The Moon is Earth‘s only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia),[16] it is the largest natural satellite in the Solar System relative to the size of its planet,[f] the fifth largest satellite in the Solar System overall, and is larger than any known dwarf planet. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under geophysical definitions of the term.[17] It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth’s (0.1654 g); Jupiter‘s moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.
Orbiting Earth at an average distance of 384,400 km (238,900 mi),[18] or about 30 times Earth’s diameter, its gravitational influence slightly lengthens Earth’s day and is the main driver of Earth’s tides. The Moon’s orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration.
The Moon with its fully illuminated near side (north is at top)
The most widely accepted origin explanation posits that the Moon formed about 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria (“seas”), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon’s apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.
Both the Moon’s prominence in the earthly sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union‘s Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States‘ Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon’s origins, internal structure, and subsequent history.
The usual English proper name for Earth’s natural satellite is simply the Moon, with a capital M.[20][21] The noun moon is derived from Old English mōna, which (like all its Germanic cognates) stems from Proto-Germanic *mēnōn,[22] which in turn comes from Proto-Indo-European *mēnsis “month”[23] (from earlier *mēnōt, genitive *mēneses) which may be related to the verb “measure” (of time).[24]
Occasionally, the name Luna /ˈluːnə/ is used in scientific writing[25] and especially in science fiction to distinguish the Earth’s moon from others, while in poetry “Luna” has been used to denote personification of the Moon.[26] Cynthia /ˈsɪnθiə/ is another poetic name, though rare, for the Moon personified as a goddess,[27] while Selene /səˈliːniː/ (literally “Moon”) is the Greek goddess of the Moon.
The usual English adjective pertaining to the Moon is “lunar”, derived from the Latin word for the Moon, lūna. The adjective selenian /səliːniən/,[28] derived from the Greek word for the Moon, σελήνη selēnē, and used to describe the Moon as a world rather than as an object in the sky, is rare,[29] while its cognate selenic was originally a rare synonym[30] but now nearly always refers to the chemical element selenium.[31] The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography, the study of the physical features of the Moon, as well as the element name selenium.[32][33]
The Greek goddess of the wilderness and the hunt, Artemis, equated with the Roman Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia, from her legendary birthplace on Mount Cynthus.[34] These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits such as apolune, pericynthion and selenocentric.
Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System.[35][36] Historically, several formation mechanisms have been proposed,[37] but none satisfactorily explained the features of the Earth–Moon system. A fission of the Moon from Earth’s crust through centrifugal force[38] would require too great an initial rotation rate of Earth.[39] Gravitational capture of a pre-formed Moon[40] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon.[39] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon.[39] None of these hypotheses can account for the high angular momentum of the Earth–Moon system.
The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The impact blasted material into orbit about the Earth and then the material accreted and formed the Moon[42][43] just beyond the Earth’s Roche limit of ~2.56 R🜨.[44] This theory best explains the evidence.
Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations also show that most of the Moon derived from the impactor, rather than the proto-Earth.[45] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth.[46][47][48][49] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two,[50] although this is debated.[51]
The impact released energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean.[52][53] Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).
While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon’s composition.[54]
In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks.[55] The rocks from the Apollo program had the same isotopic signature as rocks from Earth, differing from almost all other bodies in the Solar System. This observation was unexpected, because most of the material that formed the Moon was thought to come from Theia and it was announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures.[56] Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth,[57] which conflicts with what is expected if the Moon formed far from Earth or is derived from Theia. These discrepancies may be explained by variations of the giant-impact theory. For instance, a high-speed drive-by hit by the impactor allowed it to return to earth a second time but more slowly, and mix more thoroughly.[58] A hit-and-run-and-return scenario might be more likely.
The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This ‘fossil bulge’ indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit.
The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition.[62] It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi).[63][64] 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.[65]
Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust atop.[66] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements.[1] Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite.[15] The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.[1] The crust is on average about 50 kilometres (31 mi) thick.[1]
The Moon is the second-densest satellite in the Solar System, after Io.[67] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less,[1] around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon’s time-variable rotation suggest that it is at least partly molten.[68] The pressure at the lunar core is estimated to be 5 GPa (49,000 atm).
