light-concentrating optics such as binoculars is very hazardous without an appropriate filter that blocks UV and substantially dims the sunlight. An attenuating (ND) filter might not filter UV and so is still dangerous.HP Compaq nx7300 Adapter Attenuating filters to view the Sun should be specifically designed for that use: some improvised filters pass UV or IR rays that can harm the eye at high brightness levels. Unfiltered binoculars can deliver over 500 times as much energy to the retina as using the naked eye, killing retinal cells almost instantly. ^{Canon POWERSHOT G2 Battery} Even brief glances at the midday Sun through unfiltered binoculars can cause permanent blindness. Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. Dell Latitude E6500 Battery During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives about ten times more light than it would looking at the non-eclipsed Sun. ^{Dell LATITUDE D630 Battery} This can damage or kill those cells, resulting in small permanent blind spots for the viewer.The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.Asus R33030 Adapter During sunrise and sunset sunlight is attenuated due to Rayleigh scattering and Mie scattering from a particularly long passage through Earth's atmosphere,and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics (provided there is no risk of bright sunlight suddenly appearing through a break between clouds). Dell Mini 1011 Battery Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation. A rare optical phenomenon may occur shortly after sunset or before sunrise, known as a green flash. The flash is caused by light from the Sun just below the horizon being bent (usually through atemperature inversion) towards the observer. toshiba pa3332u-1brs Battery Light of shorter wavelengths (violet, blue, green) is bent more than that of longer wavelengths (yellow, orange, red) but the violet and blue light is scatteredmore, leaving light that is perceived as green. Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water.HP Compaq 6710b Adapter It also causes sunburn, and has other medical effects such as the production of vitamin D. Ultraviolet light is strongly attenuated by Earth's ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in humanskin color in different regions of the globe.^{HP Compaq nx6325 Adapter}

Terminology

Like other natural phenomena, the Sun has been an object of veneration in many cultures throughout human history, and was the source of the word Sunday. Its formal name in the English language is, per the International Astronomical Union, the Sun (capitalized as a proper noun).Acer Aspire 5720 Adapter The Latin name Sol ( /?s?l/), for the Sun god of the same name, is widely known but not common in general English language use; the adjectival form is the related word solar. "Sol" is, however, the modern word for "Sun" in many European languages. ^{Toshiba Equium A110-252 Battery} The term sol is also used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars. A mean Earth solar day is approximately 24 hours, while a mean Martian 'sol' is 24 hours, 39 minutes, and 35.244 seconds. See also Timekeeping on Mars.^{Acer Aspire 5052ANWXMi Battery} The Solar System consists of the Sun and the astronomical objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the many objects that orbit the Sun, most of themass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. Dell Mini 1011v Battery The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; ^{HP Compaq nx7300 Adapter} the two outermost planets,Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". The Solar System is also home to two regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, ^{Dell Vostro 1510 Battery} is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these two regions, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.^{Toshiba PA3534U-1BRS Battery I}n addition to thousands of small bodies^[e] in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions. The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc.^{Asus M50 Battery} The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. Six of the planets and three of the dwarf planets are orbited by natural satellites,^[b] usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.^{Asus Eee PC 901 Battery}

Discovery and exploration

For many thousands of years, humanity, with a few notable exceptions, did not recognize the existence of the Solar System. People believed the Earth to be stationary at the center of the universe and categorically different from the divine or ethereal objects that moved through the sky. Dell Inspiron 6400 Battery Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos,^[1]Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors, Galileo Galilei, HP Compaq Business NC6220 Battery Johannes Kepler and Isaac Newton, developed an understanding of physics that led to the gradual acceptance of the idea that the Earth moves around the Sun and that the planets are governed by the same physical laws that governed the Earth. In more recent times,Sony VGP-BPS9 Battery improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena such as mountains and craters, and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.Sony VGP-BPS13 Battery

Structure

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86 percent of the system's known mass and dominates it gravitationally.^[3] The Sun's four largest orbiting bodies, the gas giants, account for 99 percent of the remaining mass, with Jupiter and Saturn together comprising more than 90 percent.HP Compaq nx7300 Adapter Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are frequently at significantly greater angles to it.All the planets and most other objects orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above the Sun's north pole). HP Compaq 2510p Adapter For exceptions, see retrograde motion. The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the outer Kuiper belt of icy objects. Dell Studio PP39L Battery Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets. Since the discovery of the Kuiper belt, Nikon COOLPIX 5700 Battery the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Kepler's laws of planetary motion describe the orbits of objects about the Sun. According to Kepler's laws, each object travels along an ellipse with the Sun at one focus. ^{Dell Studio XPS 16 Battery} Objects closer to the Sun (with smaller semi-major axes) travel more quickly, as they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion. ^{HP Compaq nx7400 Adapter} The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects follow highly elliptical orbits. Due to the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. HP Pavilion DV6500 Adapter For example, Venus is approximately 0.33 astronomical units (AU)^[d] farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius-Bode law),^[8] but no such theory has been accepted.Dell T117C Battery Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which are larger than the planetMercury), or, in the case of the four gas giants, by planetary rings; thin bands of tiny particles that orbit them in unison. Acer PA-1650-02 Adapter Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. The objects of the inner Solar System are composed mostly of rock,^[9] the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula.Acer Aspire 5920G Adapter Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapor pressure such asmolecular hydrogen, helium, and neon, which were always in the gaseous phase in the nebula.^{Asus S1 Adapter} Ices, like water, methane, ammonia, hydrogen sulfide and carbon dioxide,^[9] have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature. They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase.HP Pavilion dv6000 Battery Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.^[9][11] Together, gases and ices are referred to as volatiles.Ibm Thinkpad T30 Battery

