Moon

Both the Moon's natural prominence in the earthly sky and its regular cycle of phases (Opens in a new window) as seen from Earth have provided cultural references and influences for human societies and cultures throughout history. Such cultural influences can be found in language, calendar systems, art, and mythology.

Name and etymology

The Moon, tinted reddish, during a lunar eclipse (Opens in a new window)

During the lunar phases (Opens in a new window), only portions of the Moon can be observed from Earth (Opens in a new window).

The usual English (Opens in a new window) proper name for Earth's natural satellite is simply the Moon, with a capital M.[16] (Opens in a new window)[17] (Opens in a new window) The noun moon is derived from Old English (Opens in a new window)mōna, which (like all its Germanic (Opens in a new window) cognates) stems from Proto-Germanic (Opens in a new window) *mēnōn,[18] (Opens in a new window) which in turn comes from Proto-Indo-European (Opens in a new window) *mēnsis"month"[19] (Opens in a new window) (from earlier *mēnōt, genitive *mēneses) which may be related to the verb "measure" (of time).[20] (Opens in a new window)

Occasionally, the name Luna /ˈluːnə/ (Opens in a new window) is used in scientific writing[21] (Opens in a new window) 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.[22] (Opens in a new window) Cynthia /ˈsɪnθiə/ (Opens in a new window) is another poetic name, though rare, for the Moon personified as a goddess,[23] (Opens in a new window) while Selene (Opens in a new window) /səˈliːniː/ (Opens in a new window) (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/ (Opens in a new window),[24] (Opens in a new window) 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,[25] (Opens in a new window) while its cognate selenic was originally a rare synonym[26] (Opens in a new window) but now nearly always refers to the chemical element selenium (Opens in a new window).[27] (Opens in a new window) The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography (Opens in a new window), the study of the physical features of the Moon, as well as the element name selenium.[28] (Opens in a new window)[29] (Opens in a new window)

The Greek goddess of the wilderness and the hunt, Artemis (Opens in a new window), equated with the Roman Diana (Opens in a new window), one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia (Opens in a new window), from her legendary birthplace on Mount Cynthus (Opens in a new window).[30] (Opens in a new window) These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits (Opens in a new window) such as apolune, pericynthion and selenocentric.

The Moon

Near side of the Moon (Opens in a new window)

Far side of the Moon (Opens in a new window)

Lunar north pole (Opens in a new window)

Lunar south pole (Opens in a new window)

Formation

Main articles: Origin of the Moon (Opens in a new window), Giant-impact hypothesis (Opens in a new window), and Circumplanetary disk (Opens in a new window)

The Moon formed 4.51 billion years ago,[f] (Opens in a new window) or even 100 million years earlier, some 50 million years after the origin of the Solar System, as research published in 2019 suggests.[31] (Opens in a new window) Several forming mechanisms have been proposed,[32] (Opens in a new window) including the fission of the Moon from Earth's crust through centrifugal force (Opens in a new window)[33] (Opens in a new window) (which would require too great an initial rotation rate of Earth),[34] (Opens in a new window) the gravitational capture of a pre-formed Moon[35] (Opens in a new window) (which would require an unfeasibly extended atmosphere of Earth (Opens in a new window) to dissipate (Opens in a new window) the energy of the passing Moon),[34] (Opens in a new window) and the co-formation of Earth and the Moon together in the primordial (Opens in a new window) accretion disk (Opens in a new window) (which does not explain the depletion of metals in the Moon).[34] (Opens in a new window) These hypotheses also cannot account for the high angular momentum (Opens in a new window) of the Earth–Moon system.[36] (Opens in a new window)

The evolution of the Moon and a tour of the Moon

The prevailing theory is that the Earth–Moon system formed after a giant impact (Opens in a new window) of a Mars (Opens in a new window)-sized body (named Theia (Opens in a new window)) with the proto-Earth (Opens in a new window). The impact blasted material into Earth's orbit and then the material accreted and formed the Moon.[37] (Opens in a new window)[38] (Opens in a new window)

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.[39] (Opens in a new window)

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.[40] (Opens in a new window) However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth.[41] (Opens in a new window)[42] (Opens in a new window)[43] (Opens in a new window)[44] (Opens in a new window) Other bodies of the inner Solar System such as Mars and Vesta (Opens in a new window) have, according to meteorites from them, very different oxygen and tungsten isotopic (Opens in a new window) 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,[45] (Opens in a new window) although this is debated.[46] (Opens in a new window)

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.[47] (Opens in a new window)[48] (Opens in a new window) Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean (Opens in a new window); its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).[47] (Opens in a new window)

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.[49] (Opens in a new window)

Oceanus Procellarum (Opens in a new window) ("Ocean of Storms")

Ancient rift valleys (Opens in a new window) – rectangular structure (visible – topography – GRAIL gravity gradients (Opens in a new window))

Ancient rift valleys (Opens in a new window)– context

Ancient rift valleys (Opens in a new window) – closeup (artist's concept)

In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures (Opens in a new window) of lunar rocks.[50] (Opens in a new window) 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 (Opens in a new window) and it was announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures.[51] (Opens in a new window) Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth,[52] (Opens in a new window) which conflicts (Opens in a new window) 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 (Opens in a new window) 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.[53] (Opens in a new window)

Internal structure

Main article: Internal structure of the Moon (Opens in a new window)

Lunar surface chemical composition[54] (Opens in a new window)CompoundFormulaCompositionMariaHighlandssilica (Opens in a new window)SiO245.4%45.5%alumina (Opens in a new window)Al2O314.9%24.0%lime (Opens in a new window)CaO11.8%15.9%iron(II) oxide (Opens in a new window)FeO14.1%5.9%magnesia (Opens in a new window)MgO9.2%7.5%titanium dioxide (Opens in a new window)TiO23.9%0.6%sodium oxide (Opens in a new window)Na2O0.6%0.6% 99.9%100.0%

The Moon is a differentiated (Opens in a new window) body. It has a geochemically (Opens in a new window) distinct crust (Opens in a new window), mantle (Opens in a new window), and core (Opens in a new window). 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).[55] (Opens in a new window)[56] (Opens in a new window) This structure is thought to have developed through the fractional crystallization (Opens in a new window) of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.[57] (Opens in a new window)

Crystallization of this magma ocean would have created a mafic (Opens in a new window) mantle from the precipitation (Opens in a new window) and sinking of the minerals olivine (Opens in a new window), clinopyroxene (Opens in a new window), and orthopyroxene (Opens in a new window); after about three-quarters of the magma ocean had crystallised, lower-density plagioclase (Opens in a new window) minerals could form and float into a crust atop.[58] (Opens in a new window) The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible (Opens in a new window) and heat-producing elements.[1] (Opens in a new window)

Consistent with this perspective, geochemical mapping made from orbit suggests the crust of mostly anorthosite (Opens in a new window).[12] (Opens in a new window) The Moon rock (Opens in a new window) 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] (Opens in a new window) The crust is on average about 50 kilometres (31 mi) thick.[1] (Opens in a new window)

The Moon is the second-densest satellite in the Solar System, after Io (Opens in a new window).[59] (Opens in a new window) However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less,[1] (Opens in a new window) 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.[60] (Opens in a new window)

Surface geology

Main articles: Topography of the Moon (Opens in a new window), Geology of the Moon (Opens in a new window), Moon rock (Opens in a new window), and List of lunar features (Opens in a new window)

The Topographic Globe of the Moon

Geological features of the Moon (near side / north pole at left, far side / south pole at right)