The Moon has an external magnetic field of generally less than 0.2 nanoteslas,[70] or less than one hundred thousandth that of Earth. The Moon does not currently have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating.[71][72] However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today.[70] This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized.[70] Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins.
The topography of the Moon has been measured with laser altimetry and stereo image analysis.[74] Its most extensive topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System.[75][76] At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon.[75][77] The highest elevations of the Moon’s surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin.[78] Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess regionally low elevations and elevated rims.[75] The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side.[1]
The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years.[79] Similar shrinkage features exist on Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn’t have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat.
The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria (Latin for “seas”; singular mare), as they were once believed to be filled with water;[81] they are now known to be vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water.[82] The majority of these lava deposits erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side “maria”.
Geological features of the Moon (near side / north pole at left, far side / south pole at right)
Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side[84] compared with 2% of the far side.[85] This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.[66][86][87] 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.[88] As of 2003, crater counting studies of the youngest eruptions appeared to suggest they formed no earlier than 1.2 billion years ago.[89]
In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old.[90] Moonquakes and releases of gas also indicate some continued lunar activity.[90] Evidence of recent lunar volcanism has been identified at 70 irregular mare patches, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements.[91][92][93][94] Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell,[95][96] inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin.[97][98]
The lighter-colored regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean.[88][89] In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[99]
The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon’s formation.[100][101] Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth.
A major geologic process that has affected the Moon’s surface is impact cratering,[103] with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon’s near side.[104] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale; structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon.[105] The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because 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.[105] 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 period of increased impacts.[106]
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. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder.[107] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 km (6.2–12.4 mi) in the highlands and 3–5 km (1.9–3.1 mi) in the maria.[108] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick.[109]
High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years.[110][111] This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.
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, partly caused by the dense mare basaltic lava flows that fill those basins.[113][114] The 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.
Lunar swirls are enigmatic features found across the Moon’s surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and have often a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts. Well known swirls include the Reiner Gamma feature and Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the solar wind, resulting in slower space weathering.
Lunar crater Daedalus on the Moon’s far side
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. 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 persist in cold, permanently shadowed craters at either pole on the Moon.[117][118] Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow.[119] 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.[120]
In years since, signatures of water have been found to exist on the lunar surface.[121] In 1994, the bistatic radar experiment located on the Clementine spacecraft, 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.[122] In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions.[123] Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior.[124]
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. The spacecraft showed that concentrations may possibly be as high as 1,000 ppm.[125] Using the mapper’s reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018.[126] In 2009, LCROSS sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material.[127][128] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb).[129]
In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported,[130] the famous high-titanium “orange glass soil” of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth’s upper mantle. Although 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.
Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time “definitive evidence” for water-ice on the lunar surface.[131][132] The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances.[133] The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun.[131][133]
In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA).
The surface of the Moon is an extreme environment with temperatures that range from 140 °C down to −171 °C, an atmospheric pressure of 10−10 Pa, and high levels of ionizing radiation from the Sun and cosmic rays. The exposed surfaces of spacecraft are considered unlikely to harbor bacterial spores after just one lunar orbit.[138] The surface gravity of the Moon is approximately 1.625 m/s2, about 16.6% that on Earth’s surface or 0.166 ɡ.
The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 tonnes (9.8 long tons; 11 short tons).[141] 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, a product of the bombardment of lunar soil by solar wind ions.[15][142] Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io); helium-4 and neon[143] from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle.[144][145] 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.[144] Water vapor 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 sublimation of water ice in the regolith.[146] These gases either return into the regolith because of the Moon’s gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind’s magnetic field.[144]
Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.