Sun

The Sun is the Solar System's star, and by far its chief component. Its large mass (332,900 Earth masses) produces temperatures and densities in its core great enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation, peaking in the 400-700 nm band we call visible light.Dell Studio PP39L Battery The Sun is classified as a type G2 yellow dwarf, but this name is misleading as, compared to the majority of stars in our galaxy, the Sun is rather large and bright. Stars are classified by the Hertzsprung-Russell diagram, a graph that plots the brightness of stars with their surface temperatures. Generally, hotter stars are brighter.^{HP Pavilion dv5 Adapter} Stars following this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarfs, are common, making up 85 percent of the stars in the galaxy.HP Compaq Business NC6400 Battery It is believed that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.HP Compaq 490306-001 Battery The Sun is a population I star; it was born in the later stages of the universe's evolution, and thus contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars. Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars,^{Compaq Presario CQ35-100 Battery} so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a planetary system, because planets form from accretion of "metals".^{HP Compaq HSTNN-LB51 Battery}

Interplanetary medium

Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (seeheliopause).^{HP Pavilion ZE4500 Battery} This is known as the interplanetary medium. Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturb the heliosphere, creating space weather and causing geomagnetic storms. The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium. ^{HP G62-100EB Battery} Earth's magnetic field stops its atmosphere from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space. Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. ^{HP Pavilion DV6-1211AX Battery} The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into the Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles. Cosmic rays originate outside the Solar System. HP Pavilion DV6-1220SO Battery The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.HP EliteBook 6930P Battery The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.HP Pavilion DV6-1210SA Battery The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.

Inner Solar System

The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids.Compaq Presario CQ71 Battery Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.

Inner planets

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. HP Pavilion DV7-1290EO Battery They are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather;HP G6000 Battery all have impact craters and tectonic surface features such as rift valleys andvolcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).HP HDX X18-1005EA Battery

Mercury

Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history.^{HP Pavilion DV6-3013SL Battery}

Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.^[33] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.HP Compaq HSTNN-IB62 Battery

Venus

Venus (0.7 AU from the Sun) is close in size to Earth (0.815 Earth masses), and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. ^{HP Pavilion DV6100 Battery}

It is the hottest planet, with surface temperatures over 400 C, most likely due to the amount of greenhouse gases in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, ^{HP EliteBook 2730P Battery}

which suggests that its atmosphere is regularly replenished by volcanic eruptions.

Earth

Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and is the only place in the universe where life is known to exist.HP Pavilion DV9000 Battery

Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.HP Pavilion DV7 Battery

It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars

Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It possesses an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6 percent that of the Earth's).^{HP Pavilion DV7-1290EO Battery}

Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxide (rust) in its soil.^{HP Compaq Business 6735S Battery}

Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.

Asteroid belt

Asteroids are mostly small Solar System bodies^[e] composed mainly of refractory rocky and metallic minerals.^{HP 540 Battery} The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.^{HP Compaq Business NW9440 Battery} Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta and Hygiea may be reclassed as dwarf planets if they are shown to have achievedhydrostatic equilibrium.^{HP Compaq Business TC4400 Battery} The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth. The main belt is very sparsely populated; spacecraft routinely pass through without incident.HP Compaq 491278-001 Battery Asteroids with diameters between 10 and 10^?4 m are called meteoroids.

Ceres

Ceres (2.77 AU) is the largest body in the asteroid belt and is classified as a dwarf planet.^[e] It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. HP HSTNN-LB31 Battery Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids.^[50] It was again reclassified in 2006 as a dwarf planet.

Asteroid groups

Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics. HP Pavilion DV3-4000 Battery Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets, which may have been the source of Earth's water.HP Pavilion TX1000 Battery Trojan asteroids are located in either of Jupiter's L₄ or L₅ points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.HP Pavilion DV6-1210SA Battery The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.^[53]

Outer Solar System

The outer region of the Solar System is home to the gas giants and their large moons. HP 448007-001 Battery Many short period comets, including the centaurs, also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia and methane, than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid.HP Compaq Business 6530B Battery

Outer planets

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun.^[c] Jupiter and Saturn are each many tens of times the mass of the Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. ^{HP Pavilion DV5-1015EA Battery} For these reasons, some astronomers suggest they belong in their own category, "ice giants."^[54] All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused withsuperior planet, which designates planets outside Earth's orbit and thus includes both the outer planets and Mars.HP Pavilion DV6-1230SB Battery

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot.HP Pavilion ZD8181EA Battery

Jupiter has 63 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating.^[55] Ganymede, the largest satellite in the Solar System, is larger than Mercury.HP Pavilion DV9500 Battery

Saturn

Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 Earth masses, making it the least dense planet in the Solar System. HP Pavilion DV4-1125BR Battery

The rings of Saturn are made up of small ice and rock particles.

