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Human presence

Human impact Beside the traces of human activity on the Moon there have been some intended permanent installations like the Moon Museum art piece Apollo 11 goodwill messages Lunar plaque the Fallen Astronaut memorial and other artifacts. 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 Humans have stayed for days on the Moon such as during Apollo 17. 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". This contamination poses a danger since the fine lunar dust can cause health issues. 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.

Name and etymology

The usual English proper name for Earth's natural satellite is simply the 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 /ˈluːnə/ 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 Earth's moon. Cynthia /ˈsɪnθiə/ is another poetic name though rare for the Moon personified as a goddess 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/ 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 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.

In culture

Mythology The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon the rabbit and the buffalo among others. In many prehistoric and ancient cultures the Moon was personified as a deity or other supernatural phenomenon and astrological views of the Moon continue to be propagated. In Proto-Indo-European religion the Moon was personified as the male god *Meh1not. The ancient Sumerians believed that the Moon was the god Nanna who was the father of Inanna the goddess of the planet Venus and Utu the god of the Sun. Nanna was later known as Sîn and was particularly associated with magic and sorcery. In Greco-Roman mythology the Sun and the Moon are represented as male and female respectively (Helios/Sol and Selene/Luna); this is a development unique to the eastern Mediterranean and traces of an earlier male moon god in the Greek tradition are preserved in the figure of Menelaus. In Mesopotamian iconography the crescent was the primary symbol of Nanna-Sîn. In ancient Greek art the Moon goddess Selene was represented wearing a crescent on her headgear in an arrangement reminiscent of horns. The star and crescent arrangement also 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. An iconographic tradition of representing Sun and Moon with faces developed in the late medieval period. The splitting of the Moon (Arabic: انشقاق القمر‎) is a miracle attributed to Muhammad. A song titled 'Moon Anthem' was released on the occasion of landing of India's Chandrayan-II on the Moon. Calendar The Moon's regular phases make it a convenient timepiece and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks notched bones dating as far back as 20–30 000 years ago are believed by some to mark the phases of the Moon. The ~30-day month is an approximation of the lunar cycle. The English noun month and its cognates in other Germanic languages stem from Proto-Germanic *mǣnṓth- which is connected to the above-mentioned Proto-Germanic *mǣnōn indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar. The PIE root of moon *méh1nōt derives from the PIE verbal root *meh1- "to measure" "indicat[ing] a functional conception of the Moon i.e. marker of the month" (cf. the English words measure and menstrual) and echoing the Moon's importance to many ancient cultures in measuring time (see Latin mensis and Ancient Greek μείς (meis) or μήν (mēn) meaning "month"). Most historical calendars are lunisolar. 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. 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 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. 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.

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 also restricts the use of the Moon to peaceful purposes explicitly banning military installations and weapons of mass destruction. The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation but 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." 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.


The Moon formed 4.51 billion years ago [g] or even 100 million years earlier some 50 million years after the origin of the Solar System as research published in 2019 suggests. Several forming mechanisms have been proposed including the fission of the Moon from Earth's crust through centrifugal force (which would require too great an initial rotation rate of Earth) the gravitational capture of a pre-formed Moon (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon) and the co-formation of Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metals in the Moon). These hypotheses also cannot 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 Earth's orbit and then the material accreted and formed the Moon just beyond the Earth's Roche limit of ~2.56 R⊕. This theory best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins Bill Hartmann Roger Phillips and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data go back to your computer do whatever you have to but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona Hawaii the giant-impact hypothesis emerged as the most consensual. Before the conference there were partisans of the three "traditional" theories plus a few people who were starting to take the giant impact seriously and there was a huge apathetic middle who didn't think the debate would ever be resolved. Afterward there were essentially only two groups: the giant impact camp and the agnostics. 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. 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 released a lot of 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. 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. 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 announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures. Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth 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.

