Moon

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This article is about Earth's natural satellite. For moons in general, see Natural satellite. For other uses, see Moon (disambiguation).

Moon
☽
orbit plane[2]
  • 24° to Earth's equator [9]
    • North pole right ascension
    • 17h 47m 26s
    • 266.86°[10]
    North pole declination
    		65.64°[10]
    Albedo0.136[11]
    Surface temp. min mean max
    Equator 100 K[12] 250 K 390 K[12]
    85°N  150 K 230 K[13]
    Surface absorbed dose rate13.2 μGy/h (during lunar daytime)[14]
    Surface equivalent dose rate57.0 μSv/h (during lunar daytime)[14]
    0.2[15]
    29.3 to 34.1 arcminutes[4][d]
    Atmosphere[16]
    Surface pressure
    • 10−7 Pa (1 picobar)  (day)
    • 10−10 Pa (1 femtobar)   
      (night)      [e]
    Composition by volume

    The Moon is

    satellite planet under the geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's at 0.1654 g—Jupiter's moon Io
    is the only satellite in the Solar System known to have a higher surface gravity and density.

    The Moon orbits Earth at an

    synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. Due to cyclical shifts in perspective (libration
    ), 59% of the lunar surface is visible from Earth.

    The most widely accepted

    impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is fairly non-reflective, with the reflectance of lunar soil being comparable to that of asphalt. However, due to its large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse
    .

    Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's uncrewed Luna 2 spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history. The Moon is the only celestial body visited by humans.

    Names and etymology

    See also: Moon § Mythology and art

    The usual

    Proto-Indo-European *mēnsis "month"[22] (from earlier *mēnōt, genitive *mēneses) which may be related to the verb "measure" (of time).[23]

    Occasionally, the name Luna /ˈluːnə/ is used in scientific writing[24] and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon.[25] Cynthia /ˈsɪnθiə/ is another poetic name, though rare, for the Moon personified as a goddess,[26] while Selene /səˈliːniː/ (literally "Moon") is the Greek goddess of the Moon.

    The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. The adjective selenian /səliːniən/,[27] 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,[28] while its cognate selenic was originally a rare synonym[29] but now nearly always refers to the chemical element selenium.[30] The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography, the study of the physical features of the Moon, as well as the element name selenium.[31][32]

    The Greek goddess of the wilderness and the hunt,

    Mount Cynthus.[33] These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits
    such as apolune, pericynthion and selenocentric.

    The

    ☾
    , for example in M☾ 'lunar mass' (also ML).

    Natural history

    Lunar geologic timescale

    Main article: Lunar geologic timescale
    Early ImbrianLate ImbrianPre-NectarianNectarianEratosthenianCopernican period
    Millions of years before present


    The lunar geological periods are named after their characteristic features, from most

    Tycho
    .

    Formation

    Main articles: Origin of the Moon, Giant-impact hypothesis, and Circumplanetary disk
    The far side of the Moon, lacking the near side's characteristic large dark areas of maria, resembling how the near side of the Moon might have looked early in the Moon's history[34][35]

    dissipate the energy of the passing Moon.[40] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon.[40] None of these hypotheses can account for the high angular momentum of the Earth–Moon system.[42]

    The prevailing theory is that the Earth–Moon system formed after a

    proto-Earth. The impact blasted material into orbit about the Earth and the material accreted and formed the Moon[43][44] just beyond the Earth's Roche limit of ~2.56 R🜨.[45]

    Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth.[46] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth.[47][48][49][50] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two,[51] although this is debated.[52]

    The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system.[53][54] Similarly, the newly formed Moon would have had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).[53]

    While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.

    example needed]Above a high resolution threshold for simulations, a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.[56]

    Natural development

    Artist's depiction of the Moon as it might have appeared in Earth's sky after the Late Heavy Bombardment around 4 billion years ago, when its orbit was half as close to Earth and it appeared 2.8 times larger than today.[57]

    After the Moon's formation it settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other,

    tidal effects were stronger.[57]
    Due to tidal acceleration, the Moon's orbit around Earth has become significantly larger, with a longer period.

    Since cooling and stripped of most of

    lunar highlands on the far side are also not well understood. One explanation suggests that large meteorites were hitting the Moon in its early history, leaving large craters which then were filled with lava. Other explanations suggest processes of lunar volcanism.[61]

    Physical characteristics

    The Moon is a very slightly

    scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit.[62]

    Size and mass

    Further information: List of natural satellites
    subsurface oceans
    and one, Titan, having a considerable atmosphere.

    The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizeable as one of its

    primary planets.[g]

    The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of Australia.[17] The whole surface area of the Moon is about 38 million square kilometers, between the size of the Americas (North and South America) and Africa.