Saturn has 62 confirmed satellites; two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice.^[56] Titan, the second largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.HP Pavilion DV5-1111EA Battery

Uranus

Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants, and radiates very little heat into space.HP Pavilion DV6-1234TX Battery

Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn.^{[58]HP Pavilion DV3-2008TX Battery}

Neptune has 13 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.^[59] Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune Trojans, that are in 1:1 resonance with it.^{HP Pavilion DV8 Battery}

Comets

Comets are small Solar System bodies,^[e] typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, HP Pavilion DV6-1215SA Battery its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye. Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years.^{HP Compaq 490306-001 Battery} Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent.Compaq Presario CQ50 Battery Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.^[61] Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.^[62]

Centaurs

The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter's (5.5 AU) and less than Neptune's (30 AU). HP Pavilion DV6-1223EO Battery The largest known centaur, 10199 Chariklo, has a diameter of about 250 km.^[63] The first centaur discovered, 2060 Chiron, has also been classified as comet (95P) since it develops a coma just as comets do when they approach the Sun.^[64]

Trans-Neptunian region

The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. HP Compaq 490306-001 Battery It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.Compaq Presario CQ35-100 Battery

Kuiper belt

The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice.^[65] It extends between 30 and 50 AU from the Sun. Though it contains at least three dwarf planets, it is composed mainly of small Solar System bodies. However, many of the largest Kuiper belt objects, such as Quaoar, ^{HP Compaq HSTNN-LB51 Battery} Varuna, and Orcus, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth.HP Pavilion ZE4500 Battery Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic. The Kuiper belt can be roughly divided into the "classical" belt and the resonances. Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). HP G62-100EB Battery The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB₁, and are still in near primordial, low-eccentricity orbits.^{HP Pavilion DV6-1211AX Battery}

Pluto and Charon

Pluto (39 AU average), a dwarf planet, is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. HP EliteBook 6930P Battery

Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.HP Pavilion DV9500 Battery

Charon, Pluto's largest moon, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycenter of gravity above their surfaces (i.e., they appear to "orbit each other"). Beyond Charon, two much smaller moons,Nix and Hydra, orbit within the system.Compaq Evo N610C Battery

Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.

Haumea and Makemake

Haumea (43.34 AU average), and Makemake (45.79 AU average), while smaller than Pluto, are the largest known objects in the classical Kuiper belt (that is, they are not in a confirmed resonance with Neptune). ^{HP Pavilion DM3-1050EO Battery}

Haumea is an egg-shaped object with two moons. Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated 2003 EL₆₁ and 2005 FY₉ respectively, they were given names and designated dwarf planets in 2008. Their orbits are far more inclined than Pluto's, at 28 and 29 .^{Compaq Presario 1245 Battery}

Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends much further outwards, is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration.HP G6000 Battery Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."HP Compaq 451086-661 Battery Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.

Eris

Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is 25% more massive than Pluto and about the same diameter. HP Pavilion DV2166EA Battery It is the most massive of the known dwarf planets. It has one moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.^{HP Compaq 464119-143 Battery}

Farthest regions

The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun;Compaq Presario CQ71 Battery this heliopause is considered the beginning of the interstellar medium.^[22] However, the Sun's Roche sphere, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.^[77]

Heliopause

The heliosphere is divided into two separate regions. HP Pavilion DV9565EA Battery The solar wind travels at roughly 400 km/s until it collides with the interstellar wind; the flow of plasma in the interstellar medium. The collision occurs at the termination shock, which is roughly 80-100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.^{HP Pavilion DV6203TX Battery} Here the wind slows dramatically, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath. This structure is believed to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; but evidence from the HP Pavilion DV7-1290EO Battery Cassini andInterstellar Boundary Explorer spacecraft has suggested that it is in fact forced into a bubble shape by the constraining action of the interstellar magnetic field.^[79] Both Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively.HP HSTNN-OB51 Battery The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.^[22] The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium^[78] as well as solar magnetic fields prevailing to the south, HP Pavilion DV7-2185DX Battery e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere. Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through theMilky Way. No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space.HP Compaq Business 6830S Battery It is expected that NASA's Voyager spacecraft will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth.^[83] How well the heliosphere shields the Solar System from cosmic rays is poorly understood.HP PP2200 Battery A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.

Oort cloud

The hypothetical Oort cloud is a spherical cloud of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (LY)), Compaq CQ50 Battery and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions,HP Pavilion DV8 Battery the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.