Topography of the Moon

STL 3D model (Opens in a new window) of the Moon with 10× elevation exaggeration rendered with data from the Lunar Orbiter Laser Altimeter of the Lunar Reconnaissance Orbiter (Opens in a new window)

The topography of the Moon (Opens in a new window) has been measured with laser altimetry (Opens in a new window) and stereo image analysis (Opens in a new window).[61] (Opens in a new window) Its most visible topographic feature (Opens in a new window) is the giant far-side South Pole–Aitken basin (Opens in a new window), 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 (Opens in a new window).[62] (Opens in a new window)[63] (Opens in a new window) At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon.[62] (Opens in a new window)[64] (Opens in a new window) 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.[65] (Opens in a new window) Other large impact basins such as Imbrium (Opens in a new window), Serenitatis (Opens in a new window), Crisium (Opens in a new window), Smythii (Opens in a new window), and Orientale (Opens in a new window) also possess regionally low elevations and elevated rims.[62] (Opens in a new window) The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side.[1] (Opens in a new window)

The discovery of fault scarp (Opens in a new window) cliffs by the Lunar Reconnaissance Orbiter (Opens in a new window) suggest that the Moon has shrunk within the past billion years, by about 90 metres (300 ft).[66] (Opens in a new window) Similar shrinkage features exist on Mercury (Opens in a new window). 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.[67] (Opens in a new window)

Volcanic features

Main article: Volcanism on the Moon (Opens in a new window)

Lunar nearside (Opens in a new window) with major maria (Opens in a new window) and craters (Opens in a new window) labeled

The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria (Opens in a new window) (Latin (Opens in a new window) for "seas"; singular mare), as they were once believed to be filled with water;[68] (Opens in a new window) they are now known to be vast solidified pools of ancient basaltic (Opens in a new window) lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water.[69] (Opens in a new window) The majority of these lavas erupted or flowed into the depressions associated with impact basins (Opens in a new window). Several geologic provinces (Opens in a new window) containing shield volcanoes (Opens in a new window) and volcanic domes (Opens in a new window) are found within the near side "maria".[70] (Opens in a new window)

Evidence of young lunar volcanism (Opens in a new window)

Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side,[71] (Opens in a new window) compared with 2% of the far side.[72] (Opens in a new window) This is thought to be due to a concentration of heat-producing elements (Opens in a new window) under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector (Opens in a new window)'s gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.[58] (Opens in a new window)[73] (Opens in a new window)[74] (Opens in a new window) Most of the Moon's mare basalts (Opens in a new window) 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.[75] (Opens in a new window) Until recently, the youngest eruptions, dated by crater counting (Opens in a new window), appeared to have been only 1.2 billion years ago.[76] (Opens in a new window) In 2006, a study of Ina (Opens in a new window), a tiny depression in Lacus Felicitatis (Opens in a new window), found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old.[77] (Opens in a new window) Moonquakes (Opens in a new window) and releases of gas also indicate some continued lunar activity.[77] (Opens in a new window) In 2014 NASA announced "widespread evidence of young lunar volcanism" at 70 irregular mare patches (Opens in a new window) 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.[78] (Opens in a new window)[79] (Opens in a new window)[80] (Opens in a new window)[81] (Opens in a new window) Just prior to this, evidence has been presented for 2–10 million years younger basaltic volcanism inside the crater Lowell,[82] (Opens in a new window)[83] (Opens in a new window) 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.[84] (Opens in a new window)[85] (Opens in a new window)

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 (Opens in a new window)cumulates (Opens in a new window) of the lunar magma ocean.[75] (Opens in a new window)[76] (Opens in a new window) In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[86] (Opens in a new window)

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.[87] (Opens in a new window)[88] (Opens in a new window)

Impact craters

Further information: List of craters on the Moon (Opens in a new window)

Lunar crater Daedalus (Opens in a new window) on the Moon's far side (Opens in a new window)

The other major geologic process that has affected the Moon's surface is impact cratering (Opens in a new window),[89] (Opens in a new window) 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.[90] (Opens in a new window) The lunar geologic timescale (Opens in a new window) is based on the most prominent impact events, including Nectaris (Opens in a new window), Imbrium (Opens in a new window), and Orientale (Opens in a new window), 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 (Opens in a new window).[91] (Opens in a new window) 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 (Opens in a new window) 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.[91] (Opens in a new window) The radiometric ages of impact-melted rocks collected during the Apollo missions (Opens in a new window) cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment (Opens in a new window) of impacts.[92] (Opens in a new window)

Blanketed on top of the Moon's crust is a highly comminuted (Opens in a new window) (broken into ever smaller particles) and impact gardened (Opens in a new window) surface layer called regolith (Opens in a new window), formed by impact processes. The finer regolith, the lunar soil (Opens in a new window) of silicon dioxide (Opens in a new window) glass, has a texture resembling snow and a scent resembling spent gunpowder (Opens in a new window).[93] (Opens in a new window)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.[94] (Opens in a new window) Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick.[95] (Opens in a new window)

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 (Opens in a new window) 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.[96] (Opens in a new window)[97] (Opens in a new window)

Lunar swirls at Reiner Gamma (Opens in a new window)

Lunar swirls

Main article: Lunar swirls (Opens in a new window)

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

Main article: Lunar water (Opens in a new window)

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation (Opens in a new window) and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets (Opens in a new window) or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind (Opens in a new window), leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon.[98] (Opens in a new window)[99] (Opens in a new window) Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow.[100] (Opens in a new window) The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation (Opens in a new window) as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.[101] (Opens in a new window)

In years since, signatures of water have been found to exist on the lunar surface.[102] (Opens in a new window) In 1994, the bistatic radar experiment (Opens in a new window) located on the Clementine (Opens in a new window) spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo (Opens in a new window), suggest these findings may rather be rocks ejected from young impact craters.[103] (Opens in a new window) In 1998, the neutron spectrometer (Opens in a new window) 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.[104] (Opens in a new window) Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior.[105] (Opens in a new window)

The 2008 Chandrayaan-1 (Opens in a new window) spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper (Opens in a new window). The spectrometer observed absorption lines common to hydroxyl (Opens in a new window), 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 (Opens in a new window).[106] (Opens in a new window) Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018.[107] (Opens in a new window) In 2009, LCROSS (Opens in a new window) sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed (Opens in a new window) polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material.[108] (Opens in a new window)[109] (Opens in a new window) Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb).[110] (Opens in a new window)

In May 2011, 615–1410 ppm water in melt inclusions (Opens in a new window) in lunar sample 74220 was reported,[111] (Opens in a new window) the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 (Opens in a new window) 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 (Opens in a new window). 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.[112] (Opens in a new window)[113] (Opens in a new window) The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances.[114] (Opens in a new window) 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.[112] (Opens in a new window)[114] (Opens in a new window)

In October 2020, astronomers reported detecting molecular water (Opens in a new window) on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (Opens in a new window) (SOFIA).[115] (Opens in a new window)[116] (Opens in a new window)[117] (Opens in a new window)[118] (Opens in a new window)

Gravitational field

Main article: Gravity of the Moon (Opens in a new window)

GRAIL (Opens in a new window)'s gravity map of the Moon

The gravitational field (Opens in a new window) of the Moon has been measured through tracking the Doppler shift (Opens in a new window) of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons (Opens in a new window), 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.[119] (Opens in a new window)[120] (Opens in a new window) 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.[121] (Opens in a new window)

Magnetic field

Main article: Magnetic field of the Moon (Opens in a new window)