Because of tidal locking, the rotation of the Moon around its own axis is synchronous to its orbital period around the Earth. The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days,[g] its sidereal period. However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days;[h] its synodic period.[84][150]
Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane than to the planet’s equatorial plane. 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 orbit gradually rotates once every 18.61 years,[151] which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini’s laws.[152]
The Moon’s axial tilt with respect to the ecliptic is only 1.5427°,[8][153] much less than the 23.44° of Earth. Because of this, the Moon’s solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects.[154] From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of the crater Peary at the Moon’s north pole may remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters,[119] and these “craters of eternal darkness” are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F)[155] and just 26 K (−247 °C; −413 °F) close to the winter solstice in the north polar crater Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto.[154] Average temperatures of the Moon’s surface are reported, but temperatures of different areas will vary greatly depending upon whether they are in sunlight or shadow.
The Moon is an exceptionally large natural satellite relative to Earth: Its diameter is more than a quarter and its mass is 1/81 of Earth’s.[84] It is the largest moon in the Solar System relative to the size of its planet,[i] though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto’s mass.[j][157] The Earth and the Moon’s barycentre, their common center of mass, is located 1,700 km (1,100 mi) (about a quarter of Earth’s radius) beneath the Earth’s surface.
The Earth revolves around the Earth-Moon barycentre once a sidereal month, with 1/81 the speed of the Moon, or about 12.5 metres (41 ft) per second. This motion is superimposed on the much larger revolution of the Earth around the Sun at a speed of about 30 kilometres (19 mi) per second.
The surface area of the Moon is slightly less than the areas of North and South America combined.
The synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. However, because of the effect of libration, about 59% of the Moon’s surface can actually be seen from Earth. The side of the Moon that faces Earth is called the near side, and the opposite the far side. The far side is often inaccurately called the “dark side”, but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During new moon, the near side is dark.[158]
The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth.[159] With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA Lunar Prospector mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth.
The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun.[84][k] This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon.[161] Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because 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 center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. 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.[162] The full Moon’s angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see § Eclipses).
The Moon’s highest altitude at culmination varies by its phase and time of year. The full moon is highest in the sky during winter (for each hemisphere). The orientation of the Moon’s crescent also depends on the latitude of the viewing location; an observer in the tropics can see a smile-shaped crescent Moon.[163] The Moon is visible for two weeks every 27.3 days at the North and South Poles. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end.
The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) to 406,700 km (252,700 mi) at perigee (closest) and apogee (farthest), respectively. On 14 November 2016, it was closer to Earth when at full phase than it has been since 1948, 14% closer than its farthest position in apogee.[165] Reported as a “supermoon“, this closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because its angular diameter is 14% greater and {\displaystyle \scriptstyle 1.14^{2}\approx 1.30}. At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:
When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase.[171]
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. 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 (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago.
The Moon’s appearance, like the Sun’s, can be affected by Earth’s atmosphere. Common optical effects are the 22° halo ring, formed when the Moon’s light is refracted through the ice crystals of high cirrostratus clouds, and smaller coronal rings when the Moon is seen through thin clouds.
The illuminated area of the visible sphere (degree of illumination) is given by
,
where {\displaystyle e} is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).
The gravitational attraction that masses have for one another decreases inversely with the square of the distance of those masses from each other. As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces. Tidal forces affect both the Earth’s crust and oceans.
The most obvious effect of tidal forces is to cause two bulges in the Earth’s oceans, one on the side facing the Moon and the other on the side opposite. This results in elevated sea levels called ocean tides.[177] As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place “under” the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours.[177] Since the Moon is orbiting the Earth in the same direction of the Earth’s rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon’s time to orbit the Earth. The Sun has the same tidal effect on the Earth, but its forces of attraction are only 40% that of the Moon’s; the Sun’s and Moon’s interplay is responsible for spring and neap tides.[177] If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects: the frictional coupling of water to Earth’s rotation through the ocean floors, the inertia of water’s movement, ocean basins that grow shallower near land, the sloshing of water between different ocean basins.[183] As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.
The libration of the Moon over a single lunar month. Also visible is the slight variation in the Moon’s visual size from Earth.
