The Moon is Earth's only natural satellite. It has no formal English name other than "the Moon", although it is occasionally called Luna (Latin: moon) to distinguish it from the generic term "moon" (referring to any of the various natural satellites of other planets). Its symbol is a crescent (☽). The related adjective for the Moon is lunar (from the Latin root), but this is not found in combination with words using the prefix seleno- or suffix -selene (from the Greek deity Selene).

The average distance from the Earth to the Moon is 384,399 kilometres (238,854 miles), which is about 30 times the diameter of the Earth. At this distance, it takes sunlight reflected from the lunar surface approximately 1.3 seconds to reach Earth. The Moon's diameter is 3,474 kilometres (2,159 miles),^[1] which is about 3.7 times smaller than the Earth, making it the Solar System's fifth largest moon, both by diameter and mass, ranking behind Ganymede, Titan, Callisto, and Io. The gravitational attraction of the Moon is responsible for the tides on Earth. The Moon makes one complete orbit about the Earth every 27.3 days, and the periodic variations in the geometry of the Earth-Moon-Sun system are responsible for the lunar phases that repeat every 29.5 days.

The Moon is the only celestial body(other than the Earth) that human beings have orbited and landed on. The Soviet Union's (USSR) Luna program was the first to reach the Moon with unmanned spacecraft. 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, 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.^[1] The United States' Apollo program achieved the first (and only) manned missions to the Moon. The first manned mission to orbit the Moon was Apollo 8 in 1968, and the first people to land and walk on the Moon came aboard Apollo 11 in 1969.^[1]

Manned exploration of the moon ceased with the conclusion of the Apollo program, although several countries have recently announced plans to send either spacecraft or humans to the Moon in the near future. On December 4, 2006, NASA outlined plans for a permanent base on the Moon as part of preparation for a voyage to Mars. Construction of the base is scheduled to take approximately five years, with the first preliminary missions beginning by 2020.^[2]

The lunar surface

The two sides

The Moon is in synchronous rotation, meaning that it keeps nearly the same face turned toward Earth at all times. Early in the Moon's history, its rotation slowed and became locked in this configuration as a result of frictional effects associated with tidal deformations caused by the Earth.^[3] Nevertheless, small variations resulting from the finite eccentricity of the lunar orbit, termed optical librations,^[4] allow up to about 59% of the lunar surface to be visible from Earth.^[1]

The side of the Moon that faces Earth is called the near side, and the opposite side is called the far side. The far side should not be confused with the dark side, which is the hemisphere that is not illuminated by the Sun. Spacecraft are cut off from direct radio communication with Earth when behind the Moon (see line-of-sight propagation). The far side of the Moon was first photographed by the Soviet probe Luna 3 in 1959. One distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features.

Maria

The dark and relatively featureless lunar plains are called maria, Latin for seas, since they were believed by ancient astronomers to be filled by water. They are actually vast ancient basaltic lava flows, many of which filled the topographic depressions associated with large impact basins (Oceanus Procellarum is a major exception in that it does not correspond to any known impact basin). Maria are found almost exclusively on the lunar nearside, with the lunar far side having only a few scattered patches. Only about 2% of the surface of the far side is covered by maria,^[5] compared to about 31% on the near side.^[1] The most likely explanation for this difference is related to a higher concentration of heat producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer.^[6][7] Several provinces containing shield volcanoes and volcanic domes are found within the near side maria.^[8]

Terrae

The lighter-colored regions of the Moon are called terrae, or more commonly just highlands since they are topographically higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are believed to be the surviving remnants of the impact basin's outer rims.^[9] In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.

Using images taken by the Clementine mission, it appears that four mountainous regions on the rim of the 73 km-wide Peary crater at the Moon's north pole remain illuminated for the entire lunar day, at least during the hemisphere's summer season. These unnamed mountains of eternal light are possible because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of Shackleton crater is illuminated for 80% of the lunar day. Another consequence of the Moon's small axial tilt is that there are regions that remain in permanent shadow at the bottoms of many polar craters.^[10]

Impact craters

The Moon's surface everywhere shows evidence of having been affected by the process of impact cratering.^[11] Most impact craters formed when asteroids and comets collided with the lunar surface over the first billion or so years, and globally, about half a million craters with diameters greater than 1 km can be found on the lunar surface. Since impact craters accumulate at a nearly constant rate, the number of craters per unit area on a geologic unit can be used to estimate the age of its surface (see crater counting). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those found on Earth.