Sedna

90377 Sedna (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete.^{Sony VGP-BPS13 Battery} Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object 2000 CR₁₀₅,^{HP DV9700 Battery} which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years. Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun.^[89] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.^{Compaq CQ35-100 Battery}

Boundaries

Much of our Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.^{[HP Pavilion DV6-1223EO Battery]} Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.^[91] Objects may yet be discovered in the Solar System's uncharted regions.Sony VGP-BPS11 Battery

Galactic context

The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars.^[92] Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur.HP 448007-001 Battery The Sun lies between 25,000 and 28,000 light years from the Galactic Centre,^[94] and its speed within the galaxy is about 220 kilometres per second, so that it completes one revolution every 225-250 million years. This revolution is known as the Solar System's galactic year.Sony Vaio VGN-CR190E/L Battery The solar apex, the direction of the Sun's path through interstellar space, is near the constellation of Hercules in the direction of the current location of the bright star Vega.^[96] The plane of the ecliptic lies at an angle of about 60 to thegalactic plane.^[f] The Solar System's location in the galaxy is very likely a factor in the evolution of life on Earth. Dell Inspiron 6400 AC Adapter Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous supernovae, this has given Earth long periods of interstellar stability for life to evolve.^{Dell LATITUDE D800 Battery} The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. Apple A1280 Battery The intense radiation of the galactic centre could also interfere with the development of complex life. Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun as radioactive dust grains and larger, comet-like bodies.Apple A1281 Battery

Neighbourhood

The immediate galactic neighbourhood of the Solar System is known as the Local Interstellar Cloud or Local Fluff, an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light years across. HP Pavilion DV7-1290EO Battery The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae. There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light years away. ^{HP Pavilion DV6-1020ED Battery} Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 light years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). ^{Dell Latitude E6400 Battery Battery} The largest star within ten light years is Sirius, a bright main sequence star roughly twice the Sun's mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system Luyten 726-8 (8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).^{[HP Pavilion DM4 Battery} Our closest solitary sun-like star is Tau Ceti, which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity.^[101] The closest known extrasolar planet to the Sun lies around the star Epsilon Eridani, a star slightly dimmer and redder than the Sun, which lies 10.5 light years away.HP Pavilion DM4-1000 Battery Its one confirmed planet, Epsilon Eridani b, is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years.^[102] The Solar System formed from the gravitational collapse of a giant molecular cloud 4.568 billion years ago.^[103] This initial cloud was likely several light-years across and probably birthed several stars.^{[HP Pavilion DM4T Battery]} As the region that would become the Solar System, known as the pre-solar nebula,^[105] collapsed, conservation of angular momentum made it rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.^{[HP 6510B Battery]} As the contracting nebula rotated, it began to flatten into a spinning protoplanetary disc with a diameter of roughly 200 AU^[104] and a hot, dense protostar at the centre.^[106][107] At this point in its evolution, the Sun is believed to have been a T Tauri star. HP 6530B Battery Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001-0.1 solar masses, with the vast majority of the mass of the nebula in the star itself.^[108] The planets formed by accretion from this disk.^{[HP 6710B Battery]} Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.^[110] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction.HP NC4400 Battery At this point the Sun became a full-fledged main sequencestar. The Solar System as we know it today will last until the Sun begins its evolution off of the main sequence of the Hertzsprung-Russell diagram. As the Sun burns through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself. HP NC6000 Battery This increase in pressure heats the core, so it burns even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years. Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main sequence phase.HP NC6100 Battery As the hydrogen reactions shut down, the core will contract further, increasing pressure and temperature, causing fusion to commence via the helium process. Helium in the core burns at a much hotter temperature, and the energy output will be much greater than during the hydrogen process. HP NC6200 Battery At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest). HP NC6400 Battery Eventually, helium in the core will exhaust itself at a much faster rate than the hydrogen, and the Sun's helium burning phase will be but a fraction of the time compared to the hydrogen burning phase. The Sun is not massive enough to commence fusion of heavier elements, and nuclear reactions in the core will dwindle. HP NW8200 Battery Its outer layers will fall away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth. The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.HP NW8440 Battery

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields.It has a diameter of about 1,392,000 km, about 109 times that of Earth, and its mass (about 2 10³⁰ kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System.HP 586006-321 Battery Chemically, about three quarters of the Sun's mass consists ofhydrogen, while the rest is mostly helium. Less than 2% consists of heavier elements, including oxygen, carbon, neon, iron, and others. The Sun's stellar classification, based on spectral class, is G2V, and is informally designated as a yellow dwarf, HP 586006-361 Battery because its visible radiation is most intense in the yellow-green portion of the spectrum and although its color is white, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.In the spectral class label, G2 indicates its surface temperature of approximately 5778 K (5505 C),HP 586028-341 Battery and V indicates that the Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogennuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen each second. Once regarded by astronomers as a small and relatively insignificant star,HP h588178-141 Battery the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs.The absolute magnitude of the Sun is +4.83; however, as the star closest to Earth, the Sun is the brightest object in the sky with anapparent magnitude of ?26.74.HP 593553-001 Battery The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.HP 593554-001 Battery The Sun is currently traveling through the Local Interstellar Cloud in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way galaxy. Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being a red dwarf named Proxima Centauri at approximately 4.2 light years away), the Sun ranks fourth in mass.HP HSTNN-CB0W Battery The Sun orbits the center of the Milky Way at a distance of approximately 24,000-26,000light years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225-250 million years. Since our galaxy is moving with respect to the cosmic microwave background radiation (CMB) in the direction of constellation Hydra with a speed of 550 km/s,HP HSTNN-CB0X Battery the sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo. The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though the distance varies as the Earth moves fromperihelion in January to aphelion in July.HP HSTNN-I79C Battery At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. Theenergy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather. The enormous effect of the Sun on the Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. HP HSTNN-I81C Battery An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little knowledge of the Sun's physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the Sun's behavior that remain unexplained.HP HSTNN-IB0N Battery