The Moon has an external magnetic field (Opens in a new window) of generally less than 0.2 nanoteslas (Opens in a new window),[122] (Opens in a new window) or less than one hundred thousandth that of Earth (Opens in a new window). The Moon does not currently have a global dipolar (Opens in a new window) magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating.[123] (Opens in a new window)[124] (Opens in a new window) However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today.[122] (Opens in a new window) This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized.[122] (Opens in a new window) 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 (Opens in a new window) of the giant impact basins.[125] (Opens in a new window)

Atmosphere

Main article: Atmosphere of the Moon (Opens in a new window)

Sketch by the Apollo 17 astronauts. The lunar atmosphere was later studied by LADEE (Opens in a new window).[126] (Opens in a new window)[127] (Opens in a new window)

The Moon has an atmosphere (Opens in a new window) so tenuous as to be nearly vacuum (Opens in a new window), with a total mass of less than 10 tonnes (9.8 long tons; 11 short tons).[128] (Opens in a new window) The surface pressure of this small mass is around 3 × 10−15 atm (Opens in a new window) (0.3 nPa (Opens in a new window)); it varies with the lunar day. Its sources include outgassing (Opens in a new window) and sputtering (Opens in a new window), a product of the bombardment of lunar soil by solar wind ions.[12] (Opens in a new window)[129] (Opens in a new window) Elements that have been detected include sodium (Opens in a new window) and potassium (Opens in a new window), produced by sputtering (also found in the atmospheres of Mercury (Opens in a new window) and Io (Opens in a new window)); helium-4 (Opens in a new window) and neon (Opens in a new window)[130] (Opens in a new window) from the solar wind; and argon-40 (Opens in a new window), radon-222 (Opens in a new window), and polonium-210 (Opens in a new window), outgassed after their creation by radioactive decay (Opens in a new window) within the crust and mantle.[131] (Opens in a new window)[132] (Opens in a new window) The absence of such neutral species (atoms or molecules) as oxygen (Opens in a new window), nitrogen (Opens in a new window), carbon (Opens in a new window), hydrogen (Opens in a new window) and magnesium (Opens in a new window), which are present in the regolith (Opens in a new window), is not understood.[131] (Opens in a new window) Water vapor has been detected by Chandrayaan-1 (Opens in a new window) and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation (Opens in a new window) of water ice in the regolith.[133] (Opens in a new window) 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.[131] (Opens in a new window)

Dust

A permanent asymmetric Moon dust (Opens in a new window) 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 (Opens in a new window)'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 (Opens in a new window), Quadrantid (Opens in a new window), Northern Taurid (Opens in a new window), and Omicron Centaurid (Opens in a new window) meteor showers (Opens in a new window), 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.[134] (Opens in a new window)[135] (Opens in a new window)

Past thicker atmosphere

In October 2017, NASA scientists at the Marshall Space Flight Center (Opens in a new window) and the Lunar and Planetary Institute (Opens in a new window) in Houston (Opens in a new window) announced their finding, based on studies of Moon magma samples retrieved by the Apollo (Opens in a new window) 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 (Opens in a new window). The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.[136] (Opens in a new window)

Seasons

The Moon's axial tilt (Opens in a new window) with respect to the ecliptic (Opens in a new window) is only 1.5424°,[137] (Opens in a new window) 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.[138] (Opens in a new window) From images taken by Clementine (Opens in a new window) in 1994, it appears that four mountainous regions on the rim of the crater Peary (Opens in a new window) at the Moon's north pole may remain illuminated for the entire lunar day (Opens in a new window), creating peaks of eternal light (Opens in a new window). No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters,[100] (Opens in a new window) and these "craters of eternal darkness (Opens in a new window)" are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F)[139] (Opens in a new window) and just 26 K (−247 °C; −413 °F) close to the winter solstice in the north polar crater Hermite (Opens in a new window). This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto (Opens in a new window).[138] (Opens in a new window) 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.[140] (Opens in a new window)

Rotation

The Moon is rotating around its own axis. This rotation is due to tidal locking (Opens in a new window) synchronous to its orbital period (Opens in a new window) around Earth.

The rotation period (Opens in a new window) depends on the frame of reference. There are sidereal rotation periods (or sidereal day (Opens in a new window), in relation to the stars), and synodic rotation periods (or synodic day (Opens in a new window), in relation to the Sun). A lunar day (Opens in a new window) 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.[141] (Opens in a new window)

Earth–Moon system

Lunar distance

Main article: Lunar distance (astronomy) (Opens in a new window)

This section is an excerpt from Lunar distance (astronomy) (Opens in a new window)[edit (Opens in a new window)]

Lunar distance (Opens in a new window) (LD or ), also called Earth–Moon distance, Earth–Moon characteristic distance, or distance to the Moon, is a unit of measure (Opens in a new window) in astronomy (Opens in a new window). It is the average distance from the center of Earth (Opens in a new window) to the center of the Moon. More technically, it is the mean semi-major axis (Opens in a new window) of the geocentric lunar orbit (Opens in a new window). It may also refer to the time-averaged distance between the centers of the Earth and the Moon, or less commonly, the instantaneous Earth–Moon distance. The lunar distance is approximately 400,000 km (Opens in a new window), which is a quarter of a million miles (Opens in a new window) or 1.28 light-seconds (Opens in a new window). This is roughly Earth's circumference (Opens in a new window) times ten, its diameter (Opens in a new window) times thirty or 1/389 of Earth's distance to the Sun (the astronomical unit (Opens in a new window)).A lunar distance, 384402 km (Opens in a new window), is the Moon's average distance to Earth. The actual distance varies over the course of its orbit (Opens in a new window). The image compares the Moon's apparent size (Opens in a new window) when it is closest (Opens in a new window)and farthest from Earth.

Scale model (Opens in a new window) of the Earth–Moon system: Sizes and distances are to scale.

Minimum, mean and maximum distances of the Moon from Earth with its angular diameter as seen from Earth's surface, to scale

Orbit

Main articles: Orbit of the Moon (Opens in a new window) and Lunar theory (Opens in a new window)

Animation of Moon's orbit around Earth from 2018 to 2027
Moon · Earth (Opens in a new window)

Earth–Moon system (schematic)

DSCOVR satellite (Opens in a new window) sees the Moon passing in front of Earth

The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days[g] (Opens in a new window) (its sidereal period (Opens in a new window)). 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 (Opens in a new window) to Earth, which is about 29.5 days[h] (Opens in a new window) (its synodic period (Opens in a new window)).[71] (Opens in a new window) Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane (Opens in a new window) than to the planet's equatorial plane (Opens in a new window). The Moon's orbit is subtly perturbed (Opens in a new window) by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates (Opens in a new window) once every 18.61 years,[142] (Opens in a new window) which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws (Opens in a new window).[143] (Opens in a new window)

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.[71] (Opens in a new window) It is the largest moon in the Solar System relative to the size of its planet,[i] (Opens in a new window) though Charon (Opens in a new window) is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass.[j] (Opens in a new window)[144] (Opens in a new window) The Earth and the Moon's barycentre (Opens in a new window), 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 (Opens in a new window) combined.

