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The Moon is
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's only
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
. It
orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...
around Earth at an average distance of (; about 30 times Earth's diameter). The Moon rotates, with a rotation period (
lunar day A lunar day is the time it takes for Earth's Moon to complete on its axis one synodic rotation, meaning with respect to the Sun. Informally, a lunar day and a lunar night is each approximately 14 Earth days. The formal lunar day is therefore t ...
) that is synchronized to its orbital period (
lunar month In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Euro ...
) of 29.5 Earth days. This is the product of Earth's gravitation having tidally pulled on the Moon until one part of it stopped rotating away from the near side, making always the same lunar surface face Earth. Conversley, the gravitational pull of the Moon, on Earth, is the main driver of Earth's
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
s. In geophysical terms, the Moon is a
planetary-mass object A planetary-mass object (PMO), planemo, or planetary body (sometimes referred to as a world) is, by geophysical definition of celestial objects, any celestial object massive enough to achieve hydrostatic equilibrium, but not enough to sustain c ...
or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is , roughly one-quarter of Earth's (about as wide as the
contiguous United States The contiguous United States, also known as the U.S. mainland, officially referred to as the conterminous United States, consists of the 48 adjoining U.S. states and the District of Columbia of the United States in central North America. The te ...
). Within the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
, it is the largest and most massive satellite in relation to its parent planet, the fifth-largest and fifth-most massive moon overall, and larger and more massive than all known
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
s. Its
surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experi ...
is about one-sixth of Earth's, about half that of
Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
, and the second-highest among all moons in the Solar System, after
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
's moon Io. The body of the Moon is differentiated and terrestrial, with no significant
hydrosphere The hydrosphere () is the combined mass of water found on, under, and above the Planetary surface, surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to ch ...
,
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
, or
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
. The
lunar surface The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is the structure and composition of the Moon, which is quite different from that of Earth. The Moon lacks a true atmosphe ...
is covered in lunar dust and marked by
mountains A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher ...
,
impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...
s, their ejecta, ray-like streaks,
rille Rille (German for 'groove') is typically used to describe any of the long, narrow depressions in the surface of the Moon that resemble channels. The Latin term is ''rima'', plural ''rimae''. Typically, a rille can be several kilometers w ...
s and, mostly on the near side of the Moon, by dark maria ('seas'), which are plains of cooled lava. These maria were formed when molten lava flowed into ancient impact basins. The Moon formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called
Theia Theia (; , also rendered Thea or Thia), also called Euryphaessa (, "wide-shining"), is one of the twelve Titans, the children of the earth goddess Gaia and the sky god Uranus in Greek mythology. She is the Greek goddess of sight and vision, an ...
. The Moon is, except when passing through
Earth's shadow Earth's shadow (or Earth shadow) is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period (both early dusk and late dawn), the shadow's visible fringe – someti ...
during a
lunar eclipse A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, ...
, always illuminated by the Sun, but from Earth the visible illumination shifts during its orbit, producing the
lunar phase A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...
s. The Moon is the brightest celestial object in Earth's
night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...
. This is mainly due to its large
angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual an ...
, while the
reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
of the lunar surface is comparable to that of
asphalt Asphalt most often refers to: * Bitumen, also known as "liquid asphalt cement" or simply "asphalt", a viscous form of petroleum mainly used as a binder in asphalt concrete * Asphalt concrete, a mixture of bitumen with coarse and fine aggregates, u ...
. The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun completely during a total
solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season i ...
. From Earth about 59% of the lunar surface is visible due to cyclical shifts in perspective (
libration In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see ...
), making parts of the far side of the Moon visible. The Moon has been an important source of inspiration and knowledge for humans, having been crucial to
cosmography The term cosmography has two distinct meanings: traditionally it has been the protoscience of mapping the general features of the cosmos, heaven and Earth; more recently, it has been used to describe the ongoing effort to determine the large-sca ...
, mythology,
religion Religion is a range of social system, social-cultural systems, including designated religious behaviour, behaviors and practices, morals, beliefs, worldviews, religious text, texts, sanctified places, prophecies, ethics in religion, ethics, or ...
, art, time keeping,
natural science Natural science or empirical science is one of the branches of science concerned with the description, understanding and prediction of natural phenomena, based on empirical evidence from observation and experimentation. Mechanisms such as peer ...
, and
spaceflight Spaceflight (or space flight) is an application of astronautics to fly objects, usually spacecraft, into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such ...
. The first human-made objects to fly to an extraterrestrial body were sent to the Moon, starting in 1959 with the flyby of the Soviet Union's ''
Luna 1 ''Luna 1'', also known as ''Mechta'' ( , ''Literal translation, lit.'': ''Dream''), ''E-1 No.4'' and ''First Lunar Rover'', was the first spacecraft to reach the vicinity of Earth's Moon, the first spacecraft to leave Earth's orbit, and the fi ...
'' and the intentional impact of ''
Luna 2 ''Luna 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 spacecraft Moon landi ...
''. In 1966, the first
soft landing A soft landing is any type of aircraft, rocket or spacecraft landing that does not result in significant damage to or destruction of the vehicle or its payload, as opposed to a hard landing. The average vertical speed in a soft landing should b ...
(by ''
Luna 9 Luna 9 (Луна-9), internal designation Ye-6 No.13, was an uncrewed space mission of the Soviet Union's Luna programme. On 3 February 1966, the Luna 9 spacecraft became the first spacecraft to achieve a soft landing on the Moon and return ima ...
'') and orbital insertion (by ''
Luna 10 Luna 10 (or Lunik 10) was a 1966 Soviet lunar robotic spacecraft mission in the Luna program. It was the first artificial satellite of the Moon, and any other body other than Earth and the Sun (in heliocentric orbit). Luna 10 conducted extens ...
'') followed. On July 20, 1969, humans for the first time stepped on an extraterrestrial body, landing on the Moon at
Mare Tranquillitatis Mare Tranquillitatis (Latin for Sea of Tranquillity or Sea of Tranquility) is a lunar mare that sits within the Tranquillitatis basin on the Moon. It contains Tranquility Base, the first location on another celestial body to be visited by huma ...
with the lander ''
Eagle Eagle is the common name for the golden eagle, bald eagle, and other birds of prey in the family of the Accipitridae. Eagles belong to several groups of Genus, genera, some of which are closely related. True eagles comprise the genus ''Aquila ( ...
'' of the United States'
Apollo 11 Apollo 11 was a spaceflight conducted from July 16 to 24, 1969, by the United States and launched by NASA. It marked the first time that humans Moon landing, landed on the Moon. Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin l ...
mission. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the
Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...
crew. Since then,
exploration of the Moon The physical exploration of the Moon began when ''Luna 2'', a space probe launched by the Soviet Union, made a deliberate impact on the surface of the Moon on 14 September, 1959. Prior to that the only available means of lunar exploration had ...
has continued robotically, and crewed missions are being planned to return beginning in the late 2020s.


