Moon
The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia), it is the fifth largest satellite in the Solar System, the largest satellite in the Solar System relative to its major planet, and 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 the geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (). 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 , or about 30 times Earth's diameter, its gravitational influence very slowly 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 most widely accepted origin explanation posits that the Moon formed 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. Name and etymology. The usual English proper name for Earth's natural satellite is simply "Moon", with a capital M. The noun "moon" is derived from Old English "mōna", which (like all its Germanic cognates) stems from Proto-Germanic "*mēnōn", which  in turn comes from Proto-Indo-European "*mēnsis" "month" (from earlier "*mēnōt", genitive "*mēneses") which may be related to the verb "measure" (of time). Occasionally, the name Luna is used in scientific writing 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. Cynthia is another poetic name, though rare, for the Moon personified as a goddess, while Selene (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" , derived from the Greek word for the Moon, "selēnē", and used to describe the Moon as a world rather than as an object in the sky, is rare, while its cognate "selenic" was originally a rare synonym but now nearly always refers to the chemical element selenium. 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". 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. These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits such as "apolune", "pericynthion" and "selenocentric". The astronomical symbol for the Moon is a crescent, , for example in "M" 'lunar mass' (also "M"). Formation. Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. Historically, several formation mechanisms have been proposed, but none satisfactorily explains the  features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force would require too great an initial rotation rate of Earth. Gravitational capture of a pre-formed Moon depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon. A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon. 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 the material accreted and formed the Moon just beyond the Earth's Roche limit of ~. 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 show that most of the Moon derived from the impactor, rather than the proto-Earth. However , more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. 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, although this is debated. The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a  magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system. Similarly, the newly formed Moon would have had its own lunar magma ocean; its depth is estimated from about to . While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks. 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 asserted in 2007 that there was less than a 1% chance that Theia and Earth would have had identical isotopic signatures. Other Apollo lunar samples analyzed in 2012 showed the same titanium isotope composition as Earth, which conflicts with what would be expected if the Moon formed far from Earth or was principally 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 could have allowed it to return to Earth a second time but more slowly, and mix more thoroughly. This hit-and-run-and-return scenario could explain some of the apparent contradictions between material evidence and other theories. Physical characteristics. 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. Size and mass. The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizeable as one of its planetary-mass moons, making it a satellite planet under the geophysical definitions of the term. It is smaller than Mercury and considerably larger than the largest dwarf planet of the Solar System, Pluto. While the minor-planet moon Charon of the Pluto-Charon system is larger relative to Pluto, the Moon is the largest  natural satellite of the Solar System relative to their primary planets. The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of Australia. The whole surface area of the Moon is about 38 million square kilometers, slightly less than the area of the Americas (North and South America). The Moon's mass is 1/81 of Earth's, being the second densest among the planetary moons, having the second highest surface gravity, after Io, at and an escape velocity of . Internal structure. The Moon is a  differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition. 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 and a fluid outer core primarily made of liquid iron with a radius of roughly . Around the core is a partially molten boundary layer with a radius of about . 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, 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. 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. Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite. 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. The crust is on average about thick. 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 or less, 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. The pressure at the lunar core is  estimated to be . Magnetic field. The Moon has an external magnetic field of generally less than 0.2 nanoteslas, 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. However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized. 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. Gravitational field. 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. 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. Surface conditions. The surface of the Moon is an extreme environment with temperatures that range from down to , an atmospheric pressure of 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. The surface gravity of the  Moon is approximately 1.625 m/s, about 16.6% that on Earth's surface or 0.166 . Atmosphere. The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than . The surface pressure of this small mass is around 3 × 10 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. 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 from the  solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. 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 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. 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. 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. Dust. A permanent Moon dust cloud exists around the Moon, generated by small particles from  comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is  asymmetric, being more dense near the boundary between the Moon's dayside and nightside. Geology. Regolith. Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened mostly gray 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. The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from in the highlands and in the maria. Beneath the finely comminuted regolith layer is the "megaregolith , a layer of highly fractured bedrock many kilometers 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 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. Computer simulations suggest that up to of the surface 	Moon