Appearance from Earth

A full moon appears as a half moon during an eclipse moonset over the High Desert (Opens in a new window) in California, on the morning of the Trifecta: Full moon (Opens in a new window), Supermoon (Opens in a new window), Lunar eclipse (Opens in a new window), January 2018 lunar eclipse (Opens in a new window)

The Moon is in synchronous rotation (Opens in a new window) as it orbits (Opens in a new window) 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 (Opens in a new window), 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 (Opens in a new window), and the opposite the far side (Opens in a new window). 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 (Opens in a new window), the near side is dark.[145] (Opens in a new window)

The Moon had once rotated at a faster rate, but early in its history its rotation slowed and became tidally locked (Opens in a new window) in this orientation as a result of frictional (Opens in a new window) effects associated with tidal (Opens in a new window) deformations caused by Earth.[146] (Opens in a new window) 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 (Opens in a new window) 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.[147] (Opens in a new window)

The Moon is prominently featured in Vincent van Gogh (Opens in a new window)'s 1889 painting, The Starry Night (Opens in a new window)

The Moon has an exceptionally low albedo (Opens in a new window), giving it a reflectance (Opens in a new window) that is slightly brighter than that of worn asphalt (Opens in a new window). Despite this, it is the brightest object in the sky after the Sun (Opens in a new window).[71] (Opens in a new window)[k] (Opens in a new window) This is due partly to the brightness enhancement of the opposition surge (Opens in a new window); the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon (Opens in a new window).[148] (Opens in a new window) Additionally, color constancy (Opens in a new window) in the visual system (Opens in a new window) 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 (Opens in a new window), because of the reflective properties (Opens in a new window) of lunar soil (Opens in a new window), which retroreflects (Opens in a new window) 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 (Opens in a new window), first described in the 7th century BC.[149] (Opens in a new window) The full Moon's angular diameter (Opens in a new window) is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see § Eclipses (Opens in a new window)).

14 November 2016 supermoon (Opens in a new window) was 356,511 kilometres (221,526 mi) away[152] (Opens in a new window) from the center of Earth, the closest occurrence since 26 January 1948. It will not be closer until 25 November 2034.[153] (Opens in a new window)

The distance between the Moon and Earth (Opens in a new window) varies from around 356,400 km (221,500 mi) to 406,700 km (252,700 mi) at perigee (Opens in a new window) (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.[154] (Opens in a new window) Reported as a "supermoon (Opens in a new window)", 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 .[155] (Opens in a new window)[156] (Opens in a new window)[157] (Opens in a new window) At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:[158] (Opens in a new window)[159] (Opens in a new window)

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.[160] (Opens in a new window)

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 (Opens in a new window), or inadequate drawings. However, outgassing (Opens in a new window) does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena (Opens in a new window). 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.[161] (Opens in a new window)[162] (Opens in a new window)

The Moon's appearance, like the Sun's, can be affected by Earth's atmosphere (Opens in a new window). Common optical effects are the 22° halo ring (Opens in a new window), formed when the Moon's light is refracted through the ice crystals (Opens in a new window) of high cirrostratus (Opens in a new window) clouds, and smaller coronal rings (Opens in a new window) when the Moon is seen through thin clouds.[163] (Opens in a new window)

The monthly changes in the angle between the direction of sunlight and view from Earth, and the phases of the Moon (Opens in a new window) that result, as viewed from the Northern Hemisphere (Opens in a new window). The Earth–Moon distance (Opens in a new window) is not to scale.

The illuminated area of the visible sphere (degree of illumination) is given by , where is the elongation (Opens in a new window) (i.e., the angle between Moon, the observer (on Earth) and the Sun).

Tidal effects

Main articles: Tidal force (Opens in a new window), Tidal acceleration (Opens in a new window), Tide (Opens in a new window), and Theory of tides (Opens in a new window)

The libration (Opens in a new window) of the Moon over a single lunar month. Also visible is the slight variation in the Moon's visual size from Earth.

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 (Opens in a new window). 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 (Opens in a new window).[164] (Opens in a new window) 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.[164] (Opens in a new window) 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 (Opens in a new window).[164] (Opens in a new window) 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 (Opens in a new window) of water's movement, ocean basins that grow shallower near land, the sloshing of water between different ocean basins.[165] (Opens in a new window) 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 (Opens in a new window) in opposition to the Earth's rotation. This "drains" angular momentum (Opens in a new window) and rotational kinetic energy (Opens in a new window) from Earth's rotation, slowing the Earth's rotation.[164] (Opens in a new window)[166] (Opens in a new window) That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration (Opens in a new window)), 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 (Opens in a new window), and the Earth's rotation is slowing in reaction.[166] (Opens in a new window) Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments (Opens in a new window)) have found that the Moon's distance increases by 38 mm (1.5 in) per year[167] (Opens in a new window) (roughly the rate at which human fingernails grow).[168] (Opens in a new window) Atomic clocks (Opens in a new window)also show that Earth's day lengthens by about 15 microseconds (Opens in a new window) every year,[169] (Opens in a new window) slowly increasing the rate at which UTC (Opens in a new window) is adjusted by leap seconds (Opens in a new window). Left to run its course, 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. As a result, the Moon would be suspended in the sky over one meridian, as is already currently the case with Pluto and its moon Charon. However, the Sun will become a red giant (Opens in a new window) engulfing the Earth-Moon system long before this occurrence.[170] (Opens in a new window)[171] (Opens in a new window) If it were to 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 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. Eventually it would come within the Roche limit (Opens in a new window) and be broken up into a ring (Opens in a new window).

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 (Opens in a new window), and a varying component from the Sun.[166] (Opens in a new window) The Earth-induced component arises from libration (Opens in a new window), a result of the Moon's orbital eccentricity (if the Moon's orbit were perfectly circular, there would only be solar tides).[166] (Opens in a new window) 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.[71] (Opens in a new window) The cumulative effects of stress built up by these tidal forces produces moonquakes (Opens in a new window). 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 (Opens in a new window) placed on the Moon by Apollo (Opens in a new window) astronauts (Opens in a new window) from 1969 through 1972.[172] (Opens in a new window)

According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining Earth's magnetic field (Opens in a new window).[173] (Opens in a new window)

Eclipses

Main articles: Solar eclipse (Opens in a new window), Lunar eclipse (Opens in a new window), and Eclipse cycle (Opens in a new window)

From Earth, the Moon and the Sun appear the same size, as seen in the 1999 solar eclipse (Opens in a new window) (left), whereas from the STEREO-B (Opens in a new window) spacecraft in an Earth-trailing orbit, the Moon appears much smaller than the Sun (right).[174] (Opens in a new window)

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy (Opens in a new window)"). Solar eclipses (Opens in a new window) occur at new moon (Opens in a new window), when the Moon is between the Sun and Earth. In contrast, lunar eclipses (Opens in a new window) 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 (Opens in a new window) (with the Moon appearing larger than the Sun) and annular (Opens in a new window) (with the Moon appearing smaller than the Sun) solar eclipses.[175] (Opens in a new window) In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona (Opens in a new window) becomes visible to the naked eye (Opens in a new window). Because the distance between the Moon and Earth is very slowly increasing over time,[164] (Opens in a new window) the angular diameter of the Moon is decreasing. Also, as it evolves toward becoming a red giant (Opens in a new window), the size of the Sun, and its apparent diameter in the sky, are slowly increasing.[l] (Opens in a new window)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.[176] (Opens in a new window)

Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun (Opens in a new window), 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.[177] (Opens in a new window) The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros (Opens in a new window), which has a period of approximately 18 years.[178] (Opens in a new window)

Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,[m] (Opens in a new window)[179] (Opens in a new window) the related phenomenon of occultation (Opens in a new window) 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 (Opens in a new window) of the lunar orbit, each year different stars are occulted.[180] (Opens in a new window)

Observation and exploration

Main articles: Exploration of the Moon (Opens in a new window), List of spacecraft that orbited the Moon (Opens in a new window), List of missions to the Moon (Opens in a new window), and List of lunar probes (Opens in a new window)

