From a distance, the day and night phases of the lunar day are visible as thelunar phases, and when the Moon passes throughEarth's shadow alunar eclipse is observable. The Moon'sapparent size in Earth's sky is about the same as that of the Sun, which causes it to cover the Sun completely during a totalsolar eclipse. The Moon is thebrightest celestial object in Earth'snight sky because of its large apparent size, while thereflectance (albedo) of its surface is comparable to that ofasphalt. About 59% of the surface of the Moon is visible from Earth owing to the different angles at which the Moon can appear in Earth's sky (libration), making parts of thefar side of the Moon visible.
The English proper name for Earth's natural satellite is typically written asMoon, with a capitalM.[19][20] The nounmoon is derived fromOld Englishmōna, which stems fromProto-Germanic *mēnōn,[21] which in turn comes fromProto-Indo-European*mēnsis ('month')[22] – from earlier*mēnōt (genitive*mēneses), which may be related to a verb meaning 'to measure [time]'.[23]
The Latin name for the Moon islūna. The English adjectivelunar was ultimately borrowed from Latin, likely through French. In scientific writing and science fiction, the Moon is sometimes referred to asLuna/ˈluːnə/[24] to distinguish it from other moons. In poetry,Luna may also refer to thepersonification of the Moon as a woman.[25]
The Ancient Greek wordselḗnē referred to the Moon as a celestial body, and also to the moon goddessSelene/səˈliːniː/. The rare English adjectiveselenian/səliːniən/[26] is used to describe the Moon as a world, as opposed to a celestial object.[27] Its cognateselenic, originally a rare synonym,[28] now almost always refers to the chemical elementselenium.[29] The corresponding prefixseleno- appears in terms includingselenography (the study of the lunar surface).[30][31]
Artemis, the Greek goddess of the wilderness and the hunt, also came to be identified with Selene, and was sometimes calledCynthia after her birthplace onMount Cynthus.[32] Her Roman equivalent isDiana.
Theastronomical symbols for the Moon are the crescent and decrescent, for example inM☾ 'lunar mass'.
This not-to-scale animation shows (in an Earth and Sun stationary rotating frame of reference) the formation of the Moon through the collision of the hypothetical Mars-sized planet namedTheia with Earth. After orbiting for tens of millions of years at the L4Lagrange point as an Earth-trojan, Theia's orbit comes in "loops" finally too close to Earth.
Isotope dating of lunar samples suggests the Moon formed around 50 million years after theorigin of the Solar System.[33][34] Historically, several formation mechanisms have been proposed,[35] but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust throughcentrifugal force[36] would require too great an initial rotation rate of Earth.[37] Gravitational capture of a pre-formed Moon[38] depends on an unfeasibly extendedatmosphere of Earth todissipate the energy of the passing Moon.[37] A co-formation of Earth and the Moon together in theprimordialaccretion disk does not explain the depletion of metals in the Moon.[37] None of these hypotheses can account for the highangular momentum of the Earth–Moon system.[39]
The prevailing theory is that the Earth–Moon system formed after agiant impact of aMars-sized body (namedTheia) with theproto-Earth. The oblique impact blasted material into orbit about the Earth and the material accreted and formed the Moon[40][41] just beyond the Earth'sRoche limit of ~2.56 R🜨.[42]
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.[43] However, models from 2007 and later suggest a larger fraction of the Moon derived from the proto-Earth.[44][45][46][47] Other bodies of the inner Solar System such as Mars andVesta have, according to meteorites from them, very different oxygen and tungstenisotopic 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,[48] although this is debated.[49]
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.[50][51] The newly formed Moon would have hadits own magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).[50]
While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.[52] Models that have the Moon acquiring a significant amount of the proto-Earth are more difficult to reconcile with geochemical data for the isotopes of zirconium, oxygen, silicon, and other elements.[53] A study published in 2022, using high-resolution simulations (up to108 particles), found that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth'sRoche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.[54]
On November 1, 2023, scientists reported that, according to computer simulations, remnants of Theia could still be present inside the Earth.[55][56]
Natural development
Artist's depiction of the Moon in Earth's sky as it might have appeared around 4 billion years ago, after theLate Heavy Bombardment. At that time the Moon orbited the Earth at a third of its current distance, making it appear 2.8 times larger inapparent size than it does today.[57]The Moon was after its formation for billions of years volcanically active, with lava flooding the lunar low lying terrane over time forming today's basalt mare, with their ages marked in this map (yellow to red: youngest; blue to purple: oldest).
The newly formed Moon settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other,eclipses were more frequent, andtidal effects were stronger.[57]Due totidal acceleration, the Moon's orbit around Earth has become significantly larger, with a longer period.[58]
Following formation, the Moon has cooled and most ofits atmosphere has been stripped.[59] Thelunar surface has since been shaped by largeimpact events and many small ones, forming a landscape featuringcraters of all ages.
The Moon wasvolcanically active until 1.2 billion years ago, which laid down the prominentlunar maria. Most of themare basalts erupted during theImbrian period, 3.3–3.7 billion years ago, though some are as young as 1.2 billion years[60] and some as old as 4.2 billion years.[61] The distribution of the mare basalts is uneven, with the basalts predominantly appearing on the Moon's near-side hemisphere. The reasons for this are not yet known, although the relative thinness of the crust on the near side of the Moon is hypothesized to be a factor.[62] Causes of the distribution of thelunar highlands on thefar side are also not well understood. Topological measurements show the near side crust is thinner than the far side. One possible scenario then is that large impacts on the near side may have made it easier for lava to flow onto the surface.[63]
The lunar geological periods are named after their characteristic features, from mostimpact craters outside the darkmare, to the mare and later craters, and finally the young, still bright and therefore readily visible craters withray systems likeCopernicus orTycho.
In 5 billion years the Moon will have wandered 40% further away from Earth than it is now. However at a similar time the Sun will have become ared giant. Assuming the Sun envelopes the Earth-Moon system, the consequent drag from the Sun's atmosphere may cause the orbital distance between the Earth and the Moon to decay to the point where the Moon comes within the Earth'sRoche limit, leading it to disintegrate.[64]
Physical characteristics
The Moon is a very slightly scaleneellipsoid 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 restorehydrostatic equilibrium at its current orbital distance.[65] Today tidal crust deformation is limited to lobate thrust fault scarps formation.[66]
Size comparison of the main moons of the Solar System with Earth to scale. Nineteen moons are large enoughto be round, several havingsubsurface oceans and one, Titan, having a considerable atmosphere.
