TheInfoList

The Moon is
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

's only
natural satellite A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...

. At about one-quarter the diameter of Earth (comparable to the width of
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
), it is the largest natural satellite in the
Solar System The Solar SystemCapitalization Capitalization ( North American English) or capitalisation ( British English) is writing a word with its first letter as a capital letter (uppercase letter) and the remaining letters in lower case, in writin ...

relative to the size of its planet, the fifth largest satellite in the Solar System overall, and is larger than any known
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dw ...
. The Moon is a
planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...
that formed a differentiated
rocky ''Rocky'' is a 1976 American sports drama film A sports film is a film genre that uses sport as the theme (arts), theme of the film. It is a production in which a sport, sporting event, athlete (and their sport), or follower of sport (and the ...
body, making it a satellite planet under geophysical definitions of the term. It lacks any significant
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...
,
hydrosphere The hydrosphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...
, or
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...
. Its surface gravity is about one-sixth of Earth's ();
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant A gas giant is a giant planet composed mainly of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and at ...

's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density. Orbiting Earth at an average distance of , or about 30 times Earth's diameter, its gravitational influence slightly lengthens Earth's day and is the main driver of Earth's
tide (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift. Tides are the rise and fall of sea level ...

s. The
Moon's orbit The Moon The Moon is Earth's only proper natural satellite. At one-quarter the diameter of Earth (comparable to the width of Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign countr ...
around Earth has a
sidereal period The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or b ...
of 27.3 days. During each
synodic period The orbital period is the time a given astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy ...

of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in
lunar phase s in 2022 as viewed from the Southern Hemisphere The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Sout ...
s that form the basis for the months of a
lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly on the solar year. Th ...
. The Moon is
tidally locked and Charon are tidally locked to each other. Charon is massive enough that the barycenter of Pluto's system lies outside of Pluto; thus Pluto and Charon are sometimes considered to be a binary system. Tidal locking (also called gravitational loc ...
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 A lunar day is the period of time for Earth's Moon The Moon is Earth's only proper natural satellite. At one-quarter the diameter of Earth (comparable to the width of Australia Australia, officially the Commonwealth of Australia, ...
is the same as the synodic period. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to
libration File:MoonVisibleLibration.jpg, Theoretical extent of visible lunar surface (in green) due to libration, compared to the extent of the visible lunar surface without libration (in yellow). The projection is the Winkel tripel projection, Winkel ...

. The most widely accepted origin explanation posits that the Moon formed about 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized
Mars Mars is the fourth planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to ...

-sized body called
Theia In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. Mo ...
. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic
maria Maria may refer to: People * Maria (given name), a popular given name in many languages Placenames Extraterrestrial *170 Maria, a Main belt S-type asteroid discovered in 1877 *Lunar maria (plural of ''mare''), large, dark basaltic plains on Eart ...
("seas"), which fill the spaces between bright ancient crustal highlands and prominent
impact crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity collision, impact of a smaller b ...

s. Most of the large
impact basin An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity collision, impact of a smaller b ...
s and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a
reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion ...

just slightly brighter than that of worn
asphalt Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous The viscosity of a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or externa ...
. However, because it has a large
angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a Geometry, geometrical object in solid geometr ...

, the
full moon The full Moon of 22 October 2010, as seen through a lunistice),_so_the_southern_lunar_craters.html" ;"title="lunar_standstill.html" ;"title="ecliptic_coordinate_system.html" "title="Schmidt-Cassegrain telescope. This full Moon was near its nort ...

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 A solar eclipse occurs when a portion of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The re ...

. Both the Moon's prominence in the earthly sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a that spanned during its existence from 1922 to 1991. It was nominally a of multiple national ; in practice and were highly until its final years. The ...
's
Luna 2 ''Luna 2'' ( rus, Луна 2}), originally named the Second Soviet Cosmic Rocket and nicknamed Lunik 2 in contemporaneous media, was the sixth of the Soviet Union's Luna programme spacecraft launched to the Moon, E-1 No.7. It was the first space ...
uncrewed
spacecraft File:Space Shuttle Columbia launching.jpg, 275px, The US Space Shuttle flew 135 times from 1981 to 2011, supporting Spacelab, ''Mir'', the Hubble Space Telescope, and the ISS. (''Columbia'' STS-1, maiden launch, which had a white external tank, ...

in 1959; this was followed by the first successful soft landing by
Luna 9 Luna 9 (Луна-9), internal designation Ye-6 No.13, was an of the 's . On 3 February 1966, the Luna 9 spacecraft became the first spacecraft to achieve a survivable landing on a . Spacecraft The lander had a mass of and consisted of a sphe ...

in 1966. The only human lunar missions to date have been those of the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country in . It consists of 50 , a , five major , 326 , and some . At , it is the world's . The United States shares significan ...

'
Apollo program The Apollo program, also known as Project Apollo, was the third United States human spaceflight program carried out by the NASA, National Aeronautics and Space Administration (NASA), which succeeded in Moon landing, landing the first humans ...

, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.