The largest crater on the Moon, which also has the distinction of being the largest known crater in the Solar System, is the South Pole-Aitken basin. This impact basin is located on the far side, between the South Pole and equator, and is some 2,240 kilometres in diameter and 13 kilometres in depth.^[12] Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.

Regolith

Blanketed atop the Moon's crust is a highly comminuted (meaning it has been broken into ever smaller particles) and "impact gardened" surficial layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3 to 5 metres (10 to 16 ft) in the maria to about 10 to 20 metres (33 to 66 ft) in the highlands.^[13] Beneath the finely comminuted regolith layer is what is generally referred to as the "megaregolith." This layer is much thicker (on the order of tens of kilometers) and consists of highly fractured bedrock.^[14]

Presence of water

The continuous bombardment of the Moon by comets and meteoroids have added some amount of water to the lunar surface. Energy from sunlight usually splits much of this water into its constituent elements hydrogen and oxygen, which generally escape to space. Attesting to the dryness of lunar rocks, the samples collected by Apollo astronauts near the equator have been found to contain only traces of water. However, because of the very slight axial tilt of the Moon's spin axis to the ecliptic plane (only 1.5°), some deep craters near the poles never receive any light from the Sun, and are permanently shadowed (see Shackleton crater). Thus, any water molecules that eventually ended up in these craters could be stable for long periods of time.

Clementine has mapped craters at the lunar south pole^[15] that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow.^[10] Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper meter of the regolith near the polar regions.^[16] Estimates for the total quantity of water ice are close to one cubic kilometer. Recently, observations with the Arecibo planetary radar showed that some of the near polar Clementine radar returns might instead be associated with rocks ejected from young craters. If true, this would indicate that the neutron results are primarily from hydrogen in forms other than ice, such as trapped hydrogen molecules or organics. Nevertheless, the interpretation of these data are non unique (ice or surface roughness could give rise to the observed signature), and it appears that these results do not exclude the possibility of water ice in permanently shadowed craters.^[17]

Water ice can be mined and then split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive.

Physical characteristics

Internal structure

The Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust (see Origin and geologic evolution below).

Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition,^[18] consistent with the magma ocean hypothesis. In terms of elements, the lunar crust is composed primarily of oxygen, silicon, magnesium, iron, calcium, and aluminium, but important minor and trace elements such as titanium, uranium, thorium, potassium, and hydrogen are present as well. Based on geophysical techniques, the crust is estimated to be on average about 50 km thick.^[19]

Partial melting within the mantle of the Moon gave rise to the eruption of mare basalts on the lunar surface. Analyses of these basalts indicate that the mantle is composed predominantly of the minerals olivine, orthopyroxene and clinopyroxene, and that the lunar mantle is more iron rich than that of the Earth. Some lunar basalts contain high abundances of titanium (present in the mineral ilmenite), suggesting that the mantle is highly heterogeneous in composition. Moonquakes have been found to occur deep within the mantle of the Moon about 1000 km below the surface. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth. A few shallow moonquakes with hypocenters located about 100 km below the surface have also been detected, but these occur more infrequently and appear to be unrelated to the lunar tides.^[19]

The Moon has a mean density of 3,346.4 kg/m³, making it the second densest moon in the Solar System after Io. Nevertheless, several lines of evidence imply that the lunar core is small, with a radius of about 350 km or less.^[19] The size of the lunar core is only about 20% the size of the Moon, in contrast to about 50% as is the case for most other terrestrial bodies. The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloyed with a small amount of sulfur and nickel. Analyses of the Moon's time-variable rotation indicate that the core is at least partly molten.^[20]

Topography

The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the Clementine mission. The most visible topographic feature is the giant far side South Pole-Aitken basin, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims.