The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizable as one of itsplanetary-mass moons, making it a satellite planet under thegeophysical definitions of the term.[18] It is smaller thanMercury but considerably larger than the largestdwarf planet of the Solar System,Pluto. The Moon is the largest natural satellite in the Solar System relative to itsprimary planet.[g][h][67]
The Moon's diameter is about 3,500 km, more than one-quarter of Earth's, with the face of the Moon comparable to the width of eithermainland Australia,[68] Europe or thecontiguous United States.[69] The whole surface area of the Moon is about 38 million square kilometers, comparable to that of theAmericas.[70]
The Moon's mass is1⁄81 of Earth's,[71] being the second densest among the planetary moons, and having the second highestsurface gravity, afterIo, at0.1654 g and an escape velocity of 2.38 km/s (8600 km/h;5300 mph).
Moon's internal structure: solid inner core (iron-metallic), molten outer core, hardened mantle and crust. The crust on the Moon's near side permanently facing Earth is thinner, featuring larger areas flooded by material of the once molten mantle forming today'slunar mare.
The Moon is adifferentiated body that was initially inhydrostatic equilibrium but has since departed from this condition.[72] It has ageochemically distinctcrust,mantle, andcore. 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).[73][74] This structure is thought to have developed through thefractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.[75]
Crystallization of this magma ocean would have created amafic mantle from theprecipitation and sinking of the mineralsolivine,clinopyroxene, andorthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-densityplagioclase minerals could form and float into a crust atop.[76] The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance ofincompatible and heat-producing elements.[1] Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostlyanorthosite.[17] TheMoon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.[1] The crust is on average about 50 kilometres (31 mi) thick.[1]
The Moon is the second-densest satellite in the Solar System, afterIo.[77] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less,[1] around 20% of the radius of the Moon. Its composition is not well understood but is probably metallic iron alloyed with a small amount of sulfur and nickel. Analyses of the Moon's time-variable rotation suggest that it is at least partly molten.[78] The pressure at the lunar core is estimated to be 5 GPa (49,000 atm).[79]
Gravitational field
AstronautJohn Young jumping on the Moon, illustrating that thegravitational pull of the Moon is approximately 1/6 of Earth's. The jumping height is limited by the EVA space suit's weight on the Moon of about 13.6 kg (30 lb) and by the suit's pressurization resisting the bending of the suit, as needed for jumping.[80][81]
On average the Moon'ssurface gravity is1.62 m/s2[5] (0.1654 g;5.318 ft/s2), about half of the surface gravity ofMars and about a sixth of Earth's.
TheMoon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking theDoppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features aremascons, 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.[82][83] 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.[84]
Thesphere of influence, of the Moon's gravity field, in which it dominates over Earth's has aHill radius of 60,000 km (i.e., extending less than one-sixth the distance of the 378,000 km between the Moon and the Earth),[85] extending to the Earth-Moonlagrange points. This space is calledcislunar space.[86]
Magnetic field
The Moon hasan external magnetic field of less than 0.2nanoteslas,[87] or less than one hundred thousandththat of Earth. The Moon does not have a globaldipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating.[88][89] Early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today.[90][87] This early dynamo field apparently expired by about one billion years ago, after the lunar core had crystallized.[87] 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 theantipodes of the giant impact basins.[91]
The Moon has anatmosphere consisting of only anexosphere,[94] which is so tenuous as to be nearlyvacuum, with a total mass of less than 10 tonnes (9.8 long tons; 11 short tons).[95] The surface pressure of this small mass is around 3 × 10−15atm (0.3 nPa); it varies with the lunar day. Its sources includeoutgassing andsputtering, a product of the bombardment of lunar soil by solar wind ions.[17][96] Elements that have been detected includesodium andpotassium, produced by sputtering (also found in the atmospheres of Mercury andIo);helium-4 andneon[97] from the solar wind; andargon-40,radon-222, andpolonium-210, outgassed after their creation byradioactive decay within the crust and mantle.[98][99] The absence of such neutral species (atoms or molecules) asoxygen,nitrogen,carbon,hydrogen andmagnesium, which are present in theregolith, is not understood.[98] Water vapor has been detected byChandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from thesublimation of water ice in the regolith.[100] 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.[98]
A permanentMoon dust cloud exists around the Moon, generated by small particles from comets. 5 tons of comet particles are estimated to strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon for 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 byLADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during theGeminid,Quadrantid,Northern Taurid, andOmicron Centauridmeteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being denser near the boundary between the Moon's dayside and nightside.[101][102]
Studies of Moon magma samples retrieved by theApollo 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-dayMars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.[59]
Surface conditions
AstronautCharles Duke working on the Moon with his suit covered inlunar dust. Lunar dust is highly abrasive and can cause damage to human lungs and nervous and cardiovascular systems.[103]
Ionizing radiation fromcosmic rays, their resultingneutron radiation,[104] and the Sun results in an average radiation level of 1.369millisieverts per day during lunardaytime,[15] which is about 2.6 times more than the level on theInternational Space Station, 5–10 times more than the level during a trans-Atlantic flight, and 200 times more than the level on Earth's surface.[105] For further comparison, radiation levels average about 1.84 millisieverts per day on aflight to Mars and about 0.64 millisieverts per day on Mars itself, with some locations on Mars possibly having levels as low as 0.342 millisieverts per day.[106][107]Solar radiation alsoelectrically charges the highly abrasivelunar dust and makes it levitate. This effect contributes to the easy spread of the sticky, lung- and gear-damaging lunar dust.[108]
The Moon'saxial tilt with respect to theecliptic is only 1.5427°,[9][109] much less than the 23.44° of Earth. This small axial tilt means that the Moon's solar illumination varies much less withseason than Earth's, and it also allows for the existence of somepeaks of eternal light at theMoon's north pole, at the rim of the craterPeary.