Name and etymology

The usual
English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventually become the World language, leading lan ...

proper name for Earth's natural satellite is simply ''the Moon'', with a capital M. The noun ''moon'' is derived from
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventu ...
''mōna'', which (like all its
Germanic Germanic may refer to: * Germanic peoples, an ethno-linguistic group identified by their use of the Germanic languages ** List of ancient Germanic peoples and tribes * Germanic languages :* Proto-Germanic language, a reconstructed proto-language of ...

cognates) stems from
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed Reconstruction may refer to: Politics, history, and sociology *Reconstruction (law), the transfer of a company's (or several companies') business to a new ...
''*mēnōn'', which in turn comes from
Proto-Indo-European Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
''*mēnsis'' "month" (from earlier ''*mēnōt'', genitive ''*mēneses'') which may be related to the verb "measure" (of time). Occasionally, the name Luna is used in scientific writing and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. Cynthia is another poetic name, though rare, for the Moon personified as a goddess, while
Selene In Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical cosmology, nature of ...

(literally "Moon") is the Greek goddess of the Moon. The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, ''lūna''. The adjective ''selenian'' , derived from the Greek word for the Moon, ''selēnē'', and used to describe the Moon as a world rather than as an object in the sky, is rare, while its cognate ''selenic'' was originally a rare synonym but now nearly always refers to the chemical element
selenium Selenium is a chemical element with the Symbol (chemistry), symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic tabl ...

. The Greek word for the Moon does however provide us with the prefix ''seleno-'', as in ''
selenography 345px, alt=Topography of the Moon measured from the Lunar Orbiter Laser Altimeter on the mission Topography of the Moon. The topography Topography is the study of the forms and features of land surfaces. The topography of an area could ref ...

'', the study of the physical features of the Moon, as well as the element name ''selenium''. The Greek goddess of the wilderness and the hunt,
Artemis Artemis (; grc-gre, Ἄρτεμις Artemis, ) is the Greek goddess Greek mythology is the body of myth Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or ori ...

, equated with the Roman Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called
Cynthia Cynthia is a feminine given name of Greek language, Greek origin: Κυνθία, ''Kynthía'', "from Mount Cynthus" on Delos island. There are various spellings for this name, and it can be abbreviated as Cindy (given name), Cindy, Cyndi, or as Cyn ...

, from her legendary birthplace on Mount Cynthus. These names – Luna, Cynthia and Selene – are reflected in technical terms for
lunar orbit In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ma ...

s such as ''apolune'', ''pericynthion'' and ''selenocentric''.

Formation

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. Historically, several formation mechanisms have been proposed, but none satisfactorily explained the features of the Earth–Moon system. A fission of the Moon from Earth's crust through
centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force A fictitious force (also called a pseudo force, d'Alembert force, or inertial force) is a force that appears to act on a mass whose motion is described using a non-inertial refe ...
would require too great an initial rotation rate of Earth. Gravitational capture of a pre-formed Moon depends on an unfeasibly extended
atmosphere of Earth The atmosphere of Earth is the layer of es, commonly known as air, retained by , surrounding the planet and forming its planetary . The atmosphere of Earth protects on Earth by creating allowing for to exist on the Earth's , absorbing , ...

to
dissipate In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous Thermodynamic system, thermodynamic systems. In a dissipative process, energy (Internal energy, internal, bulk flow Kinetic energy, kinetic, or s ...
the energy of the passing Moon. A co-formation of Earth and the Moon together in the primordial
accretion disk An accretion disk is a structure (often a ) formed by diffuse material in al motion around a massive . The central body is typically a . causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional ...

does not explain the depletion of metals in the Moon. None of these hypotheses can account for the high
angular momentum In , angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of . It is an important quantity in physics because it is a —the total angular momentum of a closed system remains constant. In three , the ...

of the Earth–Moon system. The prevailing theory is that the Earth–Moon system formed after a giant impact of a
Mars Mars is the fourth planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to ...

-sized body (named ''
Theia In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. Mo ...
'') with the proto-Earth. The impact blasted material into orbit about the Earth and then the material accreted and formed the Moon just beyond the Earth's
Roche limit In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forces ...
of ~. Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations also show that most of the Moon derived from the impactor, rather than the proto-Earth. However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated. The impact released energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean. Similarly, the newly formed Moon would also have been affected and had its own
lunar magma ocean The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of the Moon The Moon is Earth's only proper natural satellite. At one-quarter the diameter of Earth (comparable to the width o ...
; its depth is estimated from about to . While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks. The rocks from the Apollo program had the same isotopic signature as rocks from Earth, differing from almost all other bodies in the Solar System. This observation was unexpected, because most of the material that formed the Moon was thought to come from
Theia In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. Mo ...
and it was announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures. Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth, which
conflicts Conflict may refer to: Arts, entertainment, and media Films * ''Conflict'' (1936 film), an American boxing film starring John Wayne * Conflict (1938 film), ''Conflict'' (1938 film), a French drama film directed by Léonide Moguy * Confli ...
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. For instance, a high-speed drive-by hit by the impactor allowed it to return to earth a second time but more slowly, and mix more thoroughly. A hit-and-run-and-return scenario might be more likely.

Physical characteristics

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

Internal structure

The Moon is a differentiated body that was initially in
hydrostatic equilibrium In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, more specifically the relationships among fo ...

but has since departed from this condition. It has a geochemically distinct crust, mantle, and
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
. The Moon has a solid iron-rich inner core with a radius possibly as small as and a fluid outer core primarily made of liquid iron with a radius of roughly . Around the core is a partially molten boundary layer with a radius of about . This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. Crystallization of this magma ocean would have created a
mafic A mafic mineral or rock is a silicate mineral Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science c ...
mantle from the
precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. L ...
and sinking of the minerals
olivine The mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. R ...