Before spaceflight

Main article: Exploration of the Moon: Before spaceflight (Opens in a new window)

Map of the Moon by Johannes Hevelius (Opens in a new window) from his Selenographia (Opens in a new window) (1647), the first map to include the libration (Opens in a new window)zones

A study of the Moon in Robert Hooke's (Opens in a new window) Micrographia (Opens in a new window), 1665

One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 (Opens in a new window) at Knowth (Opens in a new window), Ireland.[181] (Opens in a new window)[182] (Opens in a new window)

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC, Babylonian astronomers (Opens in a new window) had recorded the 18-year Saros cycle (Opens in a new window) of lunar eclipses (Opens in a new window),[183] (Opens in a new window) and Indian astronomers (Opens in a new window) had described the Moon's monthly elongation.[184] (Opens in a new window) The Chinese astronomer (Opens in a new window) Shi Shen (Opens in a new window)(fl. 4th century BC) gave instructions for predicting solar and lunar eclipses.[185] (Opens in a new window)(p411) Later, the physical form of the Moon and the cause of moonlight (Opens in a new window)became understood. The ancient Greek (Opens in a new window) philosopher Anaxagoras (Opens in a new window) (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.[186] (Opens in a new window)[185] (Opens in a new window)(p227) Although the Chinese of the Han Dynasty (Opens in a new window) believed the Moon to be energy equated to qi (Opens in a new window), their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (Opens in a new window) (78–37 BC) noted the sphericity of the Moon.[185] (Opens in a new window)(pp413–414) In the 2nd century AD, Lucian (Opens in a new window) wrote the novel A True Story (Opens in a new window), in which the heroes travel to the Moon and meet its inhabitants. In 499 AD, the Indian astronomer Aryabhata (Opens in a new window) mentioned in his Aryabhatiya (Opens in a new window) that reflected sunlight is the cause of the shining of the Moon.[187] (Opens in a new window) The astronomer and physicist Alhazen (Opens in a new window) (965–1039) found that sunlight (Opens in a new window) 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.[188] (Opens in a new window) Shen Kuo (Opens in a new window) (1031–1095) of the Song dynasty (Opens in a new window) 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.[185] (Opens in a new window)(pp415–416)

Galileo (Opens in a new window)'s sketches of the Moon from Sidereus Nuncius (Opens in a new window)

In Aristotle (Opens in a new window)'s (384–322 BC) description of the universe (Opens in a new window), the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether (Opens in a new window), an influential philosophy (Opens in a new window) that would dominate for centuries.[189] (Opens in a new window) However, in the 2nd century BC, Seleucus of Seleucia (Opens in a new window) correctly theorized that tides (Opens in a new window) were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun (Opens in a new window).[190] (Opens in a new window) In the same century, Aristarchus (Opens in a new window) computed the size and distance (Opens in a new window) of the Moon from Earth, obtaining a value of about twenty times the radius of Earth (Opens in a new window) for the distance. These figures were greatly improved by Ptolemy (Opens in a new window) (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.[191] (Opens in a new window) Archimedes (Opens in a new window) (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System.[192] (Opens in a new window)

During the Middle Ages (Opens in a new window), before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".[193] (Opens in a new window)

In 1609, Galileo Galilei (Opens in a new window) drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius (Opens in a new window) and noted that it was not smooth but had mountains and craters. Thomas Harriot (Opens in a new window) 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 (Opens in a new window) and Francesco Maria Grimaldi (Opens in a new window) led to the system of naming of lunar features in use today. The more exact 1834–36 Mappa Selenographica of Wilhelm Beer (Opens in a new window) and Johann Heinrich Mädler (Opens in a new window), and their associated 1837 book Der Mond, the first trigonometrically (Opens in a new window) 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.[194] (Opens in a new window) Lunar craters, first noted by Galileo, were thought to be volcanic (Opens in a new window) until the 1870s proposal of Richard Proctor (Opens in a new window)that they were formed by collisions.[71] (Opens in a new window) This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert (Opens in a new window), and from comparative studies from 1920 to the 1940s,[195] (Opens in a new window) leading to the development of lunar stratigraphy (Opens in a new window), which by the 1950s was becoming a new and growing branch of astrogeology (Opens in a new window).[71] (Opens in a new window)

1959–1970s

Between the first human arrival with the robotic Soviet (Opens in a new window) Luna program (Opens in a new window) in 1958, to the 1970s with the last Missions of the crewed U.S. (Opens in a new window) Apollo landings (Opens in a new window) and last Luna mission in 1976, the Cold War (Opens in a new window)-inspired Space Race (Opens in a new window) between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon (Opens in a new window). Once launchers had the necessary capabilities, these nations sent uncrewed probes on both flyby and impact/lander missions.

Soviet missions

Main articles: Luna program (Opens in a new window) and Lunokhod programme (Opens in a new window)

First view in history of the far side of the Moon, taken by Luna 3 (Opens in a new window), 7 October 1959

A model of Soviet Moon rover Lunokhod 1 (Opens in a new window)

Spacecraft from the Soviet Union's Luna program (Opens in a new window) were the first to accomplish a number of goals: following three unnamed, failed missions in 1958,[196] (Opens in a new window) the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1 (Opens in a new window); the first human-made object to impact the lunar surface was Luna 2 (Opens in a new window), and the first photographs of the normally occluded far side of the Moon were made by Luna 3 (Opens in a new window), all in 1959.

Stamp with a drawing of the first soft landed probe Luna 9 (Opens in a new window), next to the first view of the lunar surface photographed by the probe

Luna 24 was the last Soviet mission to the Moon.

United States missions

Main articles: Apollo program (Opens in a new window) and Moon landing (Opens in a new window)

Earthrise (Opens in a new window) (Apollo 8 (Opens in a new window), 1968, taken by William Anders (Opens in a new window))

Moon rock (Opens in a new window) (Apollo 17 (Opens in a new window), 1972)

During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study (Opens in a new window) that proposed the construction of a staffed military outpost on the Moon called Project Horizon (Opens in a new window) with the potential to conduct a wide range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test.[198] (Opens in a new window)[199] (Opens in a new window) The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex (Opens in a new window).[200] (Opens in a new window)[201] (Opens in a new window)[198] (Opens in a new window) However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA.[201] (Opens in a new window)

Following President John F. Kennedy (Opens in a new window)'s 1961 commitment to a human moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory (Opens in a new window)'s Ranger program (Opens in a new window) produced the first close-up pictures; the Lunar Orbiter program (Opens in a new window) produced maps of the entire Moon; the Surveyor program (Opens in a new window)landed its first spacecraft (Opens in a new window) four months after Luna 9. The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing (Opens in a new window), in 1968 Apollo 8 (Opens in a new window) made the first human mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race.[202] (Opens in a new window)

Neil Armstrong (Opens in a new window) working at the Lunar Module Eagle (Opens in a new window) during Apollo 11 (Opens in a new window) (1969)

"That's one small step ..." (Opens in a new window)

0:00

Problems playing this file? See media help (Opens in a new window).