The lunar surface is exposed to drastic temperature differences ranging from120 °C to−171 °C depending on thesolar irradiance.Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow,[110] making topographical details play a decisive role on localsurface temperatures.[111]Parts of many craters, particularly the bottoms of many polar craters,[112] are permanently shadowed. Thesecraters of eternal darkness have extremely low temperatures. TheLunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F)[113] and just 26 K (−247 °C; −413 °F) close to the winter solstice in the north polar craterHermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface ofPluto.[111]
Blanketed on top of the Moon's crust is a highlycomminuted (broken into ever smaller particles) andimpact gardened mostly gray surface layer calledregolith, formed by impact processes. The finer regolith, thelunar soil ofsilicon dioxide glass, has a texture resembling snow and a scent resembling spentgunpowder.[114] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–15 m (33–49 ft) in the highlands and 4–5 m (13–16 ft) in the maria.[115] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick.[116]
These extreme conditions are considered to make it unlikely for spacecraft to harbor bacterial spores at the Moon for longer than just one lunar orbit.[117]
The near side (left) and far side (right) of the Moon in visible light.Map of the Moon coloured by the thickness of the crust, with main regions marked.TheKREEP rich magma terrane (hatched), with its most prominent part labeled on the near side (left), theMare Procellarum KREEP Terrane (PKT).TheSouth Pole-Aitken Terrane (SPAT) is marked on the far side (right, largest circle).The 24 crater basins with crustal thinning greater than 200 km (120 mi) are marked (black circles) across both hemispheres.The rest is the high-thicknessFelspathic Highlands Terrane (FHT; red and white).[118]
The discovery offault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years.[125] Similar shrinkage features exist onMercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon does not have tectonic plates, its tectonic activity is slow, and cracks develop as it loses heat.[126]
Scientists have confirmed the presence of a cave on the Moon near theSea of Tranquillity, not far from the 1969Apollo 11 landing site. The cave, identified as an entry point to a collapsedlava tube, is roughly 45 meters wide and up to 80 m long. This discovery marks the first confirmed entry point to a lunar cave. The analysis was based on photos taken in 2010 by NASA'sLunar Reconnaissance Orbiter. The cave's stable temperature of around17 °C could provide a hospitable environment for future astronauts, protecting them from extreme temperatures, solar radiation, and micrometeorites. However, challenges include accessibility and risks of avalanches and cave-ins. This discovery offers potential for future lunar bases or emergency shelters.[127]
Map of the near side of the Moon, with the darklunar maria, the most prominent volcanic features of the Moon, labeled in blue (brown labels are prominentlunar craters)
The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains calledmaria (sing.mare; from Latin meaning 'seas', as they were once believed to be filled with water)[128] which are vast solidified pools of ancientbasaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water.[129] The majority of these lava deposits erupted or flowed into the depressions associated withimpact basins, though the Moon's largest expanse of basalt flooding,Oceanus Procellarum, does not correspond to an obvious impact basin. Different episodes of lava flow in maria can often be recognized by variations in surface albedo and distinct flow margins.[130]
As the maria formed, cooling and contraction of the basaltic lava createdwrinkle ridges in some areas. These low, sinuous ridges can extend for hundreds of kilometers and often outline buried structures within the mare. Another result of maria formation is the creation of concentric depressions along the edges, known asarcuate rilles. These features occur as the mare basalts sink inward under their own weight, causing the edges to fracture and separate.
In addition to the visible maria, the Moon has mare deposits covered by ejecta from impacts. Called cryptomares, these hidden mares are likely older than the exposed ones.[131] Conversely, mare lava has obscured many impact melt sheets and pools. Impact melts are formed when intense shock pressures from collisions vaporize and melt zones around the impact site. Where still exposed, impact melt can be distinguished from mare lava by its distribution, albedo, and texture.[132]
Sinuous rilles, found in and around maria, are likely extinctlava channels or collapsedlava tubes. They typically originate from volcanicvents, meandering and sometimes branching as they progress. The largest examples, such asSchroter's Valley andRima Hadley, are significantly longer, wider, and deeper than terrestrial lava channels, sometimes featuring bends and sharp turns that again, are uncommon on Earth.
Sinuous rilles features andlunar lava tube caveMarius Hills pit as observed under different solar illumination conditions.
Mare volcanism has altered impact craters in various ways, including filling them to varying degrees, and raising and fracturing their floors from uplift of mare material beneath their interiors. Examples of such craters includeTaruntius andGassendi. Some craters, such asHyginus, are of wholly volcanic origin, forming ascalderas orcollapse pits. Such craters are relatively rare and tend to be smaller (typically a few kilometers wide), shallower, and more irregularly shaped than impact craters. They also lack the upturned rims characteristic of impact craters.
Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side[71] compared with 2% of the far side.[134] This is likely due to aconcentration 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.[76][135][136] Most of the Moon'smare basalts erupted during theImbrian period, 3.3–3.7 billion years ago, though some are as young as 1.2 billion years[60] and as old as 4.2 billion years.[61]
In 2006, a study ofIna, a tiny depression inLacus 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.[137]Moonquakes and releases of gas indicate continued lunar activity.[137] Evidence of recent lunar volcanism has been identified at 70irregular 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.[138][139][140][141] Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell,[142][143] 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.[144][145]
The lighter-colored regions of the Moon are calledterrae, or more commonlyhighlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago and may representplagioclasecumulates of the lunar magma ocean.[60][61] In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[146]
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.[147][148] Alternatively, it may be a consequence of asymmetricaltidal heating when the Moon was much closer to the Earth.[149]
The heavily cratered surface of the Moon, particularly where no mare formed, is discernable when viewed close up, with crater ridges highlighted at theterminator and the two most prominent craters on the near side,Tycho (left) andCopernicus (top) visible, featuring long bright impact-streaks.A view of a larger crater, the three-kilometer-deepDaedalus on theMoon's far sideApollo 17 astronautHarrison H. Schmitt next to the large Moon boulder nicknamed "Tracy's Rock" in theTaurus–Littrow of theMare Serenitatis on the near side of the Moon
A major geologic process that has affected the Moon's surface isimpact cratering,[150] 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.[151] Lunar craters exhibit a variety of forms, depending on their size. In order of increasing diameter, the basic types are simple craters with smooth bowl shaped interiors and upturned rims,complex craters with flat floors, terraced walls and central peaks,peak ring basins, andmulti-ring basins with two or more concentric rings of peaks.[152] The vast majority of impact craters are circular, but some, likeCantor andJanssen, have more polygonal outlines, possibly guided by underlying faults and joints. Others, such as theMessier pair,Schiller, andDaniell, are elongated. Such elongation can result from highly oblique impacts,binary asteroid impacts, fragmentation of impactors before surface strike, or closely spacedsecondary impacts.[153]
Thelunar geologic timescale is based on the most prominent impact events, such as multi-ring formations likeNectaris,Imbrium, andOrientale that are between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regionalstratigraphic horizon.[154] The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a fewmulti-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.[154]However care needs to be exercised with thecrater counting technique due to the potential presence ofsecondary craters. Ejecta from impacts can create secondary craters that often appear in clusters or chains but can also occur as isolated formations at a considerable distance from the impact. These can resemble primary craters, and may even dominate small crater populations, so their unidentified presence can distort age estimates.[155]
The radiometric ages of impact-melted rocks collected during theApollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose aLate Heavy Bombardment period of increased impacts.[156]
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 bydistal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years.[157][158] This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.[159]
Wide-angle image of a lunar swirl, the 70-kilometer-longReiner Gamma
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 youngregolith), and often have a sinuous shape. Their shape is often accentuated by lowalbedo regions that wind between the bright swirls. They are located in places with enhanced surfacemagnetic fields and many are located at theantipodal point of major impacts. Well known swirls include theReiner Gamma feature andMare Ingenii. They are hypothesized to be areas that have been partially shielded from thesolar wind, resulting in slowerspace weathering.[160]
Distribution of surface ice at the Moon's South (left) and North poles (right), as viewed by NASA'sMoon Mineralogy Mapper (M3) spectrometer onboard India'sChandrayaan-1 orbiter
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known asphotodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impactingcomets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen fromsolar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon.[161][162] Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow.[112] The presence of usable quantities of water on the Moon is an important factor in renderinglunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.[163]
In years since, signatures of water have been found to exist on the lunar surface.[164] In 1994, thebistatic radar experiment located on theClementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations byArecibo, suggest these findings may rather be rocks ejected from young impact craters.[165] In 1998, theneutron spectrometer on theLunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions.[166] Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior.[167]
The 2008Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-boardMoon Mineralogy Mapper. The spectrometer observed absorption lines common tohydroxyl, 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.[168] Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018.[169] In 2009,LCROSS sent a 2,300 kg (5,100 lb) impactor into apermanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material.[170][171] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb).[172]
In May 2011, 615–1410 ppm water inmelt inclusions in lunar sample 74220 was reported,[173] the famous high-titanium "orange glass soil" of volcanic origin collected during theApollo 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'supper 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.[174][175] The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances.[176] 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.[174][176]
ASun-synchronous view of the day-side of Earth from the Sun-Earth Lagrange point L1, with Earth rotating and the Moon passing on its orbit in between the observingDSCOVR satellite and Earth, showingthe Moon's far side when it is illuminated and the Moon's day-side
The Earth and the Moon form the Earth–Moonsatellite system with a shared center of mass, orbarycenter. This barycenter is 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath the Earth's surface.