,
clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming s found in many and . Pyroxenes have the general formula , where X represents (Ca), (Na), (Fe II) or (Mg) and more rarely , or , and Y represents ions of ...
, and
orthopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming Silicate minerals#Inosilicates, inosilicate minerals found in many Igneous rock, igneous and metamorphic rock, metamorphic rock (geology), rocks. Pyroxenes have th ...
plagioclase Plagioclase is a series of tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition ...
minerals could form and float into a crust atop. The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of
incompatible Compatibility may refer to: Computing * Backward compatibility, in which newer devices can understand data generated by older devices * Compatibility card, an expansion card for hardware emulation of another device * Compatibility layer, compone ...
and heat-producing elements. Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly
anorthosite Anorthosite is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group o ...

. The
Moon rock Moon rock or lunar rock refers to rock that is found on the Moon, Earth's Moon. This includes lunar material collected during the course of human Moon landing, exploration of the Moon, and rock that has been ejected naturally from the Moon's sur ...

samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth. The crust is on average about thick. The Moon is the second-densest satellite in the Solar System, after Io. However, the inner core of the Moon is small, with a radius of about or less, around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten. The pressure at the lunar core is estimated to be .

Magnetic field

The Moon has an external
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

of generally less than 0.2 nanoteslas, or less than one hundred thousandth that of Earth. The Moon does not currently have a global magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized. Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the
antipodes 360px, The same map, from the perspective of the Western Hemisphere. Here the blue areas are the reflections of the Eastern Hemisphere. In geography, the antipode ( or ) of any spot on Earth is the antipodal point, point on Earth's surface dia ...
of the giant impact basins.

Surface geology

The topography of the Moon has been measured with laser altimetry and stereo image analysis. Its most extensive topographic feature is the giant far-side
South Pole–Aitken basin The South Pole–Aitken basin (SPA Basin, ) is an immense impact crater on the far side of the Moon. At roughly in diameter and between deep, it is one of the list of largest craters in the Solar System, largest known impact craters in the Solar ...
, some in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. At deep, its floor is the lowest point on the surface of the Moon. The highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. Other large impact basins such as , , , , and Orientale possess regionally low elevations and elevated rims. The far side of the lunar surface is on average about higher than that of the near side. The discovery of
fault scarp A fault scarp is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other. It is the topographic expression of faulting attributed to the displacement of the land surface by movement a ...
cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. Similar shrinkage features exist on
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...
. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat.

Volcanic features

The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called ''
maria Maria may refer to: People * Maria (given name), a popular given name in many languages Placenames Extraterrestrial *170 Maria, a Main belt S-type asteroid discovered in 1877 *Lunar maria (plural of ''mare''), large, dark basaltic plains on Eart ...
'' (
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the dominant la ...
for "seas"; singular ''mare''), as they were once believed to be filled with water; they are now known to be vast solidified pools of ancient
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extrusi ...

ic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. The majority of these lava deposits erupted or flowed into the depressions associated with
impact basins An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity collision, impact of a smaller b ...

. Several
geologic province A geologic province is a spatial entity with common geologic attributes. A province may include a single dominant structural element such as a basin or a fold belt, or a number of contiguous related elements. Adjoining provinces may be simila ...
s containing
shield volcano A shield volcano is a type of volcano A volcano is a rupture in the of a , such as , that allows hot , , and to escape from a below the surface. On Earth, volcanoes are most often found where are or , and most are found underwater. Fo ...

es and volcanic
domes A dome (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman ...
are found within the near side "maria". Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side compared with 2% of the far side. This is likely due to a KREEP, concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. Most of the Moon's lunar mare, mare basalts erupted during the Imbrian period, 3.0–3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years. As of 2003, crater counting studies of the youngest eruptions appeared to suggest they formed no earlier than 1.2 billion years ago. In 2006, a study of Ina (crater), Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. Moonquakes and releases of gas also indicate some continued lunar activity. Evidence of recent lunar volcanism has been identified at 70 irregular mare patches, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin. The lighter-colored regions of the Moon are called ''terrae'', or more commonly ''highlands'', because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent
plagioclase Plagioclase is a series of tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition ...
Cumulate rock, cumulates of the lunar magma ocean. In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events. The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation. Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth.

Impact craters

A major geologic process that has affected the Moon's surface is
impact crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity collision, impact of a smaller b ...

ing, with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than on the Moon's near side. The lunar geologic timescale is based on the most prominent impact events, including Nectarian, Nectaris, Lower Imbrian, Imbrium, and Orientale; structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphy, stratigraphic horizon. The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment period of increased impacts. Blanketed on top of the Moon's crust is a highly Comminution, comminuted (broken into ever smaller particles) and impact gardening, impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from in the highlands and in the maria. Beneath the finely comminuted regolith layer is the ''megaregolith'', a layer of highly fractured bedrock many kilometers thick. 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 Ejecta blanket, distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years. This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.

Gravitational field

The gravitational field of the Moon has been measured through tracking the Doppler effect, Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mass concentration (astronomy), mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.

Lunar swirls

Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and have often a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

s 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.