Names and etymology

The English proper name for Earth's natural satellite is typically written as ''Moon'', with a capital ''M''. The noun ''moon'' is derived from
Old English Old English ( or , or ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the Early Middle Ages. It developed from the languages brought to Great Britain by Anglo-S ...
, which stems from
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the linguistic reconstruction, reconstructed proto-language of the Germanic languages, Germanic branch of the Indo-European languages. Proto-Germanic eventually developed from ...
''*mēnōn'', which in turn comes from
Proto-Indo-European Proto-Indo-European (PIE) is the reconstructed common ancestor of the Indo-European language family. No direct record of Proto-Indo-European exists; its proposed features have been derived by linguistic reconstruction from documented Indo-Euro ...
''*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 a rare poetic name for the Moon personified as a goddess, while ''
Selene In ancient Greek mythology and Ancient Greek religion, religion, Selene (; , meaning "Moon")''A Greek–English Lexicon's.v. σελήνη is the goddess and personification of the Moon. Also known as Mene (), she is traditionally the daughter ...
'' (literally 'Moon') is the Greek goddess of the Moon. The English adjective pertaining to the Moon is ''lunar'', derived from the Latin word for the Moon, . ''Selenian'' is an adjective used to describe the Moon as a world, rather than as a celestial object, but its use is rare. It is derived from , the Greek word for the Moon, and its cognate ''selenic'' was originally a rare synonym but now nearly always refers to the chemical element
selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...
. The element name ''selenium'' and the prefix ''seleno-'' (as in ''
selenography Selenography is the study of the surface and physical features of the Moon (also known as geography of the Moon, or selenodesy). Like geography and areography, selenography is a subdiscipline within the field of planetary science. Historically, ...
'', the study of the physical features of the Moon) come from this Greek word.
Artemis In ancient Greek religion and Greek mythology, mythology, Artemis (; ) is the goddess of the hunting, hunt, the wilderness, wild animals, transitions, nature, vegetation, childbirth, Kourotrophos, care of children, and chastity. In later tim ...
, the Greek goddess of the wilderness and the hunt, also came to be identified with Selene, and was sometimes called Cynthia after her birthplace on Mount Cynthus. Her Roman equivalent is Diana. The names Luna, Cynthia, and Selene are reflected in technical terms for
lunar orbit In astronomy and spaceflight, a lunar orbit (also known as a selenocentric orbit) is an orbit by an object around Earth's Moon. In general these orbits are not circular. When farthest from the Moon (at apoapsis) a spacecraft is said to be at apo ...
s such as ''apolune'', ''pericynthion'' and ''selenocentric''. The
astronomical symbols Astronomical symbols are abstract pictorial symbols used to represent astronomical objects, theoretical constructs and observational events in European astronomy. The earliest forms of these symbols appear in Greek papyrus texts of late anti ...
for the Moon are the crescent and decrescent , for example in ''M'' 'lunar mass'.


Natural history


Lunar geologic timescale

The lunar geological periods are named after their characteristic features, from most
impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...
s outside the dark
mare A mare is an adult female horse or other equidae, equine. In most cases, a mare is a female horse over the age of three, and a filly is a female horse three and younger. In Thoroughbred horse racing, a mare is defined as a female horse more th ...
, to the mare and later craters, and finally the young, still bright and therefore readily visible craters with
ray system In planetary geology, a ray system comprises radial streaks of fine '' ejecta'' thrown out during the formation of an impact crater, looking somewhat like many thin spokes coming from the hub of a wheel. The rays may extend for lengths up to ...
s like
Copernicus Nicolaus Copernicus (19 February 1473 – 24 May 1543) was a Renaissance polymath who formulated a mathematical model, model of Celestial spheres#Renaissance, the universe that placed heliocentrism, the Sun rather than Earth at its cen ...
or Tycho.


Formation

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the
origin of the Solar System There is evidence that the formation of the Solar System began about 4.6 bya, billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, whil ...
. Historically, several formation mechanisms have been proposed, but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through
centrifugal force Centrifugal force is a fictitious force in Newtonian mechanics (also called an "inertial" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It appears to be directed radially away from the axi ...
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 composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weather ...
to
dissipate In thermodynamics, dissipation is the result of an irreversible process that affects a thermodynamic system. In a dissipative process, energy ( internal, bulk flow kinetic, or system potential) transforms from an initial form to a final form, wh ...
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 circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...
does not explain the depletion of metals in the Moon. None of these hypotheses can account for the high
angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
of the Earth–Moon system. The prevailing theory is that the Earth–Moon system formed after a
giant impact The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Early Earth collided wit ...
of a
Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
-sized body (named ''
Theia Theia (; , also rendered Thea or Thia), also called Euryphaessa (, "wide-shining"), is one of the twelve Titans, the children of the earth goddess Gaia and the sky god Uranus in Greek mythology. She is the Greek goddess of sight and vision, an ...
'') with the proto-Earth. The oblique impact blasted material into orbit about the Earth and 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 force ...
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 show that most of the Moon derived from the impactor, rather than the proto-Earth. However, models from 2007 and later 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 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. The newly formed Moon would have had its own magma ocean; 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. Models that have the Moon acquiring a significant amount of the proto-earth are more difficult to reconcile with geochemical data for the isotopes of zirconium, oxygen, silicon, and other elements. A study published in 2022, using high-resolution simulations (up to particles), found 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 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 force ...
. 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. On November 1, 2023, scientists reported that, according to computer simulations, remnants of Theia could still be present inside the Earth.