Another distinguishing feature of the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side. If it is assumed that the crust is in isostatic equilibrium, and that the density of the crust is everywhere the same, then the higher elevations would be associated with a thicker crust. Using gravity, topography and seismic data, the crust is thought to be on average about 50±15 km thick, with the far-side crust being on average thicker than the near side by about 15 km.^[19]

Gravity field

The gravitational field of the Moon has been determined by the tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the Doppler effect, whereby the line-of-sight spacecraft acceleration can be measured by small shifts in frequency of the radio signal, and the measurement of the distance from the spacecraft to a station on Earth. Since the gravitational field of the Moon affects the orbit of a spacecraft, it is possible to use these tracking data to invert for gravitational anomalies. However, because of the Moon's synchronous rotation it is not possible to track spacecraft much over the limbs of the Moon, and the farside gravity field is thus only poorly characterized.

The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant impact basins. These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions. They were initially discovered by the analysis of Lunar Orbiter tracking data,^[21] since pre-Apollo navigational tests were experiencing landing position errors much larger than mission specifications.

The origin of mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins. However, lava flows by themselves can not explain the entirety of the gravitional signature, and uplift of the crust-mantle interface is required as well. Based on Lunar Prospector gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism.^[22] It should be noted that the huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly.

Magnetic field

The Moon has only a very weak external magnetic field in comparison to the Earth. 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. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.^[23]

Atmosphere

The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing—the release of gases such as radon that originate by radioactive decay processes within the crust and mantle. Another important source is generated through the process of sputtering, which involves the bombardment of micrometeorites, solar wind ions, electrons, and sunlight.^[18] Gasses that are released by sputtering can either reimplant into the regolith as a cause of the Moon's gravity, or can be lost to space either by solar radiation pressure or by being swept away by the solar wind magnetic field if they are ionized. The elements sodium (Na) and potassium (K) have been detected using earth-based spectroscopic methods, whereas the element radon-222 and polonium-210 have been inferred from data obtained from the Lunar Prospector alpha particle spectrometer.^[24] Argon-40, He-4, O and/or CH₄, N₂ and/or CO, and CO₂ were detected by in-situ detectors placed by the Apollo astronauts.^[25]

Origin and geologic evolution

Formation

Several mechanisms have been suggested for the Moon's formation. Early speculation proposed that the Moon broke off from the Earth's crust because of centrifugal forces, leaving a basin (presumed to be the Pacific Ocean) behind as a scar.^[26] This fission concept, however, requires too great an initial spin of the Earth. Others speculated that the Moon formed elsewhere and was captured into Earth's orbit.^[27] However, the conditions required for this capture mechanism to work (such as an extended atmosphere of the Earth for dissipating energy) are not too probable. The coformation hypothesis posits that the Earth and the Moon formed together at the same time and place from the primordial accretion disk. In this theory, the Moon forms from material surrounding the proto-Earth, similar to the way in which the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of metallic iron in the Moon. A major deficiency with all of these hypotheses is that they cannot easily account for the high angular momentum of the Earth-Moon system.^[28]

Today, the giant impact hypothesis for forming the Earth-Moon system is widely accepted by the scientific community. In this theory, the impact of a Mars-sized body (which has been referred to as Theia or Orpheus) into the proto-Earth is postulated to have put enough material into circumterrestrial orbit to form the Moon.^[1] Given that planetary bodies are believed to have formed by the hierarchical accretion of smaller to larger sized bodies, it is now recognized that giant impact events such as this should be expected to have occurred for some planets. Computer simulations modeling this impact can account for the angular momentum of the Earth-Moon system, as well as the small size of the lunar core.^[29] Unresolved questions concerning this theory are the relative sizes of the proto-Earth and impactor, and the proportion of material from the proto-Earth and impactor that contribute to making the Moon. The formation of the Moon is believed to have occurred at 4.527 ± 0.01 billion years, about 30 to 50 million years after the origin of the solar system.^[30]