Presence of water

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon. Computer simulations suggest that up to of the surface may be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering Colonization of the Moon, lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive. In years since, signatures of water have been found to exist on the lunar surface. In 1994, the Clementine mission#Bistatic Radar Experiment, bistatic radar experiment located on the ''Clementine (spacecraft), Clementine'' spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo Telescope, Arecibo, suggest these findings may rather be rocks ejected from young impact craters. In 1998, the Lunar Prospector#Neutron Spectrometer (NS), neutron spectrometer on the ''Lunar Prospector'' spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. The 2008 ''Chandrayaan-1'' spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000 parts per million, ppm. Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. In 2009, ''LCROSS'' sent a impactor into a Crater of eternal darkness, permanently shadowed polar crater, and detected at least of water in a plume of ejected material. Another examination of the LCROSS data showed the amount of detected water to be closer to . In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. Although of considerable selenological interest, this announcement affords little comfort to would-be lunar colonists – the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument. Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun. In October 2020, astronomers reported detecting Water, molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA).

Surface conditions

The surface of the Moon is an extreme environment with temperatures that range from down to , an atmospheric pressure of 10−10 Pa, and high levels of ionizing radiation from the Sun and cosmic rays. The exposed surfaces of spacecraft are considered unlikely to harbor bacterial spores after just one lunar orbit. The surface gravity of the Moon is approximately 1.625 m/s2, about 16.6% that on Earth's surface or 0.166 .

Atmosphere

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than . The surface pressure of this small mass is around 3 × 10−15 Atmosphere (unit), atm (0.3 nanopascal, nPa); it varies with the lunar day. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions. Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury (planet), Mercury and Io); helium-4 and neon from the solar wind; and Argon, argon-40, Radon, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. Water vapor has been detected by ''Chandrayaan-1'' and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation (chemistry), sublimation of water ice in the regolith. These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. Studies of Moon magma samples retrieved by the Apollo program, 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 Mars is the fourth planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to ...

. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.

Dust

A permanent Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by Lunar Atmosphere and Dust Environment Explorer, LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminids, Geminid, Quadrantids, Quadrantid, Taurids, Northern Taurid, and Omicron Centaurids, 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.

Earth–Moon system

Lunar distance

File:Earth-moon-to-scale.svg, Scale model of the Earth–Moon system: Sizes and distances are to scale.

Orbit

Because of tidal locking, the rotation of the Moon around its own axis is synchronous to its orbital period around the Earth. The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days, its
sidereal period The orbital period is the time a given astronomical object takes to complete one orbit around another object, and applies in astronomy usually to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or b ...
. 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 lunar phase, phase to Earth, which is about 29.5 days; its
synodic period The orbital period is the time a given astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy ...

. Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbation (astronomy), perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit lunar standstill, gradually rotates once every 18.61years, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's Laws, Cassini's laws. The Moon's axial tilt with respect to the ecliptic is only 1.5427°, much less than the 23.44° of Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects. From images taken by ''Clementine (spacecraft), Clementine'' in 1994, it appears that four mountainous regions on the rim of the crater Peary (crater), Peary at the Moon's north pole may remain illuminated for the entire
lunar day A lunar day is the period of time for Earth's Moon The Moon is Earth's only proper natural satellite. At one-quarter the diameter of Earth (comparable to the width of Australia Australia, officially the Commonwealth of Australia, ...
, creating Peak of Eternal Light, peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters, and these "crater of eternal darkness, craters of eternal darkness" are extremely cold: ''Lunar Reconnaissance Orbiter'' measured the lowest summer temperatures in craters at the southern pole at and just close to the winter solstice in the north polar crater Hermite (crater), Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto. Average temperatures of the Moon's surface are reported, but temperatures of different areas will vary greatly depending upon whether they are in sunlight or shadow.

Relative size

The Moon is an exceptionally large natural satellite relative to Earth: Its diameter is more than a quarter and its mass is 1/81 of Earth's. It is the largest moon in the Solar System relative to the size of its planet, though Charon (moon), Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass. The Earth and the Moon's barycentre, their common center of mass, is located (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 per second. This motion is superimposed on the much larger revolution of the Earth around the Sun at a speed of about per second. The surface area of the Moon is slightly less than the areas of Americas, North and South America combined.

Appearance from Earth

The synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. However, because of the effect of Libration#Lunar libration, libration, about 59% of the Moon's surface can actually be seen from Earth. The side of the Moon that faces Earth is called the near side of the Moon, near side, and the opposite the far side of the Moon, far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During new moon, the near side is dark. The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidal locking, tidally locked in this orientation as a result of frictional effects associated with tidal force, tidal deformations caused by Earth. With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA ''Lunar Prospector'' mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth. The Moon has an exceptionally low albedo, giving it a diffuse reflection, reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at
full moon The full Moon of 22 October 2010, as seen through a lunistice),_so_the_southern_lunar_craters.html" ;"title="lunar_standstill.html" ;"title="ecliptic_coordinate_system.html" "title="Schmidt-Cassegrain telescope. This full Moon was near its nort ...