The Moon makes a complete orbit around Earth with respect to the fixed stars, itssidereal period, about once every 27.3 days.[i] However, because the Earth–Moon system moves at the same time in its orbit around the Sun, it takes slightly longer, 29.5 days,[j][71] to return to the samelunar phase,[182] completing a full cycle, as seen from Earth. Thissynodic period or synodic month is commonly known as thelunar month and is equal to the length of thesolar day on the Moon.[183]
Due totidal locking, the Moon has a 1:1spin–orbit resonance. Thisrotation–orbit ratio makes the Moon's orbital periods around Earth equal to its correspondingrotation periods. This is the reason for only one side of the Moon, its so-callednear side, being visible from Earth. That said, while the movement of the Moon is in resonance, it still is not without nuances such aslibration, resulting in slightly changing perspectives, making over time and location on Earth about 59% of the Moon's surface visible from Earth.[184]
Unlike most satellites of other planets, the Moon's orbital plane is closer to theecliptic plane than to the planet'sequatorial plane. The Moon's orbit is subtlyperturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbitgradually rotates once every 18.61years,[185] which affects other aspects of lunar motion. These follow-on effects are mathematically described byCassini's laws.[186]
Minimum, mean and maximum distances of the Moon from Earth with its angular diameter as seen from Earth's surface to scale
Simplified diagram of Earth bulging, being pulled and stretched toward the Moon by its gravity, which is the main driver of thetides. The Ocean and Earth are being pulled more where it is closer to the Moon, causingtidal forces to be weaker at the far-side of Earth creating a second bulge and high-tide. The animation shows the change of the Moon's position on its inclined orbit.
The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides closest to each other, resulting intidal forces.Ocean tides are the most widely experienced result of this, but tidal forces also considerably affect other mechanics of Earth, as well as the Moon and their system.
The lunar solid crust experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are insynchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun.[187] The Earth-induced variable component arises from changing distance andlibration, 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).[187] According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintainingEarth's magnetic field.[188]
The cumulative effects of stress built up by these tidal forces producesmoonquakes. 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 fromseismometers placed on the Moon byApolloastronauts from 1969 through 1972.[189]
The most commonly known effect of tidal forces is elevated sea levels called ocean tides.[190] While the Moon exerts most of the tidal forces, the Sun also exerts tidal forces and therefore contributes to the tides as much as 40% of the Moon's tidal force; producing in interplay thespring and neap tides.[190]
The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. 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. The tide under the Moon is explained by the Moon's gravity being stronger on the water close to it. The tide on the opposite side can be explained either by the centrifugal force as the Earth orbits thebarycenter or by the water's inertia as the Moon's gravity is stronger on the solid Earth close to it and it is pulled away from the farther water.[191][192]
Thus, there are two high tides, and two low tides in about 24 hours.[190] 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.
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 sloshing of water between different ocean basins[193]
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.
System evolution
Delays in the tidal peaks of both ocean and solid-body tides causetorque in opposition to the Earth's rotation. This "drains"angular momentum and rotationalkinetic energy from Earth's rotation, slowing the Earth's rotation.[190][187] That angular momentum, lost from the Earth, is transferred to the Moon in a process known astidal acceleration, which lifts the Moon into a higher orbit while lowering orbital speed around the Earth. This has resulted in the length of aanomalistic month having increased from 20 days to today's 27.55 days over the course of 3.2 billion years.[194]
Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction.[187] 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).[195][196][197]Atomic clocks show that Earth's Day lengthens by about 17 microseconds every year,[198][199][200] slowly increasing the rate at whichUTC is adjusted byleap seconds.
This tidal drag makes the rotation of the Earth, and the orbital period of the Moon very slowly match. This matching first results intidally locking the lighter body of the orbital system, as is already the case with the Moon. Theoretically, in 50 billion years,[201] the Earth's rotation will have slowed to the point of matching the Moon's orbital period, causing the Earth to always present the same side to the Moon. However, the Sun will become ared giant, most likely engulfing the Earth–Moon system long before then.[202][203]
If the Earth–Moon system isn't engulfed by the enlarged Sun, the drag from the solar atmosphere can cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of 18,470 km (11,480 mi), it will cross Earth'sRoche limit, meaning that tidal interaction with Earth would break apart the Moon, turning it into aring system. Most of the orbiting rings will begin to decay, and the debris will impact Earth. Hence, even if the Sun does not swallow up Earth, the planet may be left moonless.[204]
Libration, the slight variation in the Moon'sapparent size and viewing angle over a single lunar month as viewed from somewhere on the Earth's Northern Hemisphere.
The Moon appears in Earth's sky differently depending on the position of observation on Earth and the time oflunar year,lunar month and day on Earth. During a year the Moonculminates during a day at differentaltitudes. The Moon appears highest in the sky unlike the Sun during winter and lowest during summer, which is true for the seasons of each of Earth's northern and southern hemispheres.