Neil Armstrong (Opens in a new window) became the first person to walk on the Moon as the commander of the American mission Apollo 11 (Opens in a new window) by first setting foot on the Moon at 02:56 UTC on 21 July 1969.[203] (Opens in a new window) An estimated 500 million people worldwide watched the transmission by the Apollo TV camera (Opens in a new window), the largest television audience for a live broadcast at that time.[204] (Opens in a new window)[205] (Opens in a new window) The Apollo missions 11 to 17 (except Apollo 13 (Opens in a new window), which aborted its planned lunar landing) removed 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196 separate samples.[206] (Opens in a new window) The American Moon landing (Opens in a new window) and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation (Opens in a new window) chemistry, software engineering (Opens in a new window), and atmospheric re-entry (Opens in a new window) technology, and by highly competent management of the enormous technical undertaking.[207] (Opens in a new window)[208] (Opens in a new window)

Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations (Opens in a new window), including heat flow probes, seismometers (Opens in a new window), and magnetometers (Opens in a new window), were installed at the Apollo 12 (Opens in a new window), 14 (Opens in a new window), 15 (Opens in a new window), 16 (Opens in a new window), and 17 (Opens in a new window) landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations,[209] (Opens in a new window)[210] (Opens in a new window)but as the stations' lunar laser ranging (Opens in a new window) corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimeters, and data from this experiment are being used to place constraints on the size of the lunar core.[211] (Opens in a new window)

1970s – present

An artificially (Opens in a new window) colored mosaic constructed from a series of 53 images taken through three spectral filters (Opens in a new window) by Galileo' s imaging system as the spacecraft flew over the northern regions of the Moon on 7 December 1992.

After the Moon race the focus of astronautic exploration shifted in the 1970s with probes like Pioneer 10 (Opens in a new window) and the Voyager program (Opens in a new window) towards the outer solar system (Opens in a new window). Years of near lunar quietude followed, only broken by a beginning internationalization of space and the Moon through for example the negotiation of the Moon treaty (Opens in a new window).

Since the 1990s, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten (Opens in a new window) spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission.[212] (Opens in a new window) In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine (Opens in a new window) to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral (Opens in a new window) images of the lunar surface.[213] (Opens in a new window) This was followed in 1998 by the Lunar Prospector (Opens in a new window) 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.[214] (Opens in a new window)

As viewed by Chandrayaan-1 (Opens in a new window)'s NASA Moon Mineralogy Mapper equipment, on the right, the first time discovered water-rich minerals (light blue), shown around a small crater from which it was ejected.

The European spacecraft SMART-1 (Opens in a new window), the second ion-propelled (Opens in a new window) spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface.[215] (Opens in a new window)

The ambitious Chinese Lunar Exploration Program (Opens in a new window) began with Chang'e 1 (Opens in a new window), which successfully orbited the Moon from 5 November 2007 until its controlled lunar impact on 1 March 2009.[216] (Opens in a new window) It obtained a full image map of the Moon. Chang'e 2 (Opens in a new window), beginning in October 2010, reached the Moon more quickly, mapped the Moon at a higher resolution over an eight-month period, then left lunar orbit for an extended stay at the Earth–Sun L2 Lagrangian point (Opens in a new window), before finally performing a flyby of asteroid 4179 Toutatis (Opens in a new window) on 13 December 2012, and then heading off into deep space. On 14 December 2013, Chang'e 3 (Opens in a new window) landed a lunar lander (Opens in a new window) onto the Moon's surface, which in turn deployed a lunar rover (Opens in a new window), named Yutu (Opens in a new window) (Chinese: 玉兔; literally "Jade Rabbit"). This was the first lunar soft landing (Opens in a new window) since Luna 24 (Opens in a new window) in 1976, and the first lunar rover mission since Lunokhod 2 (Opens in a new window) in 1973. Another rover mission (Chang'e 4 (Opens in a new window)) was launched in 2019, becoming the first ever spacecraft to land on the Moon's far side. China intends to following this up with a sample return mission (Opens in a new window) (Chang'e 5 (Opens in a new window)) in 2020.[217] (Opens in a new window)

Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency (Opens in a new window)'s Kaguya (Opens in a new window) (Selene) mission, a lunar orbiter fitted with a high-definition video (Opens in a new window) camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.[218] (Opens in a new window)[219] (Opens in a new window) India's first lunar mission, Chandrayaan-1 (Opens in a new window), orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil (Opens in a new window).[220] (Opens in a new window) The Indian Space Research Organisation (Opens in a new window) planned to launch Chandrayaan-2 (Opens in a new window) in 2013, which would have included a Russian robotic lunar rover.[221] (Opens in a new window)[222] (Opens in a new window) However, the failure of Russia's Fobos-Grunt (Opens in a new window) mission has delayed this project, and was launched on 22 July 2019. The lander Vikram attempted to land on the lunar south pole region (Opens in a new window) on 6 September, but lost the signal in 2.1 km (1.3 mi). What happened after that is unknown.

Copernicus (Opens in a new window)'s central peaks as observed by the LRO (Opens in a new window), 2012

The Ina (Opens in a new window) formation, 2009

The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS (Opens in a new window) impactor and follow-up observation orbiter on 18 June 2009; LCROSScompleted its mission by making a planned and widely observed impact in the crater Cabeus (Opens in a new window) on 9 October 2009,[223] (Opens in a new window) whereas LRO is currently in operation, obtaining precise lunar altimetry (Opens in a new window) and high-resolution imagery. In November 2011, the LRO passed over the large and bright crater Aristarchus (Opens in a new window). NASA released photos of the crater on 25 December 2011.[224] (Opens in a new window)

Two NASA GRAIL (Opens in a new window) spacecraft began orbiting the Moon around 1 January 2012,[225] (Opens in a new window) on a mission to learn more about the Moon's internal structure. NASA's LADEE (Opens in a new window) probe, designed to study the lunar exosphere (Opens in a new window), achieved orbit on 6 October 2013.

Future

Upcoming lunar missions include Russia's Luna-Glob (Opens in a new window): an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt (Opens in a new window) mission.[226] (Opens in a new window) Privately funded lunar exploration has been promoted by the Google Lunar X Prize (Opens in a new window), announced 13 September 2007, which offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria.[227] (Opens in a new window) Shackleton Energy Company (Opens in a new window) is building a program to establish operations on the south pole of the Moon to harvest water and supply their Propellant Depots (Opens in a new window).[228] (Opens in a new window)

NASA began to plan to resume human missions (Opens in a new window) following the call by U.S. President George W. Bush (Opens in a new window) on 14 January 2004 for a human mission to the Moon by 2019 and the construction of a lunar base by 2024.[229] (Opens in a new window) The Constellation program (Opens in a new window) was funded and construction and testing begun on a crewed spacecraft (Opens in a new window) and launch vehicle (Opens in a new window),[230] (Opens in a new window) and design studies for a lunar base.[231] (Opens in a new window) However, that program has been canceled in favor of a human asteroid landing by 2025 and a human Mars orbit by 2035.[232] (Opens in a new window) India (Opens in a new window) has also expressed its hope to send people to the Moon by 2020.[233] (Opens in a new window)

On 28 February 2018, SpaceX (Opens in a new window), Vodafone (Opens in a new window), Nokia (Opens in a new window) and Audi (Opens in a new window) announced a collaboration to install a 4G (Opens in a new window) wireless communication network on the Moon, with the aim of streaming live footage on the surface to Earth.[234] (Opens in a new window)

Recent reports also indicate NASA's intent to send a woman astronaut to the Moon in their planned mid-2020s mission.[235] (Opens in a new window)

Planned commercial missions

In 2007, the X Prize Foundation together with Google (Opens in a new window) launched the Google Lunar X Prize (Opens in a new window) to encourage commercial endeavors to the Moon. A prize of $20 million was to be awarded to the first private venture to get to the Moon with a robotic lander by the end of March 2018, with additional prizes worth $10 million for further milestones.[236] (Opens in a new window)[237] (Opens in a new window) As of August 2016, 16 teams were reportedly participating in the competition.[238] (Opens in a new window) In January 2018 the foundation announced that the prize would go unclaimed as none of the finalist teams would be able to make a launch attempt by the deadline.[239] (Opens in a new window)