. 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 Lambert's cosine law#Lambertian scatterers, reflective properties of lunar soil, which retroreflector, retroreflects light more towards the Sun than in other directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC. The full Moon's
angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a Geometry, geometrical object in solid geometr ...

is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see ). The Moon's highest altitude (astronomy), altitude at culmination varies by lunar phase, its phase and time of year. The full moon is highest in the sky during winter (for each hemisphere). The orientation of the Moon's crescent also depends on the latitude of the viewing location; an observer in the tropics can see a wet moon, smile-shaped crescent Moon. The Moon is visible for two weeks every 27.3 days at the North Pole, North and South Poles. Zooplankton in the Arctic use moonlight when the Sun is polar night, below the horizon for months on end. The lunar distance (astronomy), distance between the Moon and Earth varies from around to at apsis, perigee (closest) and apogee (farthest), respectively. On 14 November 2016, it was closer to Earth when at full phase than it has been since 1948, 14% closer than its farthest position in apogee. Reported as a "supermoon", this closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because its angular diameter is 14% greater and $\scriptstyle1.14^2\approx1.30$. At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula: :$\text\%=100 \times \sqrt$ When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase. There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported transient lunar phenomenon, lunar transient phenomena. Recently, it has been suggested that a roughly diameter region of the lunar surface was modified by a gas release event about a million years ago. The Moon's appearance, like the Sun's, can be affected by Earth's atmosphere. Common optical effects are the 22° halo (optical phenomenon), halo ring, formed when the Moon's light is refracted through the ice crystals of high cirrostratus clouds, and smaller corona (meteorology), coronal rings when the Moon is seen through thin clouds. The illuminated area of the visible sphere (degree of illumination) is given by $\left(1-\cos e\right)/2=\sin^2\left(e/2\right)$, where $e$ is the elongation (astronomy), elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).

Eclipses

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "Syzygy (astronomy), syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total eclipse, total (with the Moon appearing larger than the Sun) and annular eclipse, annular (with the Moon appearing smaller than the Sun) solar eclipses. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time, the angular diameter of the Moon is decreasing. Also, as it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur. Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the ecliptic, orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the Saros (astronomy), saros, which has a period of approximately 18 years. Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky, the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.

Tidal effects

The gravitational attraction that masses have for one another decreases inversely with the square of the distance of those masses from each other. As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces. Tidal forces affect both the Earth's crust and oceans. The most obvious effect of tidal forces is to cause two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. This results in elevated sea levels called tide, ocean tides. As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth. The Sun has the same tidal effect on the Earth, but its forces of attraction are only 40% that of the Moon's; the Sun's and Moon's interplay is responsible for Spring tide, spring and neap tides. If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects: the frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that grow shallower near land, the sloshing of water between different ocean basins. As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory. While gravitation causes acceleration and movement of the Earth's fluid oceans, gravitational coupling between the Moon and Earth's solid body is mostly elastic and plastic. The result is a further tidal effect of the Moon on the Earth that causes a bulge of the solid portion of the Earth nearest the Moon. Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth's rotation. This "drains"
angular momentum In , angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of . It is an important quantity in physics because it is a —the total angular momentum of a closed system remains constant. In three , the ...

and rotational kinetic energy from Earth's rotation, slowing the Earth's rotation. That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration), which lifts the Moon into a higher orbit and results in its lower orbital speed about the Earth. Thus the distance between Earth and Moon is tidal acceleration, increasing, and the Earth's rotation is slowing in reaction. Measurements from laser reflectors left during the Apollo missions (lunar laser ranging experiment, lunar ranging experiments) have found that the Moon's distance increases by per year (roughly the rate at which human fingernails grow). Atomic clocks also show that Earth's day lengthens by about 17 microseconds every year, slowly increasing the rate at which Coordinated Universal Time, UTC is adjusted by leap seconds. This tidal drag would continue until the rotation of Earth and the orbital period of the Moon matched, creating mutual tidal locking between the two and suspending the Moon over one meridian (this is currently the case with Pluto and its moon Charon). However, the Sun will become a red giant#The Sun as a red giant, red giant engulfing the Earth-Moon system long before this occurrence. In a like manner, the lunar surface experiences tides of around amplitude over 27 days, with three components: a fixed one due to Earth, because they are in synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun. The Earth-induced variable component arises from changing distance and
libration File:MoonVisibleLibration.jpg, Theoretical extent of visible lunar surface (in green) due to libration, compared to the extent of the visible lunar surface without libration (in yellow). The projection is the Winkel tripel projection, Winkel ...

, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides). Libration also changes the angle from which the Moon is seen, allowing a total of about 59% of its surface to be seen from Earth over time. The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of 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 program, Apollo astronauts from 1969 through 1972. According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining Earth's magnetic field.

Observation and exploration

Before spaceflight

One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving ''Orthostat 47'' at Knowth, Ireland. Understanding of the Moon's cycles was an early development of astronomy: The ancient Greece, ancient Greek philosopher Anaxagoras reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. Elsewhere in the to , Babylonian astronomy, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, and Indian astronomy, Indian astronomers had described the Moon's monthly elongation. The Chinese astronomy, Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses. In Aristotle's (384–322 BC) On the Heavens, description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether (classical element), aether, an Aristotelian physics, influential philosophy that would dominate for centuries. Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. However, in the , Seleucus of Seleucia correctly theorized that
tide (U.S.), low tide occurs roughly at moonrise and high tide with a high Moon, corresponding to the simple gravity model of two tidal bulges; at most places however, the Moon and tides have a phase shift. Tides are the rise and fall of sea level ...