Natural development

The newly formed Moon settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other,
eclipse An eclipse is an astronomical event which occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ...
s were more frequent, and
tidal effect The tidal force or tide-generating force is the difference in gravitational attraction between different points in a gravitational field, causing bodies to be pulled unevenly and as a result are being stretched towards the attraction. It is the ...
s were stronger. Due to
tidal acceleration Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit (satel ...
, the Moon's orbit around Earth has become significantly larger, with a longer period. Following formation, the Moon has cooled and most of its atmosphere has been stripped. The
lunar surface The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is the structure and composition of the Moon, which is quite different from that of Earth. The Moon lacks a true atmosphe ...
has since been shaped by large
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
s and many small ones, forming a landscape featuring craters of all ages. The Moon was volcanically active until 1.2 billion years ago, which laid down the prominent lunar maria. Most of the mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some are as young as 1.2 billion years and some as old as 4.2 billion years. There are differing explanations for the eruption of mare basalts, particularly their uneven occurrence which mainly appear on the near-side. Causes of the distribution of the
lunar highlands The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is the structure and composition of the Moon, which is quite different from that of Earth. The Moon lacks a true atmosphe ...
on the far side are also not well understood. Topological measurements show the near side crust is thinner than the far side. One possible scenario then is that large impacts on the near side may have made it easier for lava to flow onto the surface.


Physical characteristics

The Moon is a very slightly scalene
ellipsoid An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a Surface (mathemat ...
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 restore
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...
at its current orbital distance.


Size and mass

The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizable as one of its
planetary-mass moon A planetary-mass moon is a planetary-mass object. They are large and ellipsoidal (sometimes spherical) in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in t ...
s, making it a satellite planet under the geophysical definitions of the term. It is smaller than Mercury and considerably larger than the largest
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
of the Solar System,
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
. The Moon is the largest natural satellite in the Solar System relative to its
primary Primary or primaries may refer to: Arts, entertainment, and media Music Groups and labels * Primary (band), from Australia * Primary (musician), hip hop musician and record producer from South Korea * Primary Music, Israeli record label Work ...
planet. The Moon's diameter is about 3,500 km, more than one-quarter of Earth's, with the face of the Moon comparable to the width of either
mainland Australia Mainland Australia is the main landmass of the Australia (continent), Australian continent, excluding the Aru Islands Regency, Aru Islands, New Guinea, Tasmania, and other list of islands of Australia, Australian offshore islands. The landmass ...
, Europe or the
contiguous United States The contiguous United States, also known as the U.S. mainland, officially referred to as the conterminous United States, consists of the 48 adjoining U.S. states and the District of Columbia of the United States in central North America. The te ...
. The whole surface area of the Moon is about 38 million square kilometers, comparable to that of the
Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...
. The Moon's mass is of Earth's, being the second densest among the planetary moons, and having the second highest
surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experi ...
, after Io, at and an escape velocity of .


Structure

The Moon is a differentiated body that was initially in
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...
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), everything except the appendages * Core (laboratory), a highly specialized shared research resource * Core (manufacturing), used in casting and molding * Core (optical fiber ...
. 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 or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
mantle from the
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
and sinking of the minerals
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
,
clinopyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron (Fe ...
, and orthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-density
plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
minerals could form and float into a crust atop. 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. Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly
anorthosite Anorthosite () is a phaneritic, intrusive rock, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic ...
. The
Moon rock Moon rock or lunar rock is rock originating from Earth's Moon. This includes lunar material collected during the course of human exploration of the Moon, and rock that has been ejected naturally from the Moon's surface and landed on Earth ...
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 .


Gravitational field

On average the Moon's
surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experi ...
is (; ), about half of the surface gravity of
Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
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 The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...
of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.


Magnetic field

The Moon has an external magnetic field of less than 0.2 nanoteslas, or less than one hundred thousandth that of Earth. The Moon does not have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. 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 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 In geography, the antipode () of any spot on Earth is the point on Earth's surface diametrically opposite to it. A pair of points ''antipodal'' () to each other are situated such that a straight line connecting the two would pass through Ea ...
of the giant impact basins.