In August 2016, the US government granted permission to US-based start-up Moon Express to land on the Moon.[240] (Opens in a new window) This marked the first time that a private enterprise was given the right to do so. The decision is regarded as a precedent helping to define regulatory standards for deep-space commercial activity in the future, as thus far companies' operation had been restricted to being on or around Earth.[240] (Opens in a new window)

On 29 November 2018 NASA announced that nine commercial companies would compete to win a contract to send small payloads to the Moon in what is known as Commercial Lunar Payload Services (Opens in a new window). According to NASA administrator Jim Bridenstine (Opens in a new window), "We are building a domestic American capability to get back and forth to the surface of the moon.".[241] (Opens in a new window)

Human presence

Human impact

Remains of human activity, Apollo 17's Lunar Surface Experiments Package (Opens in a new window)

Beside the traces of human activity on the Moon, there have been some intended permanent installations like the Moon Museum (Opens in a new window) art piece, Apollo 11 goodwill messages (Opens in a new window), Lunar plaque (Opens in a new window), the Fallen Astronaut (Opens in a new window) memorial, and other artifacts.

Fallen Astronaut (Opens in a new window)

Infrastructure

Main article: Moonbase (Opens in a new window)

A photo of the still in use reflector of the Lunar Laser Ranging Experiment (Opens in a new window) of Apollo 11 (Opens in a new window).

Longterm missions continuing to be active are some orbiters such as the 2009 launched Lunar Reconnaissance Orbiter (Opens in a new window) surveiling the Moon for future missions, as well as some Landers such as the 2013 launched Chang'e 3 (Opens in a new window) with its Lunar Ultraviolet Telescope still operational.[242] (Opens in a new window)

There are several missions by different agencies and companies planned (Opens in a new window) to establish a longterm human presence on the Moon, with the Lunar Gateway (Opens in a new window) as the currently most advanced project as part of the Artemis program (Opens in a new window).

Concept art of the Lunar Gateway (Opens in a new window) of the Artemis program (Opens in a new window) in 2024 serving as a communication hub, science laboratory, short-term habitation and holding area for rovers in lunar orbit (Opens in a new window).[243] (Opens in a new window)

Astronomy from the Moon

A false-color image of Earth (Opens in a new window) in ultraviolet light (Opens in a new window) taken from the surface of the Moon on the Apollo 16 (Opens in a new window) mission. The day-side reflects a large amount of UV light from the Sun, but the night-side shows faint bands of UV emission from the aurora (Opens in a new window) caused by charged particles.[244] (Opens in a new window)

For many years, the Moon has been recognized as an excellent site for telescopes.[245] (Opens in a new window) It is relatively nearby; astronomical seeing (Opens in a new window) is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes (Opens in a new window); and radio telescopes (Opens in a new window) on the far side would be shielded from the radio chatter of Earth.[246] (Opens in a new window) The lunar soil (Opens in a new window), although it poses a problem for any moving parts of telescopes (Opens in a new window), can be mixed with carbon nanotubes (Opens in a new window) and epoxies (Opens in a new window) and employed in the construction of mirrors up to 50 meters in diameter.[247] (Opens in a new window) A lunar zenith telescope (Opens in a new window) can be made cheaply with an ionic liquid (Opens in a new window).[248] (Opens in a new window)

In April 1972, the Apollo 16 (Opens in a new window) mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph (Opens in a new window).[249] (Opens in a new window)

Living on the Moon

Humans have stayed for days on the Moon, such as during Apollo 17.[250] (Opens in a new window) One particular challenge for astronauts' daily life during their stay on the surface is the lunar dust (Opens in a new window) 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".[251] (Opens in a new window) This contamination poses a danger since the fine lunar dust can cause health issues (Opens in a new window).[251] (Opens in a new window)

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 (Opens in a new window) in its Lunar Micro Ecosystem.[252] (Opens in a new window)

Legal status

Main article: Space law (Opens in a new window)

Although Luna (Opens in a new window) landers scattered pennants of the Soviet Union (Opens in a new window) on the Moon, and U.S. flags (Opens in a new window) were symbolically planted at their landing sites by the Apollo astronauts (Opens in a new window), no nation claims ownership of any part of the Moon's surface.[253] (Opens in a new window) Russia, China, India, and the U.S. are party to the 1967 Outer Space Treaty (Opens in a new window),[254] (Opens in a new window) which defines the Moon and all outer space as the "province of all mankind (Opens in a new window)".[253] (Opens in a new window) This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction (Opens in a new window).[255] (Opens in a new window) The 1979 Moon Agreement (Opens in a new window) was created to restrict the exploitation of the Moon's resources (Opens in a new window) by any single nation, but as of January 2020, it has been signed and ratified by only 18 nations,[256] (Opens in a new window) none of which engages in self-launched human space exploration (Opens in a new window). Although several individuals have made claims to the Moon (Opens in a new window) in whole or in part, none of these are considered credible.[257] (Opens in a new window)[258] (Opens in a new window)[259] (Opens in a new window)

In 2020, U.S. President Donald Trump (Opens in a new window) 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."[260] (Opens in a new window)[261] (Opens in a new window)

The Declaration of the Rights of the Moon (Opens in a new window) was created by a group of independent researchers in 2021, drawing on precedents in the Rights of Nature (Opens in a new window) movement and the concept of legal personality for non-human entities in space[262] (Opens in a new window).

In culture

Luna, the Moon, from a 1550 edition of Guido Bonatti (Opens in a new window)'s Liber astronomiae

Mythology

Further information: Lunar deity (Opens in a new window), Selene (Opens in a new window), Luna (goddess) (Opens in a new window), Man in the Moon (Opens in a new window), and Crescent (Opens in a new window)

Statue of Chandraprabha (Opens in a new window) (meaning "as charming as the moon"), the eighth Tirthankara (Opens in a new window) in Jainism (Opens in a new window), with the symbol of a crescent (Opens in a new window) moon below it

Sun and Moon with faces (1493 woodcut)

The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon (Opens in a new window), the rabbit (Opens in a new window)and the buffalo, among others. In many prehistoric and ancient cultures, the Moon was personified as a deity (Opens in a new window) or other supernatural (Opens in a new window) phenomenon, and astrological views of the Moon (Opens in a new window) continue to be propagated today.

In Mesopotamian iconography, the crescent (Opens in a new window) was the primary symbol of Nanna-Sîn.[265] (Opens in a new window) In ancient Greek art (Opens in a new window), the Moon goddess Selene (Opens in a new window) was represented wearing a crescent on her headgear in an arrangement reminiscent of horns.[266] (Opens in a new window)[267] (Opens in a new window) The star and crescent (Opens in a new window) 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 (Opens in a new window) or Hecate (Opens in a new window), and via the patronage of Hecate came to be used as a symbol of Byzantium (Opens in a new window).

An iconographic tradition of representing Sun and Moon with faces developed in the late medieval period.