17845320	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. In years since, signatures of water have been found to exist on the lunar surface. 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. 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. Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. 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. Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. In 2009, "LCROSS" sent a impactor into a permanently shadowed polar crater, and detected at least of water in a plume of ejected material. Another examination of the LCROSS data showed the amount of detected water to be closer to . In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, 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 insight does not mean that water is easily available since 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. The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. 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. 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). Surface features. The topography of the Moon has  been measured with laser altimetry and stereo image analysis. Its most extensive topographic feature is the giant far-side South Pole–Aitken basin, some in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. At deep, its floor is the lowest point on the surface of the Moon. 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. Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess  regionally low elevations and elevated rims. The far side of the lunar surface is on average about higher than that of the near side. The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. 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. Volcanic features. 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; 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. 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". Almost all maria are on the near side of the Moon, and cover 31% of  the surface of the near side compared with 2% of the far side. 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. 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. As of 2003, crater counting studies of the youngest eruptions appeared to suggest they formed no earlier than 1.2 billion years ago. 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. Moonquakes and releases of gas indicate continued lunar activity. 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. Evidence has  been found for 2–10 million years old basaltic volcanism within the crater Lowell, 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. 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. In contrast to Earth, no major lunar mountains are  believed to have formed as a result of tectonic events. 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. Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth. Impact craters. A 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. There are estimated to be roughly 300,000 craters wider than on the Moon's near side. 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. 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. 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. 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. This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts. Lunar swirls. 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 often have 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. Earth–Moon system. Orbit. While orbiting around the Sun, the Earth and Moon orbit around their common center of mass, or barycentre, with a sidereal period of 27.32 days. This barycentre is located (about a quarter of Earth's radius) beneath the Earth's surface. The instantaneous Earth–Moon distance, or distance to the Moon, is the current distance from the center of  Earth to the center of the Moon. Lunar distance, the semi-major axis of the geocentric lunar orbit, is a unit of measure in astronomy. This distance is approximately 400,000 km, which is a quarter of a million miles or 1.28 light-seconds. The orbital eccentricity, giving ovalness of the orbit, is 0.055. 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, 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; its synodic period. 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 18.61years, which affects other aspects of lunar motion. These follow-on effects are  mathematically described by Cassini's laws. The Moon's axial tilt with respect to the ecliptic is only 1.5427°, 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. 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, and these "craters of eternal darkness" are extremely cold: "Lunar Reconnaissance Orbiter" measured the lowest summer temperatures in craters at the southern pole at and just 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. 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 Earth  revolves around the Earth-Moon barycentre once a sidereal month, with 1/81 the speed of the Moon, or about per second. This motion is superimposed on the much larger revolution of the Earth around the Sun at a speed of about per second. Tidal effects. 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. 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. 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. 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. 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. 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. 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. Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments) have found that the Moon's distance increases by per year (roughly the rate at which human fingernails grow). Atomic clocks show that Earth's day lengthens by about 17 microseconds every year, 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. In a like manner, the lunar surface experiences tides of around 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. 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). Libration 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. 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. Appearance from Earth. 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. 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. 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. 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. 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's color depends on the light  the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general the lunar surface reflects a brown-tinged gray light. Viewed from Earth the air filters the reflected light, at times giving it a red colour depending on the angle of the Moon in the sky and thickness of the atmosphere, or a blue tinge depending on the particles in the air, as in cases of volcanic particles. The terms blood moon and blue moon do not necessarily refer to circumstances of red or blue moonlight, but are  rather particular cultural references such as particular full moons of a year. 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's angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see ). 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  depends on the latitude of the viewing location; an observer in the tropics can see a smile-shaped crescent Moon. The Moon is visible for two weeks every draconic month (27.2 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 orientation of the Moon depends on the hemisphere of the Earth from which it is being viewed. In the northern hemisphere it is seen upside down compared to the view in the southern hemisphere. The distance between the Moon and Earth varies from around  to 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. 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 formula_1. 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. 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 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 formula_3, where formula_4 is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun). Eclipses. Eclipses only occur when the Sun, Earth , and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. 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 vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the  perspective of Earth. 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. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time, the angular diameter of the Moon is decreasing. As it evolves toward becoming  a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. The combination of these two changes 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, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur. Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of 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, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years. Because the Moon continuously blocks the 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. In this way, a solar  eclipse is an occultation of the Sun. Because the Moon is comparatively close to 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. Observation and exploration. Before spaceflight. Since pre-historic times people have taken note of the Moon's phases, its waxing and waning, and used it to keep record of time. 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. One of  the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving "Orthostat 47" at Knowth, Ireland. The ancient Greek philosopher Anaxagoras () reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. Elsewhere in the to , Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, and Indian astronomers had described the Moon's monthly elongation. The Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses. 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. Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. In the , 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  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 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. 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. 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. 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. 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. 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 , 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 of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book , 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. 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. 1959–1970s. Between the first human arrival with the robotic Soviet "Luna" program in 1958, to the 1970s with the last Missions of the crewed U.S. Apollo landings and last Luna mission in 1976, 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 uncrewed probes on  both flyby and impact/lander missions. Soviet missions. Spacecraft from the Soviet Union's "Luna" program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958, the first human-made object to escape Earth's gravity and pass near the Moon was "Luna 1"; the first human-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  vehicle to orbit the Moon was "Luna 10", both in 1966. 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. Luna 17 deployed the first lunar rover, Lunokhod 1, in 1970. United States missions. During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study that proposed the construction of a staffed military outpost on the Moon called Project Horizon with the potential to conduct a wide  range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test. The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex. However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA. Following President John F. Kennedy's 1961 commitment to a manned Moon landing before the end of the decade, the United States, under NASA leadership, launched a  series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures; the Lunar Orbiter program produced maps of the entire Moon; the Surveyor program landed its first spacecraft four months after "Luna 9". The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit. 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. An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) removed 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, 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 landings. 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 because of budgetary considerations, 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 centimeters, and data from this experiment are being used to place constraints on the size of the lunar core. 1970s–present. In the 1970s, after the Moon race, the focus of astronautic exploration shifted, as probes like "Pioneer 10" and the Voyager program were sent towards the outer Solar System. Years of near lunar quietude followed, only broken by a beginning internationalization of space and the Moon through, for  example, the negotiation of the Moon treaty. Since the 1990s, 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. 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 the "Lunar Prospector" mission, 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. The Chinese Lunar Exploration Program began with "Chang'e 1", which successfully orbited the Moon from 5 November  2007 until its controlled lunar impact on 1 March 2009, obtaining a full image map of the Moon. The "Chang'e 2" mission began October 2010, mapping the surface at a higher resolution over an eight-month period. On 14 December 2013, "Chang'e 3" landed a lunar lander onto the Moon's surface, which deployed a lunar rover, named "Yutu" (Chinese: 玉兔; literally "Jade Rabbit"). This was the first lunar rover mission since "Lunokhod 2" in 1973 and the first lunar soft landing since "Luna 24" in 1976. Another rover mission, "Chang'e 4", was launched in 2019 and was the first  spacecraft to land on the Moon's far side. "Chang'e 5" landed on the Moon in December 2020 and carried out China's first robotic sample return mission, bringing back 1,731 grams of lunar material to Earth. "Chang'e 6", another sample return mission, is planned for 2024. 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. India's first lunar mission , "Chandrayaan-1", 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. The Indian Space Research Organisation planned to launch "Chandrayaan-2" in 2013, which would have included a Russian robotic lunar rover. However, the failure of Russia's "Fobos-Grunt" mission has delayed this project, and was launched on 22 July 2019. The lander "Vikram" attempted to land on the lunar south pole region on 6 September, but lost the signal in . What happened  after that is unknown. The U.S. co-launched the "Lunar Reconnaissance Orbiter" (LRO) and the "LCROSS" impactor and follow-up observation orbiter on 18 June 2009; "LCROSS" completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, whereas "LRO" is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the large and bright crater Aristarchus. NASA released photos of the crater on 25 December 2011. Two NASA GRAIL spacecraft began orbiting the Moon around 1 January 2012, on a mission to learn  more about the Moon's internal structure. NASA's "LADEE" probe, designed to study the lunar exosphere, achieved orbit on 6 October 2013. Future. 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. 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 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. The Constellation program was funded and construction and testing begun on a crewed spacecraft and launch vehicle, and design studies for a lunar base. 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. India had also expressed its hope to send people to the Moon by 2020. 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. Recent reports indicate NASA's planned mid-2020s mission to the moon will include a female astronaut. Planned commercial missions. 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.<ref name="https://www.space.com_2017-09-25"></ref> As of August 2016, 16 teams were reportedly participating in the competition. 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. In August 2016, the US government granted permission to US-based start-up Moon Express to land on the Moon. 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. 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.". Human presence. Human impact. 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. Pollution and contamination. While the Moon has the lowest planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed 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. While the Moon has no  significant atmosphere, traffic and impacts on the Moon causes clouds of dust that can spread far and possibly contaminate the original state of the Moon and its special scientific content. 