    The Moon's mass is 1/81 of Earth's,[64] being the second densest among the planetary moons, and having the second highest surface gravity, after Io, at 0.1654 g and an escape velocity of 2.38 km/s (8600 km/h; 5300 mph).

    Structure

    Main articles: Internal structure of the Moon and Geology of the Moon
    Moon's internal structure: solid inner core (iron-metallic), molten outer core, hardened mantle and crust. The crust on the Moon's near side permanently facing Earth is thinner, featuring larger areas flooded by material of the once molten mantle forming today's lunar mare
    .

    The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition.[65] It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi).[66][67] This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.[68]

    Crystallization of this magma ocean would have created a

    orthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-density plagioclase minerals could form and float into a crust atop.[69] The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements.[1] Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite.[16] The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.[1] The crust is on average about 50 kilometres (31 mi) thick.[1]

    The Moon is the second-densest satellite in the Solar System, after Io.[70] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less,[1] around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten.[71] The pressure at the lunar core is estimated to be 5 GPa (49,000 atm).[72]

    Gravitational field

    An astronaut jumping on the Moon, illustrating that the gravitational pull
    of the Moon is approximately 1/6 of Earth's.

    On average the Moon's surface gravity is 1.62 m/s2[4] (0.1654 g; 5.318 ft/s2), about half of the surface gravity of Mars and about a sixth of Earth's.

    The Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins.[73][74] 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.[75]

    Magnetic field

    The Moon has an external magnetic field of less than 0.2 nanoteslas,[76] or less than one hundred thousandth that of Earth. The Moon does not currently have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating.[77][78] However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today.[76] This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized.[76] Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins.[79]

    Atmosphere

    Main article: Atmosphere of the Moon
    The thin lunar atmosphere is visible on the Moon's surface at sunrise and sunset with the Lunar Horizon Glow[80] and lunar twilight rays, like Earth's crepuscular rays. This Apollo 17 sketch depicts the glow and rays[81] among the general zodiacal light.[82][83]

    The Moon has an

    sublimation of water ice in the regolith.[89] 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.[87]

    Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.[58]

    A permanent

    Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being more dense near the boundary between the Moon's dayside and nightside.[90][91]

    Surface conditions

    Gene Cernan with lunar dust stuck on his suit. Lunar dust is highly abrasive and can cause damage to human lungs, nervous, and cardiovascular systems.[92]

    millisieverts per day during lunar daytime,[14] which is about 2.6 times more than on the International Space Station with 0.53 millisieverts per day at about 400 km above Earth in orbit, 5-10 times more than during a trans-Atlantic flight, 200 times more than on Earth's surface.[94] For further comparison radiation on a flight to Mars is about 1.84 millisieverts per day and on Mars on average 0.64 millisieverts per day, with some locations on Mars possibly having levels as low as 0.342 millisieverts per day.[95][96]

    The Moon's

    peaks of eternal light at the Moon's north pole, at the rim of the crater Peary
    .

    The surface is exposed to drastic temperature differences ranging from 140 °C to −171 °C depending on the solar irradiance. Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow,[98] making topographical details play a decisive role on local surface temperatures.[99] Parts of many craters, particularly the bottoms of many polar craters,

    craters of eternal darkness" have extremely low temperatures. The Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F)[101] and just 26 K (−247 °C; −413 °F) close to the winter solstice in the north polar crater Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto.[99]

    Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened mostly gray surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder.[102] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–15 m (33–49 ft) in the highlands and 4–5 m (13–16 ft) in the maria.[103] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick.[104]

    These extreme conditions for example are considered to make it unlikely for spacecraft to harbor bacterial spores at the Moon longer than just one lunar orbit.[105]

    Surface features

    Main articles: Selenography, Lunar terrane, List of lunar features, and List of quadrangles on the Moon
    Astronaut Harrison H. Schmitt
    next to a large Moon boulder

    The

    topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System.[107][108] At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon.[107][109] The highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin.[110] Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess regionally low elevations and elevated rims.[107] The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side.[1]

    The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years.[111] Similar shrinkage features exist on Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat.[112]

    Volcanic features

    Main article: Volcanism on the Moon
    The names of the main volcanic features the maria (blue) and some crater
    (brown) features of the near side of the Moon

    The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called

    Latin for "seas", as they were once believed to be filled with water)[113] are vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water.[114] The majority of these lava deposits erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side "maria".[115]

    Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side[64] compared with 2% of the far side.[116] This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.[69][117][118] Most of the Moon's mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years[59] and as old as 4.2 billion years.[60]

    Old hardened lava flows of Mare Imbrium forming wrinkle ridges

    In 2006, a study of

    Moonquakes and releases of gas indicate continued lunar activity.[119] Evidence of recent lunar volcanism has been identified at 70 irregular mare patches, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements.[120][121][122][123] Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell,[124][125] inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin.[126][127]