1959–1970s

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

Soviet missions

Spacecraft from the Soviet Union's Luna programme, ''Luna'' program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958, the first human-made object to escape Earth's gravity and pass near the Moon was ''Luna 1''; the first human-made object to impact the lunar surface was ''
Luna 2 ''Luna 2'' ( rus, Луна 2}), originally named the Second Soviet Cosmic Rocket and nicknamed Lunik 2 in contemporaneous media, was the sixth of the Soviet Union's Luna programme spacecraft launched to the Moon, E-1 No.7. It was the first space ...
'', and the first photographs of the normally occluded far side of the Moon were made by ''Luna 3'', all in 1959. The first spacecraft to perform a successful lunar lander (spacecraft), soft landing was ''
Luna 9 Luna 9 (Луна-9), internal designation Ye-6 No.13, was an of the 's . On 3 February 1966, the Luna 9 spacecraft became the first spacecraft to achieve a survivable landing on a . Spacecraft The lander had a mass of and consisted of a sphe ...

'' and the first vehicle to orbit the Moon was ''Luna 10'', both in 1966. Moon rock, Rock and soil samples were brought back to Earth by three ''Luna'' sample return missions (''Luna 16'' in 1970, ''Luna 20'' in 1972, and ''Luna 24'' in 1976), which returned 0.3 kg total. Luna 17 deployed the first lunar rover, Lunokhod 1, in 1970.

United States missions

During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study that proposed the construction of a staffed military outpost on the Moon called Project Horizon with the potential to conduct a wide range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test. The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex Project, Lunex. However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA. Following President John F. Kennedy's 1961 commitment to a manned Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures; the Lunar Orbiter program produced maps of the entire Moon; the Surveyor program landed Surveyor 1, its first spacecraft four months after ''Luna 9''. The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet human lunar programs, Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race. Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) removed of lunar rock and soil in 2,196 separate samples. The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry, software engineering, and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking. Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived Apollo Lunar Surface Experiments Package, instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, Apollo 14, 14, Apollo 15, 15, Apollo 16, 16, and Apollo 17, 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, but as the stations' Lunar laser ranging experiment, lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimeters, and data from this experiment are being used to place constraints on the size of the lunar core.

1970s–present

In the 1970s, after the Moon race, the focus of astronautic exploration shifted, as probes like ''Pioneer 10'' and the Voyager program were sent towards the outer Solar System. Years of near lunar quietude followed, only broken by a beginning internationalization of space and the Moon through, for example, the negotiation of the Moon treaty. Since the 1990s, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its ''Hiten'' spacecraft. The spacecraft released a smaller probe, ''Hagoromo'', in lunar orbit, but the transmitter failed, preventing further scientific use of the mission. In 1994, the U.S. sent the joint Defense Department/NASA spacecraft ''Clementine (spacecraft), Clementine'' to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global Multi-spectral image, multispectral images of the lunar surface. This was followed in 1998 by the ''Lunar Prospector'' mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. The European spacecraft ''SMART-1'', the second ion propulsion, ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface. The Chinese Lunar Exploration Program began with ''Chang'e 1'', which successfully orbited the Moon from 5 November 2007 until its controlled lunar impact on 1 March 2009, obtaining a full image map of the Moon. The ''Chang'e 2'' mission began October 2010, mapping the surface at a higher resolution over an eight-month period. On 14 December 2013, ''Chang'e 3'' landed a lunar lander (spacecraft), lander onto the Moon's surface, which deployed a lunar rover, named ''Yutu (rover), Yutu'' (Chinese: 玉兔; literally "Jade Rabbit"). This was the first lunar rover mission since ''Lunokhod 2'' in 1973 and the first lunar Soft landing (rocketry), soft landing since ''Luna 24'' in 1976. Another rover mission, ''Chang'e 4'', was launched in 2019 and was the first spacecraft to land on the Moon's far side. ''Chang'e 5'' landed on the Moon in December 2020 and carried out China's first robotic sample return mission, bringing back 1,731 grams of lunar material to Earth. ''Chang'e 6'', another sample return mission, is planned for 2024. Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's ''SELENE, Kaguya (Selene)'' mission, a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit. India's first lunar mission, ''Chandrayaan-1'', orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of Lunar water, water molecules in lunar soil. The Indian Space Research Organisation planned to launch ''Chandrayaan-2'' in 2013, which would have included a Russian robotic lunar rover. However, the failure of Russia's ''Fobos-Grunt'' mission has delayed this project, and was launched on 22 July 2019. The lander ''Vikram'' attempted to land on the lunar south pole region on 6 September, but lost the signal in . What happened after that is unknown. The U.S. co-launched the ''Lunar Reconnaissance Orbiter'' (LRO) and the ''LCROSS'' impactor and follow-up observation orbiter on 18 June 2009; ''LCROSS'' completed its mission by making a planned and widely observed impact in the crater Cabeus (crater), Cabeus on 9 October 2009, whereas ''LRO'' is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the large and bright crater Aristarchus (crater), Aristarchus. NASA released photos of the crater on 25 December 2011. Two NASA Gravity Recovery and Interior Laboratory, GRAIL spacecraft began orbiting the Moon around 1 January 2012, on a mission to learn more about the Moon's internal structure. NASA's ''Lunar Atmosphere and Dust Environment Explorer, LADEE'' probe, designed to study the lunar exosphere, achieved orbit on 6 October 2013.