The lunar magma ocean

As a result of the large amount of energy liberated during both the giant impact event and the subsequent reaccretion of material in Earth orbit, it is commonly believed that a large portion of the Moon was once initially molten. The molten outer portion of the Moon is referred to as a magma ocean, and estimates for its depth range from about 500 km to the entire radius of the Moon.^[6]

As the magma ocean cooled, it fractionally crystallized and differentiated, giving rise to a geochemically distinct crust and mantle. The mantle is predicted to have formed largely by the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene. After about three-quarters of magma ocean crystallization was complete, the mineral anorthosite is predicted to have precipitated and floated to the surface because of its low density, forming the crust.^[6]

The final liquids to crystallize from the magma ocean would have been initially sandwiched between the crust and mantle, and would have contained a high abundance of incompatible and heat-producing elements. This geochemical component is referred to by the acronym KREEP, for potassium (K), rare earth elements (REE), and phosphorus (P), and appears to be concentrated within the Procellarum KREEP Terrane, which is a small geologic province that encompasses most of Oceanus Procellarum and Mare Imbrium on the near side of the Moon.^[19]

Geologic evolution

A large portion of the Moon's post-magma-ocean geologic evolution was dominated by impact cratering. The lunar geologic timescale is divided in time based on a few prominent impact events, such as Nectaris, Imbrium, Eratosthenes, and Copernicus. While not all of these craters have been definitively dated (and some ages are still being debated), they are useful for assigning relative ages based on stratigraphic grounds. The continuous effects of impact cratering are responsible for forming the regolith.

The other major geologic process that affected the Moon's surface was mare volcanism. The enhancement of heat producing elements within the Procellarum KREEP Terrane would have caused the underlying mantle to heat up, and eventually, to partially melt. Some portion of these magmas rose to the surface and erupted, accounting for the high concentration of mare basalts on the near side of the Moon.^[6] Most of the Moon's mare basalts erupted during the Imbrian period within this geologic province, around 3 to 3.5 billion years ago. Nevertheless, some dated samples are as old as 4.2 billion years,^[31] and the youngest eruptions, based on the method of crater counting, are believed to have occurred only 1.2 billion years ago.^[32]

There has been some controversy over whether features on the Moon's surface undergo changes with time. Some observers have claimed that craters either appeared or disappeared, or that other forms of transient phenomena had occurred. Today, many of these claims are thought to be illusory, resulting from observing under different lighting conditions, poor astronomical seeing, or the inadequacy of earlier drawings. Nevertheless, it is known that the phenomenon of outgassing does occasionally occur, and these events 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.^[33][34]

Orbit and relationship to Earth

The Moon makes a complete orbit about the Earth with respect to the fixed stars (its sidereal period) approximately once every 27.3 days. 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 its same phase to Earth, which is about 29.5 days (its synodic period).^[1] Unlike most satellites of other planets, the Moon orbits near the ecliptic and not the Earth's equatorial plane.

The Earth and Moon have many physical effects upon one another, including the tides. Most of the tidal effects seen on the Earth are caused by the Moon's gravitational pull, with the Sun making only a small contribution. Tidal effects result in an increase of the mean Earth-Moon distance of about 4 meters per century, or 4 centimetres per year.^[35] As a result of the conservation of angular momentum, the increasing semimajor axis of the Moon is accompanied by a gradual slowing of the Earth's rotation by about 0.002 seconds per day per century.^[36]

The Earth-Moon system is sometimes considered to be a double planet rather than a planet-moon system. This is due to the exceptionally large size of the Moon relative to its host planet; the Moon is one-fourth the diameter of Earth and 1/81 its mass. However, this definition is criticized by some since the common center of mass of the system (the barycenter) is located about 1700 km (average) beneath the surface of the Earth, or about a quarter of the Earth's radius. The surface of the Moon is less than 1/10th that of the Earth, and only about one quarter the size of Earth's land area (about as large as Russia, Canada, and the United States combined).