The apparent orientation of the Moon depends on its position in the sky and the hemisphere of the Earth from which it is being viewed. In theNorthern Hemisphere it appears upside down compared to the view from theSouthern Hemisphere.[206] Sometimes the "horns" of a crescent moon appear to be pointing more upwards than sideways. This phenomenon is called awet moon and occurs more frequently in thetropics.[207]
Thedistance between the Moon and Earth varies from around 356,400 km (221,500 mi) (perigee) to 406,700 km (252,700 mi) (apogee), making the Moon's distance and apparent size fluctuate up to 14%.[208][209] On average the Moon'sangular diameter is about 0.52°, roughly the same apparent size as the Sun (see§ Eclipses). In addition, a purely psychological effect, known as theMoon illusion, makes the Moon appear larger when close to the horizon.[210]
Rotation
Comparison between the Moon on the left, rotating tidally locked (correct), and with the Moon on the right, without rotation (incorrect)
Thetidally locked synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. The side of the Moon that faces Earth is called thenear side, and the opposite thefar 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. Duringdark moon tonew moon, the near side is dark.[211]
The Moon originally rotated at a faster rate, but early in its history its rotation slowed and becametidally locked in this orientation as a result offrictional effects associated withtidal deformations caused by Earth.[212] 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 NASALunar 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.[213]
The Moon rotates, as it orbits Earth, changing orientation toward the Sun, experiencing alunar day. A lunar day is equal to onelunar month (one synodic orbit around Earth) due to it beingtidally locked to Earth. Since the Moon is not tidally locked to the Sun, lunar daylight and night times both occur around the Moon. The changing position of the illumination of the Moon by the Sun during a lunar day is observable from Earth as the changinglunar phases, waxing crescent being the sunrise and the waning crescent the sunset phase of a day observed from afar.[214]
Lunar night is the darkest on the far side and during lunar eclipses on the near side (and darker than a moonless night on Earth). The near side is during its night illuminated byEarthlight, making the near side illuminated enough by the Earthlight to see lunar surface features from Earth where it is dark during its night phase due to Earthlight being reflected back to Earth. Earthshine makes the night on the near side about 43 times brighter, and sometimes even 55 times brighter than a night on Earth illuminated by the light of the full moon.[215]
In Earth's sky brightness and apparent size of the Moon changes also due to its ellipticorbit around Earth. Atperigee (closest), since the Moon is up to 14% closer to Earth than atapogee (most distant), it subtends asolid angle which is up to 30% larger. Consequently, given the same phase, the Moon's brightness also varies by up to 30% between apogee and perigee.[216] A full (or new) moon at such a position is called asupermoon.[208][209][217]
Each of the four intermediate phases of the Moon lasts, on average, about seven days (approximately 7.38 days), but this duration can vary by about ±11% due to the change in the distance between the Moon and Earth between apogee (farthest point) and perigee (closest point).
The number of days since the most recent new moon is called the Moon's age.[218] A complete cycle of lunar phases is known as a lunation.[219]
The approximate age and phase of the Moon for any date can be calculated by determining the number of days since a known new moon (e.g., January 1, 1900, or August 11, 1999) and dividing that value by the average length of a synodic month (29.53059 days)[220]. The remainder of the division represents the Moon's age. This method assumes a perfectly circular orbit and does not consider the exact time of the new moon, so results may be off by a few hours. Accuracy decreases as the date moves away from the reference date.
This simplified calculation is suitable for general or decorative purposes, such as moon phase clocks. More precise applications that take into account the Moon's apogee and perigee require more complex methods.
Due to lunar libration, it is sometimes possible to see slightly more than the full moon (up to about 101%) or a small part of its far side (up to 5%).[221]
As it orbits, the Moon moves an average of 13° eastward on the celestial sphere every day. This means that from new moon onward, the moon will become increasingly distant from the Sun, becoming more prominent until full moon, when it appears to be on the opposite side of the Sun. Thereafter, the Moon appears to move closer to the Sun until a new moon occurs. The position and time at which the Moon rises on the eastern horizon vary continually, primarily due to the inclination of the lunar orbit , which is more than 5° relative to the Earth's equator, which in turn is inclined more than 23° to the ecliptic.
Observational phenomena
There has been historical controversy over whether observed 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, poorastronomical seeing, or inadequate drawings. However,outgassing does occasionally occur and could be responsible for a minor percentage of the reportedlunar 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.[222][223]
The changing apparent color of the Moon, filtered by Earth's atmosphere
Albedo and true color of the surface
The Moon’s appearance is shaped by its reflective properties and surface composition. Itsalbedo is about 0.12[224], comparable to wornasphalt, making it relatively dark despite its brightness in the night sky. This low albedo represents the reflectance of the lunar regolith, a layer of fine rock fragments created by meteorite impacts. Light scattering by the regolith causes the Moon to appear much brighter at full moon than at quarter phases.
The Moon’s true color, without atmospheric effects, is a muted brownish-gray[225]. This results from silicate minerals in the regolith, with darker basaltic plains (maria) and lighter feldspar-rich highlands. The maria, formed by ancient volcanic flows, contain more iron and titanium, giving them a darker tone. From Earth, the Moon’s color can be altered by the atmosphere, appearing red during lunar eclipses or occasionally blue due to volcanic particles.
The Moon also exhibits retro-reflection[226], scattering light back toward its source and creating a uniform brightness across its disc without significant limb darkening. Its apparent size and brightness vary due to its elliptical orbit, appearing up to 30% brighter and 14% larger at perigee compared to apogee, a phenomenon known as a "Supermoon".
At times, the Moon can appear red or blue. It may appear red during alunar eclipse, because of the red spectrum of the Sun's light beingrefracted onto the Moon by Earth's atmosphere. Because of this red color, lunar eclipses are also sometimes calledblood moons. The Moon can also seem red when it appears at low angles and through a thick atmosphere.
The Moon may appear blue depending on the presence of certain particles in the air,[227] such as volcanic particles,[228] in which case it can be called ablue moon.
Because the words "red moon" and "blue moon" can also be used to refer to specificfull moons of the year, they do not always refer to the presence of red or bluemoonlight.
Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy").Solar eclipses occur atnew 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 bothtotal (with the Moon appearing larger than the Sun) andannular (with the Moon appearing smaller than the Sun) solar eclipses.[229] In a total eclipse, the Moon completely covers the disc of the Sun and thesolar corona becomes visible to thenaked eye.
Because the distance between the Moon and Earth is very slowly increasing over time,[190] the angular diameter of the Moon is decreasing. As it evolves toward becoming ared giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing.[k] 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.[230]
As the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to theorbit 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.[231] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by thesaros, which has a period of approximately 18 years.[232]
Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,[l][233] the related phenomenon ofoccultation 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 theprecession of the lunar orbit, each year different stars are occulted.[234]
TheMoon illusion is theoptical illusion of the Moon appearing larger near thehorizon than it does higher up in thesky. It has been known since ancient times and recorded by various cultures.[235][236]
It is believed by some that the oldestcave paintings from up to 40,000BP of bulls and geometric shapes,[237] or 20–30,000 year oldtally sticks were used to observe the phases of the Moon, keeping time using the waxing and waning ofthe Moon's phases.[238]Aspects of the Moon were identified and aggregated inlunar deities fromprehistoric times and were eventually documented and put into symbols from the very first instances ofwriting in the4th millennium BC. One of the earliest-discovered possible depictions of the Moon is a 3,000 BCE rock carvingOrthostat 47 atKnowth, Ireland.[239][240] Thecrescent depicting the Moon as with the lunar deityNanna/Sin have been found from the 3rd millennium BCE.[241]
InAristotle'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 ofaether, aninfluential philosophy that would dominate for centuries.[252]Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System.[253] In the2nd century BC,Seleucus of Seleucia correctly thought thattides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to theSun.[254] In the same century,Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times theradius of Earth for the distance.