Human presence

Human impact

Beside the remains of human activity on the Moon, there have been some intended permanent installations like the ''Moon Museum'' art piece, Apollo 11 goodwill messages, six Lunar plaques, the ''Fallen Astronaut'' memorial, and other artifacts.

Pollution and contamination

While the Moon has the lowest planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed and particularly understood regarding keeping the Shielded Zone of the Moon (SZM), of value for Moon#Astronomy from the Moon, astronomy from the Moon, free from any radio spectrum pollution, as well as conserving the special and scientifically interesting nature of the Moon, in face of prospecting commercial and national projects to claim and exploit the Moon. The so-called "Tardigrade affair" of the 2019 crashed Beresheet lander and its carrying of tardigrades has been discussed as an example for lacking measures and lacking international regulation for planetary protection.

Infrastructure

Image:Apollo 11 Lunar Laser Ranging Experiment.jpg, A photo of the reflector of the Lunar Laser Ranging Experiment of Apollo 11, still in use. Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang'e 3 with its Lunar Ultraviolet Telescope still operational. There are List of missions to the Moon#Future missions, several missions by different agencies and companies planned to establish a longterm human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.

Astronomy from the Moon

For many years, the Moon has been recognized as an excellent site for telescopes. It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth. The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and Epoxy, epoxies and employed in the construction of mirrors up to 50 meters in diameter. A lunar zenith telescope can be made cheaply with an ionic liquid. In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph.

Living on the Moon

Humans have stayed for days on the Moon, such as during Apollo 17 in an Apollo Lunar Module, which have been sofar the only Space habitat (facility)#Extraterrestrial surface habitat, extraterrestrial surface habitats. One particular challenge for astronauts' daily life during their stay on the surface is the #Dust, lunar dust sticking to their suits and being carried into their quarters. Subsequently, the dust was tasted and smelled by the astronauts, calling it the "Apollo aroma". This contamination poses a danger since the fine lunar dust can Lunar dust#Harmful effects of lunar dust, cause health issues. In 2019 at least one plant seed sprouted in an experiment, carried along with other small life from Earth on the Change 4#Lunar lander, Chang'e 4 lander in its ''Lunar Micro Ecosystem''.

Legal status

Although ''Luna program, Luna'' landers scattered pennants of the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a that spanned during its existence from 1922 to 1991. It was nominally a of multiple national ; in practice and were highly until its final years. The ...
on the Moon, and Lunar Flag Assembly, U.S. flags were symbolically planted at their landing sites by the List of Apollo astronauts, Apollo astronauts, no nation claims ownership of any part of the Moon's surface. Russia, China, India, and the U.S. are party to the 1967 Outer Space Treaty, which defines the Moon and all outer space as the "common heritage of mankind, province of all mankind". This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. The 1979 Moon Treaty, Moon Agreement was created to restrict the exploitation of the Lunar resources, Moon's resources by any single nation, but as of January 2020, it has been signed and ratified by only 18 nations, none of which engages in self-launched Human spaceflight, human space exploration. Although several individuals have made Extraterrestrial real estate, claims to the Moon in whole or in part, none of these are considered credible. In 2020, U.S. President Donald Trump signed an executive order called "Encouraging International Support for the Recovery and Use of Space Resources". The order emphasizes that "the United States does not view outer space as a 'global commons and calls the Moon Agreement "a failed attempt at constraining free enterprise." In the face of such increasing commercial and national interest, particularly prospecting territories, US lawmakers have introduced regulation for the conservation of historic landing sites and interest groups have argued for making such sites World Heritage Sites and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon. The ''Declaration of the Rights of the Moon'' was created by a group of "lawyers, space archaeologists and concerned citizens" in 2021, drawing on precedents in the Rights of Nature movement and the concept of legal personality for non-human entities in space.

Coordination

In light of future development on the Moon some international and multi-space agency organizations have been created: * International Lunar Exploration Working Group (ILEWG) * Moonbase#Moon Village, Moon Village Association (MVA) * International Space Exploration Coordination Group (ISECG)

In culture and life

Calendar

The Moon's regular phases make it a convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. The ~30-day month is an approximation of the lunar cycle. The English noun ''month'' and its cognates in other Germanic languages stem from Proto-Germanic ''*mǣnṓth-'', which is connected to the above-mentioned Proto-Germanic ''*mǣnōn'', indicating the usage of a
lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly on the solar year. Th ...
among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar. The Proto-Indo-European language, PIE root of ''moon'', *''méh1nōt'', derives from the PIE verbal root *''meh1''-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker of the month" (cf. the English words ''measure'' and ''menstrual''), and echoing the Moon's importance to many ancient cultures in measuring time (see the Chinese language, Chinese pictographic logogram , Latin and Ancient Greek (''meis'') or (mēn), meaning "month"). Most historical calendars are lunisolar calendar, lunisolar. The 7th-century Islamic calendar is an example of a purely
lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's lunar phase, phases (Lunar month#Synodic month, synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly on the solar year. Th ...
, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon. Many festivities celebrate or use the Moon, particularly the Full Moon of Autumnal equinox (disambiguation), autumnal equinox called Harvest moon, Harvest Moon.