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. Space debris beyond Earth around the Moon has been considered as a future challenge with increasing numbers of missions to the Moon, particularly as a danger for such missions. Infrastructure. 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. 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. Astronomy from the Moon. For many years, the Moon has been recognized as an excellent site for telescopes. 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. 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. A lunar zenith telescope can be made cheaply with an ionic liquid. In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph. Living on  the Moon. The only instances of humans living on the Moon have taken place in an Apollo Lunar Module (for example, during the Apollo 17 mission) for several days at a time. One challenge to astronauts during their stay on the surface is that lunar dust sticks to their suits and is carried into their quarters. Astronauts could taste and smell the dust, calling it the "Apollo aroma". This fine lunar dust can cause health issues. In 2019 at least one plant seed sprouted in an experiment on the Chang'e 4 lander. It was carried from Earth along with other  small life in its "Lunar Micro Ecosystem". 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 claims ownership of any part of the Moon's surface. Russia, China, India, 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". This treaty 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, leaving it to a yet unspecified international regulatory regime, but as of January 2020, it has been signed and ratified by only 18 nations, 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. 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." 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 and interest groups have argued for making such sites World Heritage Sites and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon. The "Declaration of the Rights of the Moon" 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. Coordination. In light of future development on the Moon some international and multi-space agency organizations have been created: In culture and life. Calendar. Since pre-historic times people have taken note of the Moon's phases, its waxing and waning, and used it to keep record of time. 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 counting of the days between the Moon's phases  gave eventually rise to generalized time periods of the full lunar cycle as months, and possibly of its phases as weeks. The words for the month in a range of different languages carry this relation between the period of the month and the Moon etymologically. The English "month" as well as "moon", and its cognates in other Indo-European languages (e.g. the Latin and Ancient Greek ("meis") or (mēn), meaning "month") stem from the Proto-Indo-European (PIE) root of "moon", *"méhnōt", derived from the PIE verbal root *"meh"-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker  of the month" (cf. the English words "measure" and "menstrual"). To give another example from a different language family, the Chinese language uses the same word () for "moon" as well as for "month", which furthermore can be found in the symbols for the word "week" (). This lunar timekeeping gave rise to the historically dominant, but varied, lunisolar calendars. The 7th-century Islamic calendar is an example of a purely lunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon. Of particular significance has been for a range of cultures  and calendars the occasion of the Full Moon, to use or celebrate, particularly around the autumnal equinox, the so-called Harvest Moon. Furthermore association of time with the Moon can also be found in religion, such as the ancient Egyptian temporal and lunar deity Khonsu. Cultural representation. Since prehistoric and ancient times humans have depicted and interpreted the Moon, particularly for astrology and religion, as lunar deity. For the representation of the Moon, especially its lunar phases, the crescent symbol has been particularly used by many cultures. In writing systems such as Chinese the crescent has developed into the symbol  , the word for Moon, and in ancient Egyptian it was the symbol , which is spelled like the ancient Egyptian lunar deity "Iah", meaning Moon. Iconographically the crescent was used in Mesopotamia as the primary symbol of Nanna/Sîn, the ancient Sumerian lunar deity, who was the father of Innana/Ishtar, the goddess of the planet Venus (symbolized as the eight pointed Star of Ishtar), and Utu/Shamash, the god of the Sun (symbolized as a disc, optionally with eight rays), all three often depicted next to each other. Nanna was later known as Sîn, and was particularly associated with magic  and sorcery. The crescent was further used as an element of lunar deities wearing headgears or crowns in an arrangement reminiscent of horns, as in the case of the ancient Greek Selene or the ancient Egyptian Khonsu. Selene is associated with Artemis and paralleled by the Roman Luna, which both are occasionally depicted driving a chariot, like the Hindu lunar deity Chandra. The different or sharing aspects of deities within pantheons has been observed in many cultures, especially by later or contemporary culture, particularly forming triple deities. The Moon in Roman mythology for example has been associated with Juno and  Diana, while Luna being identified as their byname and as part of a triplet ("diva triformis") with Diana and Proserpina, Hecate being identified as their binding manifestation as "trimorphos". The star and crescent (☪️) arrangement goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a symbol of Byzantium, possibly influencing the development of the Ottoman flag, specifically the combination of the Turkish crescent with a star. Since then  the heraldric use of the star and crescent proliferated becoming a popular symbol for Islam (as the "hilal" of the Islamic calendar) and for a range of nations. In Roman Catholic Marian veneration, the Virgin Mary (Queen of Heaven) has been depicted since the late middle ages on a crescent and adorned with stars. In Islam Muhammad is particularly attributed with the Moon through the so-called splitting of the Moon () miracle. The contrast between the brighter highlands and the darker maria have been seen by different cultures forming abstract shapes, which are among others the Man in the Moon  or the Moon Rabbit (e.g. the Chinese Tu'er Ye or in Indigenous American mythologies, as with the aspect of the Mayan Moon goddess). In Western alchemy silver is associated with the Moon, and gold with the Sun. Modern representation. The perception of the Moon in modern times has been informed by telescope enabled modern astronomy and later by spaceflight enabled actual human activity at the Moon, particularly the culturally impactful lunar landings. These new insights inspired cultural references, connecting romantic reflections about the Moon and speculative fiction such as science-fiction dealing with the Moon. Contemporarily the Moon has been  seen as a place for economic expansion into space, with missions prospecting for lunar resources. This has been accompanied with renewed public and critical reflection on humanity's cultural and legal relation to the celestial body, aspecially regarding colonialism, as in the 1970 poem "Whitey on the Moon". In this light the Moon's nature has been invoked, particularly for lunar conservation and as a common. A song titled "Moon Anthem" by Abhay Kumar, paralleling the proposals for an Earth Anthem, was released 2019 on the occasion of India's lunar probe Chandrayaan-2. Lunar effect. 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 associated with insanity and irrationality; the words "lunacy" and "lunatic" 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. 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.