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. Internal structure
Lunar surface chemical composition
Compound Formula Composition
Maria Highlands
silica SiO2 45.4% 45.5%
alumina Al2O3 14.9% 24.0%
lime CaO 11.8% 15.9%
iron(II) oxide FeO 14.1% 5.9%
magnesia MgO 9.2% 7.5%
titanium dioxide TiO2 3.9% 0.6%
sodium oxide Na2O 0.6% 0.6%
  99.9% 100.0%
The Moon is a differentiated body. It has a geochemically distinct crust mantle and core. The Moon has a solid iron-rich inner core with a radius 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). 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 the 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 50 kilometres (31 mi) 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 350 kilometres (220 mi) 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. Surface geology The topography of the Moon has been measured with laser altimetry and stereo image analysis. Its most visible 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. At 13 km (8.1 mi) deep its floor is the lowest point on the surface of the Moon. The highest elevations of the surface are located directly to the northeast and it has been suggested 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 also possess regionally low elevations and elevated rims. The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side. The discovery of fault scarp cliffs by the Lunar Reconnaissance Orbiter suggest that the Moon has shrunk within the past billion years by about 90 metres (300 ft). Similar shrinkage features exist on Mercury. A recent study of over 12000 images from the orbiter has observed that Mare Frigoris near the north pole a vast basin assumed to be geologically dead has been cracking and shifting. Since the Moon doesn't have tectonic plates its tectonic activity is slow and cracks develop as it loses heat over the years. 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 lavas 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 thought to be due to a concentration of heat-producing elements under the crust on the near side seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer which would have caused the underlying mantle to heat up partially melt rise to the surface and erupt. 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. Until recently the youngest eruptions dated by crater counting appeared to have been only 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 also indicate some continued lunar activity. In 2014 NASA announced "widespread evidence of young lunar volcanism" at 70 irregular mare patches identified by the Lunar Reconnaissance Orbiter 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. Just prior to this evidence has been presented for 2–10 million years younger basaltic volcanism inside the crater Lowell Orientale basin located in the transition zone between the near and far sides of the Moon. An initially hotter mantle and/or local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities also 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 their formation. Impact craters The other major geologic process that has affected the Moon's surface is impact cratering with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300 000 craters wider than 1 km (0.6 mi) on the Moon's near side alone. 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 of impacts. Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith formed by impact processes. 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 10–20 km (6.2–12.4 mi) in the highlands and 3–5 km (1.9–3.1 mi) in the maria. Beneath the finely comminuted regolith layer is the megaregolith a layer of highly fractured bedrock many kilometers thick. Comparison of high-resolution images obtained by the Lunar Reconnaissance Orbiter has shown a contemporary crater-production rate significantly higher than previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith a hundred times more quickly than previous models suggested – on a timescale of 81 000 years. 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 have often a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. 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 14 000 km2 (5 400 sq mi) of the surface may be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive. 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 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. Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb). 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 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. 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). 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. 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. Atmosphere 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). 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. 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. Dust A permanent asymmetric Moon dust cloud exists around the Moon created by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours. The particles striking the Moon's surface eject Moon dust above the Moon. 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 to 100 kilometers above the surface. The dust measurements were made by LADEE's Lunar Dust EXperiment (LDEX) between 20 and 100 kilometers above the surface during a six-month period. LDEX detected an average of one 0.3 micrometer Moon dust particle each minute. Dust particle counts peaked during the Geminid Quadrantid Northern Taurid and Omicron Centaurid meteor showers when the Earth and Moon pass through comet debris. The cloud is asymmetric more dense near the boundary between the Moon's dayside and nightside. Past thicker atmosphere In October 2017 NASA scientists at the Marshall Space Flight Center and the Lunar and Planetary Institute in Houston announced their finding based on studies of Moon magma samples retrieved by the Apollo missions 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. Seasons The Moon's axial tilt with respect to the ecliptic is only 1.5424° 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 35 K (−238 °C; −397 °F) 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. 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. Rotation The Moon is rotating around its own axis. This rotation is due to tidal locking synchronous to its orbital period around Earth. The rotation period depends on the frame of reference. There are sidereal rotation periods (or sidereal day in relation to the stars) and synodic rotation periods (or synodic day in relation to the Sun). A lunar day is a synodic day. Because of the tidal locked rotation the sidereal and synodic rotation periods correspond to the sidereal (27.3 Earth days) and synodic (29.5 Earth days) orbital periods.

Earth–Moon system

Lunar distance Scale model of the Earth–Moon system: Sizes and distances are to scale. Orbit The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days[h] (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[i] (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 18.61 years which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws. Relative size 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. It is the largest moon in the Solar System relative to the size of its planet [j] though Charon is larger relative to the dwarf planet Pluto at 1/9 Pluto's mass.[k] 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. Appearance from Earth The Moon is in synchronous rotation as it orbits Earth; it rotates about its axis in about the same time it takes to orbit Earth. This results in it always keeping nearly the same face turned towards Earth. 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 had once 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 much earlier 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. [l] 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 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 § 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. 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. 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 1.14 2 ≈ 1.30 {\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: perceived reduction % = 100 × actual reduction % 100 {\displaystyle {\text{perceived reduction))\%=100\times {\sqrt ((\text{actual reduction))\% \over 100))} 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 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 ( 1 − cos ⁡ e ) / 2 = sin 2 ⁡ ( e / 2 ) {\displaystyle (1-\cos e)/2=\sin ^{2}(e/2)} where e {\displaystyle e} is the elongation (i.e. the angle between Moon the observer on Earth and the Sun). 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 that acts as a 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 38 mm (1.5 in) per year (roughly the rate at which human fingernails grow). Atomic clocks also show that Earth's day lengthens by about 15 microseconds every year slowly increasing the rate at which UTC is adjusted by leap seconds. This tidal drag 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. If this tidal locking did happen the rotation of the Earth would continue to slow down because of the tides caused by the Sun. With the day longer than the month the Moon would then move slowly from west to east in the sky. The tides caused by the Moon would then cause the opposite effect from before and the Moon would get closer to the Earth. It would eventually come within the Roche limit and be broken up into a ring. In a like manner the lunar surface experiences tides of around 10 cm (4 in) amplitude over 27 days with two components: a fixed one due to Earth because they are in synchronous rotation and a varying component from the Sun. The Earth-induced component arises from libration a result of the Moon's orbital eccentricity (if the Moon's orbit were perfectly circular there would only be solar tides). Libration also changes the angle from which the Moon is seen allowing 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 the absence of water to damp out the seismic vibrations. 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. 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. Also as it evolves toward becoming a red giant the size of the Sun and its apparent diameter in the sky are slowly increasing.[m] 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 [n] 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 One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 at Knowth Ireland. Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses and Indian astronomers had described the Moon's monthly elongation. The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses. (p411) Later the physical form of the Moon and the cause of moonlight became understood. The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks and that the latter reflected the light of the former. (p227) 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. (pp413–414) 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. (pp415–416) 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. However in the 2nd century BC Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon and that their height depends on the Moon's position relative to the Sun. In the same century Aristarchus computed the size and distance of the Moon from Earth obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 AD): his 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. Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. During the Middle Ages before the invention of the telescope the Moon was increasingly recognised as a sphere though many believed that it was "perfectly smooth". In 1609 Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters. 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 1834–36 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. 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 luna


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