Characteristics

The Sun is a G-type main sequence star comprising about 99.8632% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 km.HP HSTNN-IB0X Battery As the Sun consists of a plasma and is not solid, it rotates faster at its equator than at its poles. This behavior is known asdifferential rotation, and is caused by convection in the Sun and the movement of mass, due to steeptemperature gradients from the core outwards. HP HSTNN-LB0W Battery This mass carries a portion of the Sun's counter-clockwise angular momentum, as viewed from the ecliptic north pole, thus redistributing the angular velocity. The period of this actual rotation is approximately 25.6 days at the equator and 33.5 days at the poles.HP HSTNN-OB0X Battery However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days.The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun.HP HSTNN-OB0Y Battery The Sun is a Population I, or heavy element-rich, star. The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae. This is suggested by a highabundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. HP HSTNN-Q61C Battery These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or bytransmutation through neutron absorption inside a massive second-generation star. The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops exponentially with increasing distance from its center.HP HSTNN-Q62C Battery Nevertheless, it has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye. HP HSTNN-YB0X Battery The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the star's inner structure.HP NBP6A174B1 Battery Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.

Core

The core of the Sun is considered to extend from the center to about 20-25% of the solar radius. It has a density of up to 150 g/cm³ (about 150 times the density of water) and a temperature of close to 13.6 million kelvin (K).HP HSTNN-YB0X Battery By contrast, the Sun's surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone. Through most of the Sun's life, energy is produced by nuclear fusion through a series of steps called the p-p (proton-proton) chain; this process converts hydrogen into helium.HP NBP6A174 Battery Less than 2% of the helium generated in the Sun comes from the CNO cycle. The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; inside 24% of the Sun's radius, 99% of the power has been generated, and by 30% of the radius, fusion has stopped nearly entirely.HP NBP6A174B1 Battery The rest of the star is heated by energy that is transferred outward from the core and the layers just outside. The energy produced by fusion in the core must then travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.HP NBP6A175 Battery The proton-proton chain occurs around 9.2 1037 times each second in the core of the Sun. Since this reaction uses four free protons (hydrogen nuclei), it converts about 3.7 10³⁸ protons to alpha particles (helium nuclei) every second (out of a total of ~8.9 10⁵⁶ free protons in the Sun), or about 6.2 10¹¹ kg per second.HP NBP6A175B1 Battery Since fusing hydrogen into helium releases around 0.7% of the fused mass as energy, the Sun releases energy at the mass-energy conversion rate of 4.26 million metric tons per second, 384.6 yotta watts (3.846 1026 W), or 9.192 10¹⁰ megatons of TNT per second. HP WD548AA Battery This mass is not destroyed to create the energy, rather, the mass is carried away in the radiated energy, as described by the concept of mass-energy equivalence. The power production by fusion in the core varies with distance from the solar center. HP WD549AA Battery At the center of the Sun, theoretical models estimate it to be approximately 276.5 watts/m³, a power production density that more nearly approximates reptile metabolism than a thermonuclear bomb. Peak power production in the Sun has been compared to the volumetric heats generated in an active compost heap. HP Envy 17 Notebook PC Battery The tremendous power output of the Sun is not due to its high power per volume, but instead due to its large size. The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; HP Envy 17T Battery and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level. The gamma rays (high-energy photons) released in fusion reactions are absorbed in only a few millimeters of solar plasma and then re-emitted again in random direction and at slightly lower energy. HP HSTNN-Q61C Battery Therefore it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years. Since energy transport in the Sun is a process which involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. HP GSTNN-Q62C Battery This is the time it would take the Sun to return to a stable state if the rate of energy generation in its core were suddenly to be changed. After a final trip through the convective outer layer to the transparent surface of the photosphere, the photons escape as visible light. HP HSTNN-IB1E Battery Each gamma ray in the Sun's core is converted into several million photons of visible light before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. HP HSTNN-IB0X Battery For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was resolved in 2001 through the discovery of the effects of neutrino oscillation: the Sun emits the number of neutrinos predicted by the theory, HP HSTNN-OB0X Battery but neutrino detectors were missing ²?₃ of them because the neutrinos had changedflavor by the time they were detected.