While gravitation causes acceleration and movement of the Earth’s fluid oceans, gravitational coupling between the Moon and Earth’s solid body is mostly elastic and plastic. The result is a further tidal effect of the Moon on the Earth that causes a bulge of the solid portion of the Earth nearest the Moon. Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth’s rotation. This “drains” angular momentum and rotational kinetic energy from Earth’s rotation, slowing the Earth’s rotation.[177][184] That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration), which lifts the Moon into a higher orbit and results in its lower orbital speed about the Earth. Thus the distance between Earth and Moon is increasing, and the Earth’s rotation is slowing in reaction.[184] Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments) have found that the Moon’s distance increases by 38 mm (1.5 in) per year (roughly the rate at which human fingernails grow).[185][186][187] Atomic clocks also show that Earth’s day lengthens by about 17 microseconds every year,[188][189][190] slowly increasing the rate at which UTC is adjusted by leap seconds. This tidal drag would continue until the rotation of Earth and the orbital period of the Moon matched, creating mutual tidal locking between the two and suspending the Moon over one meridian (this is currently the case with Pluto and its moon Charon). However, the Sun will become a red giant engulfing the Earth-Moon system long before this occurrence.[191][192]
In a like manner, the lunar surface experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun.[184] The Earth-induced variable component arises from changing distance and libration, a result of the Moon’s orbital eccentricity and inclination (if the Moon’s orbit were perfectly circular and un-inclined, there would only be solar tides).[184] Libration also changes the angle from which the Moon is seen, allowing a total of about 59% of its surface to be seen from Earth over time.[84] The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.
According to recent research, scientists suggest that the Moon’s influence on the Earth may contribute to maintaining Earth’s magnetic field.
One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 at Knowth, Ireland.[195][196]
Understanding of the Moon’s cycles was an early development of astronomy: 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.[197][198]: 227 Elsewhere in the 5th century BC to 4th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses,[199] and Indian astronomers had described the Moon’s monthly elongation.[200] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses.[198]: 411
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.[201] Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System.[202] 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.[203] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance.
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.[198]: 413–414 Ptolemy (90–168 AD) greatly improved on the numbers of Aristarchus, calculating the values of a mean distance of 59 times Earth’s radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively.[204] In the 2nd century AD, Lucian wrote the novel A True Story, in which the heroes travel to the Moon and meet its inhabitants. In 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon.[205] 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.[206] 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.[198]: 415–416
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 used an early telescope to make drawings of the Moon for his book Sidereus Nuncius, and deduced that it was not smooth but had mountains and craters. Thomas Harriot had made, but not published such drawings a few months earlier.
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 1834–1836 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, 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.[208] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions.[84] This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s,[209] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology.
Upcoming lunar missions include Russia’s Luna-Glob: an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission.[241] 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.[242]
NASA began to plan to resume human missions following the call by U.S. President George W. Bush on 14 January 2004 for a human mission to the Moon by 2019 and the construction of a lunar base by 2024.[243] The Constellation program was funded and construction and testing begun on a crewed spacecraft and launch vehicle,[244] and design studies for a lunar base.[245] That program was cancelled in 2010, however, and was eventually replaced with the Donald Trump supported Artemis program, which plans to return humans to the Moon by 2025.[246] India had also expressed its hope to send people to the Moon by 2020.[247]
On 28 February 2018, SpaceX, Vodafone, Nokia and Audi announced a collaboration to install a 4G wireless communication network on the Moon, with the aim of streaming live footage on the surface to Earth.[248]
Recent reports also indicate NASA’s intent to send a woman astronaut to the Moon in their planned mid-2020s mission.