    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

    cumulates of the lunar magma ocean.[60][59] In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[128]

    The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation.[129][130] Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth.[131]

    Impact craters

    Further information: List of craters on the Moon
    A gray, many-ridged surface from high above. The largest feature is a circular ringed structure with high walled sides and a lower central peak: the entire surface out to the horizon is filled with similar structures that are smaller and overlapping.
    A view of a three kilometer deep larger crater Daedalus on the Moon's far side

    A major geologic process that has affected the Moon's surface is

    Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment period of increased impacts.[135]

    High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years.[136][137] This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.[138]

    Lunar swirls

    Main article: Lunar swirls
    Wide angle image of a lunar swirl, the 70 kilometer long Reiner Gamma

    Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and often have a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts. Well known swirls include the Reiner Gamma feature and Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the solar wind, resulting in slower space weathering.[139]

    Presence of water

    Main article: Lunar water

    Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as

    comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon.[140][141] Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow.[100] The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.[142]

    In years since, signatures of water have been found to exist on the lunar surface.

    bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters.[144] In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions.[145] Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior.[146]

    In 2008, NASA's Moon Mineralogy Mapper equipment on India's Chandrayaan-1
    discovered, for the first time, water-rich minerals (shown in blue around a small crater from which they were ejected).

    The 2008

    permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material.[149][150] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb).[151]

    In May 2011, 615–1410 ppm water in

    melt inclusions in lunar sample 74220 was reported,[152] the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle
    . Although of considerable selenological interest, this insight does not mean that water is easily available since the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.

    Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface.[153][154] The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances.[155] 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.[153][155]

    In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA).[156][157][158][159]

    Earth–Moon system

    See also: Satellite system (astronomy), Claimed moons of Earth, and Double planet

    Orbit

    Main articles: Orbit of the Moon and Lunar theory
    DSCOVR satellite
    and Earth

    The Earth and the Moon form the Earth-Moon satellite system with a shared center of mass, or barycenter. This barycenter stays located at all times 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath the Earth's surface, making the Moon seemingly orbit the Earth.

    The orbital eccentricity is 0.055, indicating a slightly elliptical orbit.[1] The

    semi-major axis
    of the geocentric lunar orbit, is approximately 400,000 km, which is a quarter of a million miles or 1.28 light-seconds, and a unit of measure in astronomy. This is not to be confused with the instantaneous Earth–Moon distance, or distance to the Moon, the momentanous distance from the center of Earth to the center of the Moon.

    The Moon makes a complete orbit around Earth with respect to the fixed stars, its

    solar day on the Moon.[160]

    Due to

    near side, being visible from Earth. That said, while the movement of the Moon is in resonance, it still is not without nuances such as libration, resulting in slightly changing perspectives, making over time and location on Earth about 59% of the Moon's surface visible from Earth.[161]

    Unlike most satellites of other planets, the Moon's orbital plane is closer to the

    equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates once every 18.61 years,[162] which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.[163]

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

    Tidal effects

    Main articles: Tidal force, Tidal acceleration, Tide, and Theory of tides
    Simplified diagram of the Moon's gravity tidal effect on the Earth

    The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides of closest to each other, resulting in

    tidal forces. Ocean tides
    are the most widely experienced result of this, but tidal forces considerably affect also other mechanics of Earth, as well as the Moon and their system.

    The lunar solid crust experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in

    synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun.[164] The Earth-induced variable component arises from changing distance and libration, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides).[164] According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining Earth's magnetic field.[165]

    The cumulative effects of stress built up by these tidal forces produces

    moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.[166]

    The most commonly known effect of tidal forces are elevated sea levels called ocean tides.

    spring and neap tides.[167]

    The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours.[167] Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth.

    If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects:

    • the frictional coupling of water to Earth's rotation through the ocean floors
    • the inertia of water's movement
    • ocean basins that grow shallower near land
    • the sloshing of water between different ocean basins[168]

    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.

    Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's rotation, slowing the Earth's rotation.[167][164] That angular momentum, lost from the Earth, is transferred to the Moon in a process known as tidal acceleration, which lifts the Moon into a higher orbit while lowering orbital speed around the Earth.

    Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction.

    lunar ranging experiments) have found that the Moon's distance increases by 38 mm (1.5 in) per year (roughly the rate at which human fingernails grow).[169][170][171]
    Atomic clocks show that Earth's day lengthens by about 17 microseconds every year,[172][173][174] slowly increasing the rate at which UTC is adjusted by leap seconds.