Radiative zone

From about 0.25 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward.HP HSTNN-YB0X Battery This zone is free of thermalconvection; while the material gets cooler from 7 to about 2 million kelvin with increasing altitude, this temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection.Energy is transferred by radiation ions of hydrogen and helium emit photons,HP 586006-361 Battery which travel only a brief distance before being reabsorbed by other ions.The density drops a hundredfold (from 20 g/cm³ to only 0.2 g/cm³) from the bottom to the top of the radiative zone. The radiative zone and the convection form a transition layer, the tachocline. HP NBP6A174 Battery This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear a condition where successive horizontal layers slide past one another. The fluid motions found in the convection zone above,HP NBP6A174B1 Battery slowly disappear from the top of this layer to its bottom, matching the calm characteristics of the radiative zone on the bottom. Presently, it is hypothesized (see Solar dynamo), that a magnetic dynamo within this layer generates the Sun's magnetic field. HP 588178-141 Battery

Convective zone

In the Sun's outer layer, from its surface down to approximately 200,000 km (or 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the thermal energy of the interior outward through radiation; in other words it is opaque enough. HP Pavilion dv5 Adapter As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Sony NP-F330 Battery At the visible surface of the Sun, the temperature has dropped to 5,700 K and the density to only 0.2 g/m³ (about 1/6,000th the density of air at sea level). The thermal columns in the convection zone form an imprint on the surface of the Sun as the solar granulation and supergranulation.Asus Eee PC 904HA Adapter The turbulent convection of this outer part of the solar interior causes a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun. The Sun's thermal columns are B nard cells and therefore tend to be hexagonal prisms.Nikon D80 Battery

Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light.Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H^? ions, which absorb visible light easily.Sony PCGA-AC19V9 Adapter Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H^? ions. The photosphere is tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, Dell T555C Battery an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. HP Pavilion DV6-3013SL Battery The photosphere has a particle density of ~10²³ m^?3 (this is about 0.37% of the particle number per volume of Earth's atmosphere at sea level; however, photosphere particles are electrons and protons, so the average particle in air is 58 times as heavy).HP Pavilion DV7-2085EL Battery During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed helium, after the Greek Sun god Helios.HP Pavilion DV9700 Battery It was not until 25 years later that helium was isolated on Earth.

Atmosphere

Above the chromosphere, in a thin (about 200 km) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K.HP Mini 702EA Battery The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion.HP Pavilion DV7-1120EO Battery The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the extreme ultraviolet portion of the spectrum. The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. HP Compaq 491278-001 Battery The corona continuously expands into space forming the solar wind, which fills all the Solar System. The low corona, which is very near the surface of the Sun, has a particle density around 10¹⁵-10¹⁶ m^?3.The average temperature of the corona and solar wind is about 1,000,000-2,000,000 K; however, in the hottest regions it is 8,000,000-20,000,000 K.HP Pavilion DV6-1030US Battery While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection. The heliosphere, which is the cavity around the Sun filled with the solar wind plasma, extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the Solar System. HP Compaq 490306-001 Battery Its inner boundary is defined as the layer in which the flow of the solar wind becomessuperalfv nic that is, where the flow becomes faster than the speed of Alfv n waves. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfv n waves. HP Pavilion DV5-1211EM Battery The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. HP EliteBook 2530P Battery Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.

Magnetic field

The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum.HP Pavilion DV4385EA Battery The Sun's magnetic field leads to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System. Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Compaq PP2100 Battery Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth's outer atmosphere. All matter in the Sun is in the form of gas and plasma because of its high temperatures. HP Compaq KU532AA Battery This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots andsolar prominences (see magnetic reconnection). HP Pavilion DV6-1270EQ Battery This twisting action creates the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years. The solar magnetic field extends well beyond the Sun itself. The magnetized solar wind plasma carries Sun's magnetic field into the space forming what is called the interplanetary magnetic field.HP Pavilion DV4385EA Battery Since the plasma can only move along the magnetic field lines, the interplanetary magnetic field is initially stretched radially away from the Sun. Because the fields above and below the solar equator have different polarities pointing towards and away from the Sun, there exists a thin current layer in the solar equatorial plane, which is called the heliospheric current sheet.HP Compaq Business NC4400 Battery At the large distances the rotation of the Sun twists the magnetic field and the current sheet into the Archimedean spiral like structure called the Parker spiral. The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. ^{HP Pavilion ZD8227 Battery} The Sun's 50-400 ?T (in the photosphere) magnetic dipole field reduces with the cube of the distance to about 0.1 nT at the distance of the Earth. However, according to spacecraft observations the interplanetary field at the Earth's location is about 100 times greater at around 5 nT.Nikon D200 Battery

Chemical composition

The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2% of the mass. Acer PA-1650-02 Adapter The most abundant metals are oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%). The Sun inherited its chemical composition from the interstellar medium out of which it formed: the hydrogen and helium in the Sun were produced by Big Bang nucleosynthesis. Sony CYBER-SHOT DSC-T10 Battery The metals were produced by stellar nucleosynthesis in generations of stars which completed their stellar evolution and returned their material to the interstellar medium before the formation of the Sun. The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System.HP Compaq Business 6715b Battery However, since the Sun formed, the helium and heavy elements have settled out of the photosphere. Therefore, the photosphere now contains slightly less helium and only 84% of the heavy elements than the protostellar Sun did; the protostellar Sun was 71.1% hydrogen, 27.4% helium, and 1.5% metals.Kodak KLIC-7004 Battery In the inner portions of the Sun, nuclear fusion has modified the composition by converting hydrogen into helium, so the innermost portion of the Sun is now roughly 60% helium, with the metal abundance unchanged. Because the interior of the Sun is radiative, not convective (see Structure above), none of the fusion products from the core have risen to the photosphere.Acer Aspire 5920G Adapter The solar heavy-element abundances described above are typically measured both using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and thus not affected by settling of heavy elements. Canon Digital IXUS 90 IS Battery The two methods generally agree well.