In 2007, the X Prize Foundation together with Google launched the Google Lunar X Prize to encourage commercial endeavors to the Moon. A prize of $20 million was to be awarded to the first private venture to get to the Moon with a robotic lander by the end of March 2018, with additional prizes worth $10 million for further milestones.[250][251] As of August 2016, 16 teams were reportedly participating in the competition.[252] In January 2018 the foundation announced that the prize would go unclaimed as none of the finalist teams would be able to make a launch attempt by the deadline.[253]
In August 2016, the US government granted permission to US-based start-up Moon Express to land on the Moon.[254] This marked the first time that a private enterprise was given the right to do so. The decision is regarded as a precedent helping to define regulatory standards for deep-space commercial activity in the future. Previously, private companies were restricted to operating on or around Earth.[254]
On 29 November 2018 NASA announced that nine commercial companies would compete to win a contract to send small payloads to the Moon in what is known as Commercial Lunar Payload Services. According to NASA administrator Jim Bridenstine, “We are building a domestic American capability to get back and forth to the surface of the moon.”.
Beside the remains of human activity on the Moon, there have been some intended permanent installations like the Moon Museum art piece, Apollo 11 goodwill messages, six Lunar plaques, the Fallen Astronaut memorial, and other artifacts.
While the Moon has the lowest planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed[256] and particularly understood regarding keeping the Shielded Zone of the Moon (SZM), of value for astronomy from the Moon, free from any radio spectrum pollution, as well as conserving the special and scientifically interesting nature of the Moon, in face of prospecting commercial and national projects to claim and exploit the Moon.[257]
The so-called “Tardigrade affair” of the 2019 crashed Beresheet lander and its carrying of tardigrades has been discussed as an example for lacking measures and lacking international regulation for planetary protection.
Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang’e 3 with its Lunar Ultraviolet Telescope still operational.[258]
There are several missions by different agencies and companies planned to establish a longterm human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.
For many years, the Moon has been recognized as an excellent site for telescopes.[259] 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.[260] The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies and employed in the construction of mirrors up to 50 meters in diameter.[261] A lunar zenith telescope can be made cheaply with an ionic liquid.[262]
In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph.[
Humans have stayed for days on the Moon, such as during Apollo 17[264] in an Apollo Lunar Module, which have been sofar the only extraterrestrial surface habitats. One particular challenge for astronauts’ daily life during their stay on the surface is the lunar dust sticking to their suits and being carried into their quarters. Subsequently, the dust was tasted and smelled by the astronauts, calling it the “Apollo aroma”.[265] This contamination poses a danger since the fine lunar dust can cause health issues.[265]
In 2019 at least one plant seed sprouted in an experiment, carried along with other small life from Earth on the Chang’e 4 lander in its Lunar Micro Ecosystem.
A photo of the reflector of the Lunar Laser Ranging Experiment of Apollo 11, still in use.
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 claims ownership of any part of the Moon’s surface.[267] Russia, China, India, and the U.S. are party to the 1967 Outer Space Treaty,[268] which defines the Moon and all outer space as the “province of all mankind“.[267] This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction.[269] The 1979 Moon Agreement was created to restrict the exploitation of the Moon’s resources by any single nation, but as of January 2020, it has been signed and ratified by only 18 nations,[270] none of which engages in self-launched human space exploration. Although several individuals have made claims to the Moon in whole or in part, none of these are considered credible.[271][272][273]
In 2020, U.S. President Donald Trump signed an executive order called “Encouraging International Support for the Recovery and Use of Space Resources”. The order emphasizes that “the United States does not view outer space as a ‘global commons'” and calls the Moon Agreement “a failed attempt at constraining free enterprise.”[274][275]
In the face of such increasing commercial and national interest, particularly prospecting territories, US lawmakers have introduced regulation for the conservation of historic landing sites[276] and interest groups have argued for making such sites World Heritage Sites[277] and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon.[257]
The Declaration of the Rights of the Moon[278] was created by a group of “lawyers, space archaeologists and concerned citizens” in 2021, drawing on precedents in the Rights of Nature movement and the concept of legal personality for non-human entities in space.[279]
In light of future development on the Moon some international and multi-space agency organizations have been created:
The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans.
The Moon has long been particularly associated with insanity and irrationality; the words lunacy and lunatic (popular shortening loony) are derived from the Latin name for the Moon, Luna. Philosophers Aristotle and Pliny the Elder argued 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.[306] Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims.
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