Mythology and art

Since prehistoric and ancient times many cultures view Moon (astrology), the Moon astrologically and have Lunar deity, personified the Moon as a deity. The crescent (🌙) is a symbol used by many cultures, particularly as an identifier for the Moon and its appearance, especially its lunar phases, but also its pale colour, e.g. for silver by Western alchemy. For example in Mesopotamian iconography the primary symbol of Sin (mythology), Nanna/Sîn, the ancient Sumerian lunar deity. who was the father of Innana, Innana/Ishtar, the goddess of the planet Venus (symbolized as the Octagram, eight pointed Star of Ishtar), and Utu, Utu/Shamash, the god of the Sun (Solar symbol, symbolized as a disc, optionally with eight rays), all three often depicted next to each other. Nanna was later known as Sîn, and was particularly associated with magic and sorcery. The crescent was further used as an element of lunar Horned deity, deities wearing headgears or crowns in an arrangement reminiscent of horns, as in the case of the ancient Greek
Selene In Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical cosmology, nature of ...

or the ancient Egyptian Khonsu. Selene is associated with
Artemis Artemis (; grc-gre, Ἄρτεμις Artemis, ) is the Greek goddess Greek mythology is the body of myth Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or ori ...

and paralleled by the Roman Luna (goddess), Luna, which both are occasionally depicted Luna (goddess)#Chariot of the moon, driving a chariot, like the Hindu lunar deity Chandra. The different or sharing Aspect (religion), aspects of deities within Pantheon (religion), pantheons has been observed in many cultures, especially by later or contemporary culture, particularly forming triple deity, triple deities. The Moon in Roman mythology for example has been associated with Juno (mythology), Juno and Diana (goddess), Diana, while Luna being identified as their byname and as part of a triplet (''diva triformis'') with Diana and Proserpina, Hecate being identified as their binding manifestation as ''trimorphos''. The star and crescent (☪️) arrangement also goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a Byzantine flags and insignia, symbol of Byzantium, possibly influencing the development of the Flags of the Ottoman Empire, Ottoman flag, specifically the combination of the Turkish crescent with a star. Other historic states and Star and crescent#Contemporary use, contemporarily a range of municipal and national flags employ the symbol of star and crescent. Many but not all employ the star and crescent since it (as the ''wikt:hilal#Noun, hilal'' of the Islamic calendar) has been identified as a Symbols of Islam, symbol for Islam. Particularly attributed to Muhammad is also the so-called splitting of the Moon ( ar, انشقاق القمر) miracle. In Roman Catholic Marian veneration, the Virgin Mary (Queen of Heaven) has been depicted since the late middle ages on a crescent and adorned with stars. The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon, the Moon rabbit, rabbit (e.g. the Chinese Tu'er Ye or in Mythologies of the indigenous peoples of the Americas, Indigenous American mythologies, as with the aspect of the Maya moon goddess, Mayan Moon goddess) and the buffalo, among others. The European iconographic tradition of representing Sun and Moon with faces developed in the late middle ages.

The perception of the Moon in modern times has been informed by the telescope enabled modern astronomy observation of the surface of the Moon, subsequent mapping and eventual actual scientific lunar exploration by the Apollo program#Cultural impact, culturally impactful lunar landings. These new insights inspired and intertwined with established cultural references of the Moon and allowed science-fiction to become established, particularly science-fiction dealing with the Moon and its possible environment and life, but also connecting with romantic reflections about the Moon. In face of prospecting commercialization of the Moon has the Moon not only gained public interest, but has also seen public and critical reflection on humanity's cultural and subsequently Moon#Legal status, legal relation to the celestial body, questioning colonization, in this case of the Moon's nature, with reflections like ''Whitey on the Moon'' or advocacy for conservation of the Moon and for its inorganic nature as a commons, common. A song titled 'Moon Anthem' by Abhay Kumar, paralleling the proposals for an Earth Anthem, was released 2019 on the occasion of India's lunar probe Chandrayaan-2.Ahead Of Chandrayaan 2 Landing, Poet-Diplomat Writes "Moon Anthem"
NDTV, 6 Sept.2019

Lunar effect

The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. The Moon has long been associated with insanity and irrationality; the words ''lunacy'' and ''lunatic'' are derived from the Latin name for the Moon, ''Luna''. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims.

*List of natural satellites

References

* * * * (podcast and transcript) * * * * * * * * * * *

NASA images and videos about the Moon
* Albums of images and high-resolution overflight videos by Seán Doran, based on Lunar Reconnaissance Orbiter Camera, LROC data, o
Flickr
an
* *

Cartographic resources

Unified Geologic Map of the Moon
– United States Geological Survey
Moon Trek – An integrated map browser of datasets and maps for the Moon

a 3-D rendition of the Moon akin to Google Earth *
Gazetteer of Planetary Nomenclature (USGS)
List of feature names. * * 3D zoomable globes: ** ** * Maps and panoramas at Apollo landing sites

Selene, Kaguya (Selene) images
Lunar Earthside chart (4497 x 3150px)

Large image of the Moon's north pole area

Large image of Moon's south pole area (1000x1000px)

Observation tools

* * * See when the next new crescent moon is visible for any location.

General

Lunar shelterbuilding a lunar base with 3D printing
{{Authority control Moon, Moons, * Articles containing video clips Astronomical objects known since antiquity Planetary satellite systems Planetary-mass satellites