    This tidal drag makes the rotation of the Earth and the orbital period of the Moon very slowly match. This matching first results in tidally locking the lighter body of the orbital system, as is already the case with the Moon. Theoretically, in 50 billion years,[175] the Earth's rotation will have slowed to the point of matching the Moon's orbital period, causing the Earth to always present the same side to the Moon. However, the Sun will become a red giant, engulfing the Earth-Moon system, long before then.[176][177]

    Position and appearance

    See also: Lunar observation
    apparent size
    and viewing angle over a single lunar month as viewed from Earth's north

    The Moon's highest

    altitude at culmination varies by its lunar phase
    , or more correctly its orbital position, and time of the year, or more correctly the position of the Earth's axis. The full moon is highest in the sky during winter and lowest during summer (for each hemisphere respectively), with its altitude changing towards dark moon to the opposite.

    At the

    polar day of the tropical year. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end.[178]

    The apparent orientation of the Moon depends on its position in the sky and the hemisphere of the Earth from which it is being viewed. In the

    southern hemisphere.[179] Sometimes the "horns" of a crescent moon appear to be pointing more upwards than sideways. This phenomenon is called a wet moon and occurs more frequently in the tropics.[180]

    The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) (perigee) to 406,700 km (252,700 mi) (apogee), making the Moon's apparent size fluctuate. On average the Moon's angular diameter is about 0.52°, roughly the same apparent size as the Sun (see § Eclipses). In addition, a purely psychological effect, known as the Moon illusion, makes the Moon appear larger when close to the horizon.[181]

    Despite the Moon's tidal locking, the effect of libration makes about 59% of the Moon's surface visible from Earth over the course of one month.[161][64]

    Rotation

    Comparison between the Moon on the left, rotating tidally locked (correct), and with the Moon on the right, without rotation (incorrect)

    The

    near side, and the opposite the far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During dark moon to new moon, the near side is dark.[182]

    The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth.[183] With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA Lunar Prospector mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth.[184]

    Illumination and phases

    See also: Lunar phase, Moonlight, and Halo (optical phenomenon)

    Half of the Moon's surface is always illuminated by the Sun (except during a lunar eclipse). Earth also reflects light onto the Moon, observable at times as Earthlight when it is again reflected back to Earth from areas of the near side of the Moon that are not illuminated by the Sun.

    With the different positions of the Moon, different areas of it are illuminated by the Sun. This illumination of different lunar areas, as viewed from Earth, produces the different

    synodic month
    . A phase is equal to the area of the visible lunar sphere that is illuminated by the Sun. This area or degree of illumination is given by , where is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).

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

    On 14 November 2016, the Moon was at full phase closer to Earth than it had been since 1948. It was 14% closer and larger than its farthest position in apogee.[185] This closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because of its increased apparent diameter, which made it a particularly notable example of a "supermoon".[186][187][188]

    At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:[189][190]

    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.[191]

    There has been historical controversy over whether observed features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago.[192][193]

    Albedo and color

    The changing apparent color of the Moon, filtered by Earth's atmosphere

    The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun.[64][j] This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon.[194] Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon's color depends on the light the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general the lunar surface reflects a brown-tinged gray light.[195]

    Viewed from Earth the air filters the reflected light, at times giving it a red color depending on the angle of the Moon in the sky and thickness of the atmosphere, or a blue tinge depending on the particles in the air,[195] as in cases of volcanic particles.[196] The terms blood moon and blue moon do not necessarily refer to circumstances of red or blue moonlight, but are rather particular cultural references such as particular full moons of a year.

    Eclipses

    Main articles: Solar eclipse, Lunar eclipse, and Eclipse cycle
    The Moon, tinted reddish, during a lunar eclipse

    Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "

    solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time,[167] the angular diameter of the Moon is decreasing. As it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing.[k] The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur.[198]

    Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.[199] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years.[200]

    Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,[l][201] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.[202]

    History of exploration and human presence

    Main articles:
    List of spacecraft that orbited the Moon, List of missions to the Moon, and List of lunar probes

    Pre-telescopic observation (before 1609)

    Main article: Exploration of the Moon § Before spaceflight

    It is believed by some that 20–30,000 year old

    the Moon's phases.[203]
    One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 at Knowth, Ireland.[204][205]

    The

    Saros cycle of lunar eclipses,[208] and Indian astronomers had described the Moon's monthly elongation.[209] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses.[207]
    : 411 

    In

    computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth
    for the distance.

    Although the Chinese of the

    Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions.[215] Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.[207]
    : 415–416 

    During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".[216]

    Telescopic exploration (1609-1959)

    Main article: Exploration of the Moon § Before spaceflight
    Galileo's sketches of the Moon from the ground-breaking Sidereus Nuncius
    (1610), publishing among other findings the first descriptions of the Moons topography