Singly ionized iron group elements

In the 1970s, much research focused on the abundances of iron group elements in the Sun. Although significant research was done, the abundance determination of some iron group elements (e.g., cobalt and manganese) was still difficult at least as far as 1978 because of their hyperfine structures.HP 2133 Mini-Note PC Adapter The first largely complete set of oscillator strengths of singly ionized iron group elements were made available first in the 1960s,^[76] and improved oscillator strengths were computed in 1976. In 1978 the abundances of singly ionized elements of the iron group were derived.Toshiba PA3536U-1BRS Battery

Solar and planetary mass fractionation relationship

Various authors have considered the existence of a mass fractionation relationship between the isotopic compositions of solar and planetary noble gases, for example correlations between isotopic compositions of planetary and solar neon and xenon.HP Compaq Business NX6110 Battery Nevertheless, the belief that the whole Sun has the same composition as the solar atmosphere was still widespread, at least until 1983. In 1983, it was claimed that it was the fractionation in the Sun itself that caused the fractionation relationship between the isotopic compositions of planetary and solar wind implanted noble gases.HP Pavilion DV9000 Adapter

Sunspots and the sunspot cycle

When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Dell Studio 1537 Battery Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field causes strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. HP PPP009H Adapter The largest sunspots can be tens of thousands of kilometers across. The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Sony VGP-BPS8 Battery Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Sp rer's law. Sunspots usually exist as pairs with opposite magnetic polarity. Dell PA-12 Adapter The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next. The solar cycle has a great influence on space weather, and is a significant influence on the Earth's climate since luminosity has a direct relationship with magnetic activity.Sony Cyber-shot DSC-T7/B Battery Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. Dell LATITUDE M65 Battery During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures. HP Compaq 6720s Adapter

Possible long-term cycle

A recent theory claims that there are magnetic instabilities in the core of the Sun that cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles.Acer AS07B41 Battery

Life cycle

The Sun was formed about 4.57 billion years ago when a hydrogen molecular cloud collapsed. Solar formation is dated in two ways: the Sun's current main sequence age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.Dell Vostro 1310 Battery This is in close accord with the radiometric date of the oldest Solar System material, at 4.567 billion years ago. The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium.Sony VGP-BPL13 Battery Each second, more than four million metric tons of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. At this rate, the Sun has so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star.Sony VGP-BPS13 Battery The Sun does not have enough mass to explode as a supernova. Instead, in about 5 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 million K and will produce carbon, entering the asymptotic giant branch phase.Sony VGP-BPS13/B Battery Earth's fate is precarious. As a red giant, the Sun will have a maximum radius beyond the Earth's current orbit, 1 AU (1.5 10¹¹ m), 250 times the present radius of the Sun. However, by the time it is an asymptotic giant branch star, the Sun will have lost roughly 30% of its present mass due to a stellar wind, so the orbits of the planets will move outward. Sony VGP-BPS13/S Battery If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the Sun owing to tidal interactions.Even if Earth would escape incineration in the Sun, still all its water will be boiled away and most of its atmosphere would escape into space. Sony VGP-BPS13A/B Battery Even during its current life in the main sequence, the Sun is gradually becoming more luminous (about 10% every 1 billion years), and its surface temperature is slowly rising. The Sun used to be fainter in the past, which is possibly the reason life on Earth has only existed for about 1 billion years on land. Sony VGP-BPS13A/S Battery The increase in solar temperatures is such that in about another billion years the surface of the Earth will likely become too hot for liquid water to exist, ending all terrestrial life. Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula.Sony VGP-BPS13AS Battery The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.

Sunlight

Sunlight is Earth's primary source of energy.Sony VGP-BPS13B/B Battery The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,368 W/m² (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth).Sony VGP-BPS13B/Q Battery Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface closer to 1,000 W/m² in clear conditions when the Sun is near the zenith. Solar energy can be harnessed by a variety of natural and synthetic processes photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), Sony VGP-BPS13B/S Battery while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work, sometimes employing concentrating solar power (that it is measured in suns). The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.Sony VGP-BPS13Q Battery

Motion and location within the galaxy

The Sun lies close to the inner rim of the Milky Way Galaxy's Orion Arm, in the Local Fluff or the Gould Belt, at a hypothesized distance of 7.5-8.5 kpc(25,000-28,000 lightyears) from the Galactic Center, contained within the Local Bubble, a space of rarefied hot gas, possibly produced by the supernova remnant, Geminga.Sony VGP-BPS13S Battery The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone.Sony VGP-BPL21 Battery The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way, relative to other nearby stars. The general direction of the Sun's galactic motion is towards the star Vega in the constellation of Lyra at an angle of roughly 60 sky degrees to the direction of theGalactic Center.Sony VGP-BPS21 Battery The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. Sony VGP-BPS21/S Battery This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events.Sony VGP-BPS21A Battery It takes the Solar System about 225-250 million years to complete one orbit of the galaxy (a galactic year), so it is thought to have completed 20-25 orbits during the lifetime of the Sun. The orbital speed of the Solar System about the center of the Galaxy is approximately 251 km/s.Sony VGP-BPS21A/B Battery At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 AU. The Sun's motion about the centre of mass of the Solar System is complicated by perturbations from the planets. Every few hundred years this motion switches between prograde and retrograde.Sony VGP-BPS21B Battery