In 1997 the asteroid 3753 Cruithne was found to have an unusual Earth-associated horseshoe orbit. However, astronomers do not consider it to be a second moon of Earth, and its orbit is not stable in the long term.^[37] Three other near-Earth asteroids, (54509) 2000 PH5, (85770) 1998 UP1 and 2002 AA29, which exist in orbits similar to Cruithne's, have since been discovered.^[38]

Eclipses

Eclipses can occur only when the Sun, Earth, and Moon are all in a straight line. Solar eclipses can occur near a new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses can occur near a full moon, when the Earth is between the Sun and Moon. Because the Moon's orbit around the Earth is inclined by about 5° with respect 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.^[39]

The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible.^[40] 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 slightly increasing over time, the angular diameter of the Moon is decreasing. This means that hundreds of millions of years ago the Moon could always completely cover the Sun on solar eclipses so that no annular eclipses were possible. Likewise, about 600 million years from now (assuming that the angular diameter of the Sun will not change), the Moon will no longer cover the Sun completely and only annular eclipses will occur.^[39]

A phenomenon related to eclipse is occultation. The Moon is continuously blocking our view of the sky by a 1/2 degree wide circular area. When a bright star or planet passes behind the Moon it is occulted or hidden from view. A solar eclipse is an occultation of the Sun. Because the Moon is close to Earth, occultations of individual stars are not visible everywhere, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.^[41]

Observation

During the brightest full moons, the Moon can have an apparent magnitude of about −12.6. For comparison, the Sun has an apparent magnitude of −26.8. When the Moon is in a quarter phase, its brightness is not one half of a full moon, but instead is only about 1/10. This is because the lunar surface is not a perfect Lambertian reflector and because shadows projected onto the surface also diminish the amount of reflected light.

The Moon appears larger when close to the horizon. This is a purely psychological effect (see Moon illusion). The angular diameter of the Moon from Earth is about one half of one degree, and is actually about 1.5% smaller when the Moon is near the horizon than when it is high in the sky (because it is farther away by up to 1 Earth radius).

Another quirk of the visual system causes us to see the Moon as almost pure white, when in fact it reflects only about 7% of the light falling on it (about as dark as a lump of coal). It has a very low albedo. Color constancy in the visual system recalibrates the relations between colors of an object and its surroundings; however, there is nothing next to the Moon to reflect the light falling on the Moon, therefore it is perceived as the brightest object visible. We have no standard to compare it to. An example of this is that, if you used a narrow beam of light to illuminate a lump of coal in a dark room, it would look white. If you then broadened the beam of the light source to illuminate the surroundings, it would revert to black.

The highest altitude of the Moon on a day varies and has nearly the same limits as the Sun. It also depends on season and lunar phase with the full moon being highest in winter. The orientation of the Moon's crescent also depends on the latitude of the observing site. Close to the equator an observer can see a boat Moon.^[42]

Like the Sun, the Moon can also give rise to the atmospheric effects including a 22 degree halo ring and the smaller coronal rings seen more often through thin clouds. For more information on how the Moon appears in Earth's sky, see lunar phase.

Exploration

The first leap in lunar observation was caused by the invention of the telescope. Galileo Galilei made especially good use of this new instrument and observed mountains and craters on the Moon's surface.

The Cold War-inspired space race between the Soviet Union and the United States of America led to an acceleration of interest in the Moon. Unmanned probes, both flyby and impact/lander missions, were sent almost as soon as launcher capabilities would allow. The Soviet Union's (USSR) Luna program was the first to reach the Moon with unmanned spacecraft. 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, 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.^[1] Moon samples have been brought back to Earth by three Luna missions (Luna 16, 20, and 24) and the Apollo missions 11 through 17 (excepting Apollo 13, which aborted its planned lunar landing).

What was the next big step depends on the political viewpoint: in the US (and the western world in general) the landing of the first humans on the Moon in 1969 is seen 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 July 21, 1969. The last person (as of 2007) to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972. The USA Moon landing and return was enabled by several technologies where the US surpassed the Russians; for example, the US achieved considerable advances in ablation chemistry and atmospheric re-entry technology in the early 1960s.