The Chinese of theHan dynasty believed the Moon to be energy equated toqi and their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun;Jing Fang (78–37 BC) noted the sphericity of the Moon.[249]: 413–414 Ptolemy (90–168 AD) greatly improved on the numbers of Aristarchus, calculating a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters, close to the correct values of about 60 and 0.273 respectively.[255] In the 2nd century AD,Lucian wrote the novelA True Story, in which the heroes travel to the Moon and meet its inhabitants. In 510 AD, the Indian astronomerAryabhata mentioned in hisAryabhatiya that reflected sunlight is the cause of the shining of the Moon.[256][257] The astronomer and physicistIbn al-Haytham (965–1039) found thatsunlight 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.[258]Shen Kuo (1031–1095) of theSong 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.[249]: 415–416 During theMiddle Ages, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was "perfectly smooth".[259]
Telescopic exploration (1609–1959)
The first published sketches of one of the first telescopic observations of the Moon, from the ground-breakingSidereus Nuncius (1610) byGalileo Galilei, providing among other findings the first descriptions of the Moon's topography
Thetelescope was developed and reported on in 1608. The first record of telescopic astronomy and rough mapping of the Moon's features is from early summer 1609 byThomas Harriot, but which he did not publish. At the same timeGalileo Galilei too started to use telescopes to observe the sky and the Moon, recording later that year more detailed observations and crucial conclusions, such as that the Moon was not smooth, featuring mountains and craters, which he published in 1610 in his ground-breaking and soon widely discussed bookSidereus Nuncius.
Later in the 17th century, the efforts ofGiovanni Battista Riccioli andFrancesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–1836Mappa Selenographica ofWilhelm Beer andJohann Heinrich von Mädler, and their associated 1837 bookDer Mond, the firsttrigonometrically 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.[260] Lunar craters, first noted by Galileo, were thought to bevolcanic until the 1870s proposal ofRichard Proctor that they were formed by collisions.[71] This view gained support in 1892 from the experimentation of geologistGrove Karl Gilbert, and from comparative studies from 1920 to the 1940s,[261] leading to the development oflunar stratigraphy, which by the 1950s was becoming a new and growing branch ofastrogeology.[71]
AfterWorld War II the firstlaunch systems were developed and by the end of the 1950s they reached capabilities that allowed theSoviet Union and theUnited States to launchspacecraft into space. TheCold War fueled a closely followed development of launch systems by the two states, resulting in the so-calledSpace Race and its later phase the Moon Race, accelerating efforts and interest inexploration of the Moon.
After the first spaceflight ofSputnik 1 in 1957 duringInternational Geophysical Year the spacecraft of the Soviet Union'sLuna program were the first to accomplish a number of goals. Following three unnamed failed missions in 1958,[262] the first human-made objectLuna 1 escaped Earth's gravity and passed the Moon on 4 January 1959. Later that year the first human-made objectLuna 2 reached the Moon's surface byintentionally impacting. By the end of the yearLuna 3 reached as the first human-made object the normally occludedfar side of the Moon, taking the first photographs of it.The first spacecraft to perform a successful lunarsoft landing wasLuna 9 and the first vehicle to orbit the Moon wasLuna 10, both in 1966.[71]
Following PresidentJohn F. Kennedy's 1961 commitment to a crewed 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: theJet Propulsion Laboratory'sRanger program, theLunar Orbiter program and theSurveyor program. The crewedApollo 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 potentialSoviet lunar human landing, in 1968Apollo 8 made the first human mission to lunar orbit (the first Earthlings, two tortoises, had circled the Moon three months earlier on the Soviet Union'sZond 5, followed by turtles onZond 6).
The first time a person landed on the Moon and any extraterrestrial body was whenNeil Armstrong, the commander of the American missionApollo 11, set foot on the Moon at 02:56 UTC on July 21, 1969.[263] Considered the culmination of theSpace Race,[264] an estimated 500 million people worldwide watched the transmission by theApollo TV camera, the largest television audience for a live broadcast at that time.[265][266] While at the same time another mission, the robotic sample return missionLuna 15 by the Soviet Union had been in orbit around the Moon, becoming together with Apollo 11 the first ever case of two extraterrestrial missions being conducted at the same time.
The Apollo missions 11 to 17 (exceptApollo 13, which aborted its planned lunar landing) removed 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196separate samples.[267]Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-livedinstrument stations, including heat flow probes,seismometers, andmagnetometers, were installed at theApollo 12,14,15,16, and17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations,[268][269] but as the stations'lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used.[270]Apollo 17 in 1972 remains the last crewed mission to the Moon.Explorer 49 in 1973 was the last dedicated U.S. probe to the Moon until 1994.
The Soviet Union continued sending robotic missions to the Moon until 1976, deploying in 1970 withLuna 17 the first remote controlledroverLunokhod 1 on an extraterrestrial surface, and collecting and returning 0.3 kg of rock and soil samples with threeLunasample return missions (Luna 16 in 1970,Luna 20 in 1972, andLuna 24 in 1976).[272]
Negotiation in 1979 ofMoon treaty, and its subsequent ratification in 1984 was the only major activity regarding the Moon until 1990.
Renewed exploration (1990–present)
In 1990Hiten –Hagoromo,[273] the first dedicated lunar mission since 1976, reached the Moon. Sent by Japan, it became the first mission that was not a Soviet Union or U.S. mission to the Moon.
In 1994, the U.S. dedicated a mission to fly a spacecraft (Clementine) to the Moon again for the first time since 1973. This mission obtained the first near-global topographic map of the Moon, and the first globalmultispectral images of the lunar surface.[274] In 1998, this was followed by theLunar 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.[275]
The next years saw a row of first missions to the Moon by a new group of states actively exploring the Moon.Between 2004 and 2006 the first spacecraft by theEuropean Space Agency (ESA) (SMART-1) reached the Moon, recording the first detailed survey of chemical elements on the lunar surface.[276]TheChinese Lunar Exploration Program reached the Moon for the first time with the orbiterChang'e 1 (2007–2009),[277] obtaining a full image map of the Moon.India reached, orbited and impacted the Moon in 2008 for the first time with itsChandrayaan-1 andMoon Impact Probe, becoming the fifth and sixth state to do so, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence ofwater molecules in lunar soil.[278]
The U.S. launched theLunar Reconnaissance Orbiter (LRO) and theLCROSS impactor on June 18, 2009.LCROSS completed its mission by making a planned and widely observed impact in the craterCabeus on October 9, 2009,[279] whereasLRO is currently in operation, obtaining precise lunaraltimetry and high-resolution imagery.