Solar neutrino problem

For many years the number of solar electron neutrinos detected on Earth was ¹?₃ to ¹?₂ of the number predicted by the standard solar model. This anomalous result was termed the solar neutrino problem. Sony VGP-BPL8 Battery Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth.Sony VGP-BPL8A Battery Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande.Results from these observatories eventually led to the discovery that neutrinos have a very small rest massand do indeed oscillate.Sony VGP-BPL8B Battery Moreover, in 2001 the Sudbury Neutrino Observatory was able to detect all three types of neutrinos directly, and found that the Sun's total neutrino emission rate agreed with the Standard Solar Model, although depending on the neutrino energy as few as one-third of the neutrinos seen at Earth are of the electron type.Sony VGP-BPS8 Battery This proportion agrees with that predicted by the Mikheyev-Smirnov-Wolfenstein effect (also known as the matter effect), which describes neutrino oscillation in matter, and it is now considered a solved problem.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6,000 K.Sony VGP-BPS8A Battery Above it lies the solar corona, rising to a temperature of 1,000,000-2,000,000 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating.Sony VGP-BPS8B Battery The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat.HP Compaq Business 6910p Battery The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events nanoflares. Currently, it is unclear whether waves are an efficient heating mechanism. Panasonic LUMIX DMC-TZ4S Battery All waves except Alfv n waves have been found to dissipate or refract before reaching the corona. In addition, Alfv n waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. Sony NP-FH100 Battery

Faint young Sun problem

Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop.Toshiba Equium M50-192 Battery However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today. The consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases Apple MacBook Pro Battery (such as carbon dioxide, methane and/or ammonia) than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching the planet.

Present anomalies

The Sun is currently behaving unexpectedly in a number of ways. Apple PowerBook G4 Battery

• It is in the midst of an unusual sunspot minimum, lasting far longer and with a higher percentage of spotless days than normal; since May 2008.
• It is measurably dimming; its output has dropped 0.02% at visible wavelengths and 6% at EUV wavelengths in comparison with the levels at the last solar minimum.Acer BATBL50L6 Battery
• Over the last two decades, the solar wind's speed has dropped by 3%, its temperature by 13%, and its density by 20%.
• Its magnetic field is at less than half strength compared to the minimum of 22 years ago. The entire heliosphere, which fills the Solar System, Canon EOS 450D Battery
• has shrunk as a result, resulting in an increase in the level of cosmic radiation striking the Earth and its atmosphere.

Early understanding and etymology

The English proper noun sun developed from Old English sunne (around 725, attested in Beowulf), and may be related to south.Acer Travelmate 2400 Battery Cognates to English sunappear in other Germanic languages, including Old Frisian sunne, sonne ("sun"), Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, and Gothic sunn?. All Germanic terms for the Sun stem from Proto-Germanic *sunn?n.Dell Inspiron 1525 Adapter InGermanic paganism, the Sun is personified as a goddess; S l/Sunna. Theories have been proposed that Sun, as Germanic goddess, may represent an extension of an earlier Proto-Indo-European deity due to Indo-European linguistic connections between Old Norse S l, Sanskrit Surya, Gaulish Sulis, Lithuanian Saul?, and Slavic Solnitse.Compaq Presario 2100 Adapter Humanity's most fundamental understanding of the Sun is as the luminous disk in the sky, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural phenomenon.Acer Aspire 1362 Battery Worship of the Sun was central to civilizations such as the Inca of South America and the Aztecs of what is now Mexico. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer or winter solstice ( Toshiba Equium M50-192 Battery some of the most prominent megaliths are located inNabta Playa, Egypt, Mnajdra, Malta and at Stonehenge, England); Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of El Castillo at Chich n Itz in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumn equinoxes.Dell XPS M1330 Battery In the late Roman Empire the Sun's birthday was a holiday celebrated as Sol Invictus (literally "unconquered sun") soon after the winter solstice which may have been an antecedent to Christmas. Regarding the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac,Acer Travelmate 2200 Adapter and so Greek astronomers considered it to be one of the seven planets (Greek planetes, "wanderer"), after which the seven days of the week are named in some languages.

Development of scientific understanding

In the early first millennium BCE, Babylonian astronomers observed that the Sun's motion along the ecliptic was not uniform, HP Compaq 6735b Adapter though they were unaware of why this was; it is today known that this is due to the Earth moving in an elliptic orbit around the Sun, with the Earth moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.Dell Vostro 1000 Battery One of the first people to offer a scientific or philosophical explanation for the Sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus rather than the chariot of Helios, and that the Moon reflected the light of the Sun.HP Compaq nx7300 Adapter For teaching this heresy, he was imprisoned by the authorities and sentenced to death, though he was later released through the intervention of Pericles. Eratosthenesestimated the distance between the Earth and the Sun in the 3rd century BCE as "of stadia myriads 400 and 80000", the translation of which is ambiguous, Asus f82 Battery