The splitting of the moon (Opens in a new window) (Arabic (Opens in a new window): انشقاق القمر‎) is a miracle attributed to Muhammad (Opens in a new window).[268] (Opens in a new window) A song titled 'Moon Anthem' was released on the occasion of landing of India's Chandrayan-II on the Moon.[269] (Opens in a new window)

Calendar

Further information: Lunar calendar (Opens in a new window), Lunisolar calendar (Opens in a new window), Metonic cycle (Opens in a new window), Blue moon (Opens in a new window), and Movable feast (Opens in a new window)

The Moon's regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks (Opens in a new window), notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon.[270] (Opens in a new window)[271] (Opens in a new window)[272] (Opens in a new window) The ~30-day month is an approximation of the lunar cycle (Opens in a new window). 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 (Opens in a new window) among the Germanic peoples (Opens in a new window) (Germanic calendar (Opens in a new window)) prior to the adoption of a solar calendar (Opens in a new window).[273] (Opens in a new window) The PIE root (Opens in a new window) 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. (Opens in a new window) the English words measure and menstrual),[274] (Opens in a new window)[275] (Opens in a new window)[276] (Opens in a new window) and echoing the Moon's importance to many ancient cultures in measuring time (see Latin (Opens in a new window) mensis and Ancient Greek (Opens in a new window) μείς (meis) or μήν (mēn), meaning "month").[277] (Opens in a new window)[278] (Opens in a new window)[279] (Opens in a new window)[280] (Opens in a new window)Most historical calendars are lunisolar (Opens in a new window). The 7th-century Islamic calendar (Opens in a new window) is an exceptional example of a purely lunar calendar (Opens in a new window). Months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon.[281] (Opens in a new window)

Moonrise, 1884, painting by Stanisław Masłowski (Opens in a new window) (National Museum, Kraków (Opens in a new window), Gallery of Sukiennice Museum (Opens in a new window))

Lunar effect

Main article: Lunar effect (Opens in a new window)

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 (Opens in a new window) (popular shortening loony) are derived from the Latin name for the Moon, Luna. Philosophers Aristotle (Opens in a new window) and Pliny the Elder (Opens in a new window) 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.[282] (Opens in a new window) Even today, people who believe in a lunar effect (Opens in a new window) claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims.[282] (Opens in a new window)[283] (Opens in a new window)[284] (Opens in a new window)[285] (Opens in a new window)[286] (Opens in a new window)

Notes

^ (Opens in a new window) Between 18.29° and 28.58° to Earth's equator (Opens in a new window).[1] (Opens in a new window)
^ (Opens in a new window) There are a number of near-Earth asteroids (Opens in a new window), including 3753 Cruithne (Opens in a new window), that are co-orbital (Opens in a new window) 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 are quasi-satellites (Opens in a new window) – they are not moons as they do not orbit Earth. For more information, see Other moons of Earth (Opens in a new window).
^ (Opens in a new window) The maximum 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. The minimum value (for a distant new moon (Opens in a new window)) 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 the earthshine (Opens in a new window) onto such a new moon. The brightness of the earthshine is [ Earth albedo (Opens in a new window) × (Earth radius (Opens in a new window) / Radius of Moon's orbit (Opens in a new window))2 ] relative to the direct solar illumination that occurs for a full moon. (Earth albedo = 0.367; Earth radius = (polar radius × equatorial radius)½ = 6 367 km.)
^ (Opens in a new window) 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, the angular size (Opens in a new window) is 1896 arcseconds (Opens in a new window). 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.
^ (Opens in a new window) Lucey et al. (2006) give 107 particles cm−3 by day and 105 particles cm−3by night. Along with equatorial surface temperatures of 390 K (Opens in a new window) by day and 100 K by night, the ideal gas law (Opens in a new window) yields the pressures given in the infobox (rounded to the nearest order of magnitude (Opens in a new window)): 10−7 Pa (Opens in a new window) by day and 10−10 Pa by night.
^ (Opens in a new window) This age is calculated from isotope dating of lunar zircons.
^ (Opens in a new window) 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).
^ (Opens in a new window) 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).
^ (Opens in a new window) 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.
^ (Opens in a new window) With 27% the diameter and 60% the density of Earth, the Moon has 1.23% of the mass of Earth. The moon Charon (Opens in a new window) is larger relative to its primary Pluto (Opens in a new window), but Pluto is now considered to be a dwarf planet (Opens in a new window).
^ (Opens in a new window) The Sun's apparent magnitude (Opens in a new window) is −26.7, while the full moon's apparent magnitude is −12.7.
^ (Opens in a new window) See graph in Sun#Life phases (Opens in a new window). 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.
^ (Opens in a new window) On average, the Moon covers an area of 0.21078 square degrees on the night sky.

References

Citations

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External links

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Resources (Opens in a new window) from Wikiversity
NASA images and videos about the Moon (Opens in a new window)
Albums of images and high-resolution overflight videos by Seán Doran, based on LROC (Opens in a new window) data, on Flickr (Opens in a new window) and YouTube (Opens in a new window)
Video (04:56) – The Moon in 4K (NASA, April 2018) (Opens in a new window) on YouTube (Opens in a new window)
Video (04:47) – The Moon in 3D (NASA, July 2018) (Opens in a new window) on YouTube (Opens in a new window)

Cartographic resources

Unified Geologic Map of the Moon (Opens in a new window) - United States Geological Survey (Opens in a new window)
Moon Trek – An integrated map browser of datasets and maps for the Moon (Opens in a new window)
The Moon on Google Maps (Opens in a new window), a 3-D rendition of the Moon akin to Google Earth
"Consolidated Lunar Atlas" (Opens in a new window). Lunar and Planetary Institute (Opens in a new window). Retrieved 26 February 2012.
Gazetteer of Planetary Nomenclature (USGS) (Opens in a new window) List of feature names.
"Clementine Lunar Image Browser" (Opens in a new window). U.S. Navy (Opens in a new window). 15 October 2003. Retrieved 12 April 2007.
3D zoomable globes:"Google Moon" (Opens in a new window). 2007. Retrieved 12 April 2007.
"Moon" (Opens in a new window). World Wind Central. NASA. 2007. Retrieved 12 April 2007.
Aeschliman, R. "Lunar Maps" (Opens in a new window). Planetary Cartography and Graphics. Retrieved 12 April 2007. Maps and panoramas at Apollo landing sites
Japan Aerospace Exploration Agency (JAXA) (Opens in a new window) Kaguya (Selene) (Opens in a new window) images
Lunar Earthside chart (4497 x 3150px) (Opens in a new window)
Large image of the Moon's north pole area (Opens in a new window)
Large image of Moon's south pole area (1000x1000px) (Opens in a new window)

Observation tools

"NASA's SKYCAL – Sky Events Calendar" (Opens in a new window). NASA. Archived from the original (Opens in a new window) on 20 August 2007. Retrieved 27 August 2007.
"Find moonrise, moonset and moonphase for a location" (Opens in a new window). 2008. Retrieved 18 February 2008.
"HMNAO's Moon Watch" (Opens in a new window). 2005. Retrieved 24 May 2009. See when the next new crescent moon is visible for any location.

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Date March 30, 2021

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April 15, 2021

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March 29, 2021

Moon

Contents

Name and etymology

Formation

Physical characteristics

Internal structure

Surface geology

Volcanic features

Impact craters

Lunar swirls

Presence of water

Gravitational field

Magnetic field

Atmosphere

Dust

Past thicker atmosphere

Seasons

Rotation

Earth–Moon system

Lunar distance

Orbit

Relative size

Appearance from Earth

Tidal effects

Eclipses

Observation and exploration

Before spaceflight

1959–1970s

Soviet missions

United States missions

1970s – present

Future

Planned commercial missions

Human presence

Human impact

Infrastructure

Astronomy from the Moon

Living on the Moon

Legal status

In culture

Mythology

Calendar

Lunar effect

Notes

References

Citations

Further reading

External links

Cartographic resources

Observation tools

General

Navigation menu

Search

Contribute

Tools

Print/export

In other projects

Languages

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