The U.S.developed plans for returning to the Moon beginning in 2004,[282] and with the signing of the U.S.-ledArtemis Accords in 2020, theArtemis program aims to return the astronauts to the Moon in the 2020s.[283] The Accords have been joined by a growing number of countries. The introduction of the Artemis Accords has fueled a renewed discussion about the international framework and cooperation of lunar activity, building on theMoon Treaty and the ESA-ledMoon Village concept.[284][285][286]
2022 South Korea launchedDanuri successfully, its first mission to the Moon, from the US.In 2023 and 2024 India and Japan became the fourth and fifth country tosoft land a spacecraft on the Moon, following theSoviet Union and United States in the 1960s, and China in the 2010s.[287] Notably, Japan's spacecraft, theSmart Lander for Investigating Moon, survived 3 lunar nights.[288] TheIM-1 lander became the first commercially built lander to land on the Moon in 2024.[289]
Artemis II crew, with thefirst woman, person of color and non–US-citizen astronaut planned to go to the Moon, scheduled for 2026, returning humans to the Moon for the first time sinceApollo 17 in 1972.Clockwise from left:Koch,Glover,Hansen andWiseman.
Beside the progressingArtemis program and supportingCommercial Lunar Payload Services,[293] China is continuing its ambitiousChang'e program, having announced with Russia's strugglingLuna-Glob program joint missions.[294][295] Both the Chinese and US lunar programs have the goal to establish in the 2030s alunar base with their international partners, though the US and its partners will first establish an orbitalLunar Gateway station in the 2020s, from which Artemis missions will land theHuman Landing System to set up temporary surface camps.
While the Apollo missions were explorational in nature, the Artemis program plans to establish a more permanent presence. To this end, NASA is partnering with industry leaders to establish key elements such as modern communication infrastructure. A4G connectivity demonstration is to be launched aboard anIntuitive Machines Nova-C lander in 2024.[296] Another focus is onin situ resource utilization, which is a key part of theDARPA lunar programs.DARPA has requested that industry partners develop a 10–year lunar architecture plan to enable the beginning of a lunar economy.[297]
Map of all the sites ofsoft landings on the Moon (2024)
In 1959 the first extraterrestrial probes reached the Moon (Luna program), just a year into thespace age, after the first ever orbital flight. Since then, humans have sent a range of probes and people to the Moon. The first stay of people on the Moon was conducted in 1969, in a series of crewed exploration missions (theApollo Program), the last having taken place in 1972.
Uninterrupted presence has been the case through theremains of impactors, landings andlunar orbiters. Some landings and orbiters have maintained a small lunar infrastructure, providing continuous observation and communication at the Moon. Longterm missions continuing to be active are some orbiters such as the 2009-launchedLunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launchedChang'e 3 with its Lunar Ultraviolet Telescope still operational.[298]Five retroreflectors have been installed on the Moon since the 1970s and since used for accurate measurements of the physicallibrations throughlaser ranging to the Moon.
While the Moon has the lowestplanetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed.[302] If there isastronomy performed from the Moon, it will need to be free from any physical andradio pollution. 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.[303] ScholarAlice Gorman asserts that, although the Moon is inhospitable, describing the Moon as dead negates its dynamics, which requires sustainable human activity to treat the Moon's ecology as a co-participant.[304]
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.[306][307] As such lunar waste management has been raised as an issue which future lunar missions, particularly on the surface, need to tackle.[308][309]
The Moon has been used as a site for astronomical andEarth observations. The Earth appears in theMoon's sky with anapparent size of 1° 48′ to 2°,[312] three to four times the size of the Moon or Sun in Earth's sky, or about the apparent width of two little fingers at an arm's length away. Observations from the Moon started as early as 1966 with thefirst images of Earth from the Moon, taken byLunar Orbiter 1. Of particular cultural significance is the 1968 photograph calledEarthrise, taken byBill Anders ofApollo 8 in 1968. In April 1972 theApollo 16 mission set up the first dedicated telescope,[313][314] theFar Ultraviolet Camera/Spectrograph, recording various astronomical photos and spectra.[315]
The Moon is recognized as an excellent site for telescopes.[316] It is relatively nearby; certain craters near the poles are permanently dark and cold and especially useful forinfrared telescopes; andradio telescopes on the far side would be shielded from the radio chatter of Earth.[317] Thelunar soil, although it poses a problem for any moving parts oftelescopes, can be mixed withcarbon nanotubes andepoxies and employed in the construction of mirrors up to 50 meters in diameter.[318] A lunarzenith telescope can be made cheaply with anionic liquid.[319]
With increased human activity on the surface of the Moon the atmospheric circulation of lunar dust will increase, reducing the favourable conditions for astronomy from the surface depending on meassures to mitigate the spread of lunar dust.[320]
Humans have lived on the Moon in groups of two and for up to three days.[321] Altogether twelve people have stayed on the Moon over the course of six visits.[322] They all lived in one type ofsurface habitat, theApollo Lunar Module.[323] In the course of these stays some people have spent in total up to one day roaming the surface.[321]
Challenges when roaming the surface arise fromlunar dust sticking to suits and tools, and has been carried inside the habitats. Astronauts could taste and smell the dust, which smells like gunpowder and was called the "Apollo aroma".[324] This fine lunar dust cancause health issues.[324]
In 2019, at least one plant seed sprouted in an experiment on theChang'e 4 lander. It was carried from Earth along with other small life in itsLunar Micro Ecosystem.[325]
The 1967Outer Space Treaty defines the Moon and all outer space as the "province of all mankind".[326] It restricts the use of the Moon to peaceful purposes, explicitly banning military installations andweapons of mass destruction.[330] A majority of countries are parties of this treaty.[331]The 1979Moon Agreement was created to elaborate, and restrict the exploitation of theMoon's resources by any single nation, leaving it to a yet unspecified international regulatory regime.[332] As of January 2020, it has been signed and ratified by 18 nations,[333] none of which havehuman spaceflight capabilities.
Since 2020, countries have joined the U.S. in theirArtemis Accords, which are challenging the treaty. The U.S. has furthermore emphasized in a presidentialexecutive order ("Encouraging International Support for the Recovery and Use of Space Resources.") 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."[334][335]
With Australia signing and ratifying both the Moon Treaty in 1986 as well as the Artemis Accords in 2020, there has been a discussion if they can be harmonized.[285] In this light anImplementation Agreement for the Moon Treaty has been advocated for, as a way to compensate for the shortcomings of the Moon Treaty and to harmonize it with other laws and agreements such as the Artemis Accords, allowing it to be more widely accepted.[284][286]
In the face of such increasing commercial and national interest, particularly prospecting territories, U.S. lawmakers have introduced in late 2020 specific regulation for the conservation of historic landing sites[336] and interest groups have argued for making such sitesWorld Heritage Sites[337] and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon.[305]
In 2021, theDeclaration of the Rights of the Moon[338] was created by a group of "lawyers, space archaeologists and concerned citizens", drawing on precedents in theRights of Nature movement and the concept of legal personality for non-human entities in space.[339][340]
Coordination and regulation
Increasing human activity at the Moon has raised the need for coordination to safeguard international and commercial lunar activity. Issues from cooperation to mere coordination, through for example the development of a sharedLunar time, have been raised.