Scientific instrument packages were installed on the lunar surface during all of the Apollo missions. Long-lived ALSEP stations (Apollo lunar surface experiment package) were installed at the Apollo 12, 14, 15, 16, and 17 landing sites, whereas a temporary station referred to as EASEP (Early Apollo Scientific Experiments Package) was installed during the Apollo 11 mission. The ALSEP stations contained, among others, heat flow probes, seismometers, magnetometers, and corner-cube retroreflectors. Transmission of data to Earth was terminated on September 30 1977 because of budgetary considerations. Since the lunar laser ranging (LLR) corner-cube arrays are passive instruments, they are still being used to today. Ranging to the LLR stations is routinely performed from earth-based stations with an accuracy of a few centimeters, and data from this experiment are being used to place constraints on the size of the lunar core.^[43]

From the mid-1960s to the mid-1970s, there were a total of 65 Moon landings (both manned and robotic, with 10 in 1971 alone), but after Luna 24 in 1976 they stopped. The Soviet Union started focusing on Venus and space stations and the US on Mars and beyond. In 1990 Japan orbited the Moon with the Hiten spacecraft, becoming the third country to place a spacecraft into lunar orbit. The spacecraft released a smaller probe, Hagormo, in lunar orbit, but the transmitter failed rendering the mission scientifically useless.

In 1994, the US finally returned to the Moon, robotically at least, sending the Joint Defense Department/NASA spacecraft Clementine. This mission obtained the first near global topographic map of the Moon, as well as the first global multispectral images of the lunar surface. This was followed by the Lunar Prospector mission in 1998. The neutron spectrometer on Lunar Prospector indicated the presence of excess hydrogen at the lunar poles, which is likely due to the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. The European spacecraft Smart 1 was launched September 27, 2003 and was in lunar orbit from November 15, 2004 to September 3, 2006.

On January 14, 2004, US President George W. Bush called for a plan to return manned missions to the Moon by 2020 (see Vision for Space Exploration).^[44] NASA is now planning for the construction of a permanent outposts at one of the lunar poles.^[45] The People's Republic of China has expressed ambitious plans for exploring the Moon and has started the Chang'e program for lunar exploration. Japan has two planned lunar missions, LUNAR-A and Selene. India intends to launch several unmanned missions, beginning with Chandrayaan I in February 2008, followed by Chandaryaan II in 2010 or 2011; the latter is slated to include a robotic lunar rover. The US will launch the Lunar Reconnaissance Orbiter in 2008. Russia also announced to resume its previously frozen project Luna-Glob, consisting of an unmanned lander and orbiter, which is slated to land in 2012.^[46]

Human understanding

The Moon has been the subject of many works of art and literature and the inspiration for countless others. 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.^[47] In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated today.

Among the first in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras, who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile.^[48]

In Aristotle's 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. This separation was held to be part of physics for many centuries after.^[49]

By the Middle Ages, before the invention of the telescope, more and more people began to recognize the Moon as a sphere, though they believed that it was "perfectly smooth".^[50] 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. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today.

On maps, the dark parts of the Moon's surface were called maria (singular mare) or seas, and the light parts were called terrae or continents. The possibility that the Moon could contain vegetation and be inhabited by selenites was seriously considered by some major astronomers even into the first decades of the 19th century. 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, amongst others.

In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon.^[51] Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.

The far side of the Moon remained completely unknown until the Luna 3 probe was launched in 1959, and was extensively mapped by the Lunar Orbiter program in the 1960s.

Legal status

Though several flags of the Soviet Union^[a] and the United States have been symbolically planted on the Moon, the Russian and U.S. governments make no claims to any part of the Moon's surface. Russia and the U.S. are party to the Outer Space Treaty, which places the Moon under the same jurisdiction as international waters (res communis). This treaty also restricts use of the Moon to peaceful purposes, explicitly banning weapons of mass destruction (including nuclear weapons) and military installations of any kind.

A second treaty, the Moon Treaty, was proposed 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. Several individuals have made claims to the Moon in whole or in part, though none of these claims are generally considered credible (see Extraterrestrial real estate).