TheVenus of Laussel (c. 25,000BP) holding a crescent shaped horn. The 13 notches on the horn may symbolize the average number of days from menstruation to anovulation, or the approximate number of fullmenstrual cycles andlunar cycles per year (although these two phenomena are unrelated).[341][342]
Since pre-historic times people have taken note ofthe Moon's phases and itswaxing and waning cycle 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.[238][343][344] The counting of the days between the Moon's phases eventually gave rise to generalizedtime periods of lunar cycles asmonths, and possibly of its phases asweeks.[345]
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 Englishmonth as well asmoon, and its cognates in other Indo-European languages (e.g. theLatinmensis andAncient Greekμείς (meis) orμήν (mēn), meaning "month")[346][347][348][349] stem from theProto-Indo-European (PIE) root ofmoon, *méh1nōt, derived 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 wordsmeasure andmenstrual).[350][351][352] To give another example from a differentlanguage family, theChinese language uses the same word (月) formoon as formonth, which furthermore can be found in the symbols for the wordweek (星期).
This lunar timekeeping gave rise to the historically dominant, but varied,lunisolar calendars. The 7th-centuryIslamic calendar is an example of a purelylunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon.[353]
Humans have not only observed the Moon sinceprehistoric times, but have also developed intricate perceptions of the Moon. Over time the Moon has been characterized and associated in many different ways, from having aspirit or beinga deity, and anaspect thereof or an aspectin astrology, being made an important part of manycosmologies.
For the representation of the Moon, especially itslunar phases, thecrescent (🌙) has been a recurring symbol in a range of cultures since at least 3,000 BCE or possibly earlier with bull horns dating to the earliestcave paintings at 40,000BP.[237][244] Inwriting systems such as Chinese the crescent has developed into the symbol月, the word for Moon, and in ancient Egyptian it was the symbol𓇹, meaning Moon and spelled like the ancient Egyptian lunar deityIah,[356] which the other ancient Egyptian lunar deitiesKhonsu andThoth were associated with.
The particular arrangement of the crescent with a star known as thestar and crescent (☪️) goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and the planet Venus, in combination. It came to represent the selene goddessArtemis, and via the patronage ofHecate, which astriple deity under theepithettrimorphos/trivia included aspects of Artemis/Diana, came to be used as asymbol of Byzantium, withVirgin Mary (Queen of Heaven) later taking her place, becoming depicted inMarian veneration on a crescent and adorned with stars. Since then theheraldric use of the star and crescent proliferated, Byzantium's symbolism possibly influencing the development of theOttoman flag, specifically the combination of the Turkish crescent with a star,[360] and becoming a popularsymbol for Islam (as thehilal of theIslamic calendar) andfor a range of nations.[361]
Occasionally some lunar deities have been also depicteddriving a chariot across the sky, such as the HinduChandra/Soma, the Greek Artemis, which is associated with Selene, or Luna, Selene's ancient Roman equivalent.
Color and material wise the Moon has been associated in Westernalchemy withsilver, while gold is associated with the Sun.[362]
The perception of the Moon in themodern era has been informed bytelescope-enabledmodern astronomy and later byspaceflight which enabled actual human activity at the Moon, particularly theculturally impactful lunar landings. These new insights inspired cultural references, connecting romantic reflections about the Moon[365] and speculative fiction such as science-fiction dealing with the Moon.[364][366]
The lunar effect is an 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 wordslunacy andlunatic are derived from the Latin name for the Moon,Luna. PhilosophersAristotle andPliny 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.[369] 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,[369] but over 37 studies invalidate these claims.[370]Lunar cycles have significant impacts on human culture but no solid evidence connects these cycles to human biology.[371]
^There are a number ofnear-Earth asteroids, including3753 Cruithne, that areco-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term (Morais et al, 2002). These arequasi-satellites – they are not moons as they do not orbit Earth. For more information, seeOther moons of Earth.
^Themaximum value is given based on scaling of the brightness from the value of −12.74 given for an equator to Moon-centre distance of 378 000 km in the NASA factsheet reference to the minimum Earth–Moon distance given there, after the latter is corrected for Earth's equatorial radius of 6 378 km, giving 350 600 km. Theminimum value (for a distantnew moon) is based on a similar scaling using the maximum Earth–Moon distance of 407 000 km (given in the factsheet) and by calculating the brightness of theearthshine onto such a new moon. The brightness of the earthshine is[ Earthalbedo ×(Earth radius / Radius ofMoon's orbit)2 ] relative to the direct solar illumination that occurs for a full moon. (Earth albedo = 0.367;Earth radius = (polar radius × equatorialradius)½ = 6 367 km.)
^The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378 000 km, theangular size is 1896 arcseconds. The same fact sheet gives extreme Earth–Moon distances of 407 000 km and 357 000 km. For the maximum angular size, the minimum distance has to be corrected for Earth's equatorial radius of 6 378 km, giving 350 600 km.
^Lucey et al. (2006) give107 particles cm−3 by day and105 particles cm−3 by night. Along with equatorial surface temperatures of 390 K by day and 100 K by night, theideal gas law yields the pressures given in the infobox (rounded to the nearestorder of magnitude): 10−7Pa by day and 10−10 Pa by night.
^Because the Moon follows an elliptical path, its distance from Earth varies over the year, ranging from 357,000–407,000 kilometres (222,000–253,000 mi).[2]
^There is no strong correlation between the sizes of planets and the sizes of their satellites. Larger planets tend to have more satellites, both large and small, than smaller planets.
^With 27% the diameter and 60% the density of Earth, the Moon has 1.23% of the mass of Earth. The moonCharon is larger relative to its primaryPluto, but Earth and the Moon are different since Pluto is considered adwarf planet and not a planet, unlike Earth.
^More accurately, the Moon's mean sidereal period (fixed star to fixed star) is 27.321661 days(27 d 07 h 43 min 11.5 s), and its mean tropical orbital period (from equinox to equinox) is 27.321582 days(27 d 07 h 43 min 04.7 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
^More accurately, the Moon's mean synodic period (between mean solar conjunctions) is 29.530589 days(29 d 12 h 44 min 02.9 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
^See graph inSun#Life phases. At present, the diameter of the Sun is increasing at a rate of about five percent per billion years. This is very similar to the rate at which the apparent angular diameter of the Moon is decreasing as it recedes from Earth.
^On average, the Moon covers an area of0.21078 square degrees on the night sky.
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![Sumerian cylinder seal and impression, dated c. 2100 BC, of Ḫašḫamer, ensi (governor) of Iškun-Sin c. 2100 BC. The seated figure is probably king Ur-Nammu, bestowing the governorship on Ḫašḫamer, who is led before him by Lamma (protective goddess).[355]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aSumerian_Cylinder_Seal_of_King_Ur-Nammu.jpg)