Atmosphere

The Moon has an
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
consisting of only an
exosphere The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of ...
, which is so tenuous as to be nearly
vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
, with a total mass of less than . The surface pressure of this small mass is around 3 × 10−15  atm (0.3  nPa); it varies with the lunar day. Its sources include
outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (whic ...
and
sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and c ...
, a product of the bombardment of lunar soil by solar wind ions. Elements that have been detected include
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
and
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
, produced by sputtering (also found in the atmospheres of Mercury and Io);
helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consi ...
and
neon Neon is a chemical element; it has symbol Ne and atomic number 10. It is the second noble gas in the periodic table. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with approximately two-thirds the density of ...
from the solar wind; and
argon-40 Argon (Ar) has 26 known isotopes, from Ar to Ar, of which three are stable (Ar, Ar, and Ar). On Earth, Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are Ar with a half-life of 268 years, Ar with a half-life of 32.9 ye ...
,
radon-222 Radon-222 (222Rn, Rn-222, historically radium emanation or radon) is the most stable isotope of radon, with a half-life of approximately 3.8215(2) days. It is transient in the decay chain of primordial uranium-238 and is the immediate decay pro ...
, and
polonium-210 Polonium-210 (210Po, Po-210, historically radium F) is an isotope of polonium. It undergoes alpha decay to stable 206Pb with a half-life of 138.376 days (about months), the longest half-life of all naturally occurring polonium isotopes (210– ...
, outgassed after their creation by
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
within the crust and mantle. The absence of such neutral species (atoms or molecules) as
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
,
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
,
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
,
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
, which are present in the
regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
, is not understood. Water vapor has been detected by '' Chandrayaan-1'' and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. Studies of Moon magma samples retrieved by the
Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...
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 from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. 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
LADEE The Lunar Atmosphere and Dust Environment Explorer (LADEE; ) was a NASA lunar exploration and technology demonstration mission. It was launched on a Minotaur V rocket from the Mid-Atlantic Regional Spaceport on September 7, 2013. During its s ...
's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid
meteor shower A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at ext ...
s, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being denser near the boundary between the Moon's dayside and nightside.


Surface conditions

Ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
from
cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...
s, their resulting
neutron radiation Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides— ...
, and the Sun results in an average radiation level of 1.369 millisieverts per day during lunar
daytime Daytime or day as observed on Earth is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the ...
, which is about 2.6 times more than the level on the
International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...
, 510 times more than the level during a trans-Atlantic flight, and 200 times more than the level on Earth's surface. For further comparison, radiation levels average about 1.84 millisieverts per day on a flight to Mars and about 0.64 millisieverts per day on Mars itself, with some locations on Mars possibly having levels as low as 0.342 millisieverts per day. Solar radiation also electrically charges the highly abrasive lunar dust and makes it levitate. This effect contributes to the easy spread of the sticky, lung- and gear-damaging lunar dust. The Moon's
axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbita ...
with respect to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
is only 1.5427°, much less than the 23.44° of Earth. This small axial tilt means that the Moon's solar illumination varies much less with
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's axial tilt, tilted orbit around the Sun. In temperat ...
than Earth's, and it also allows for the existence of some peaks of eternal light at the Moon's north pole, at the rim of the crater Peary. The lunar surface is exposed to drastic temperature differences ranging from to depending on the
solar irradiance Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ( ...
. Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow, making topographical details play a decisive role on local surface temperatures. Parts of many craters, particularly the bottoms of many polar craters, are permanently shadowed. These craters of eternal darkness have extremely low temperatures. The ''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. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
. 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 Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
, formed by impact processes. The finer regolith, the
lunar soil Lunar regolith is the unconsolidated material found on the selenography, surface of the Moon and in the Lunar atmosphere, Moon's tenuous atmosphere. Sometimes referred to as Lunar soil, Lunar soil specifically refers to the component of regoli ...
of
silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...
glass, has a texture resembling snow and a scent resembling spent
gunpowder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, charcoal (which is mostly carbon), and potassium nitrate, potassium ni ...
. 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. These extreme conditions are considered to make it unlikely for spacecraft to harbor bacterial spores at the Moon for longer than just one lunar orbit.


Surface features

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, 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, reaching at in a crater within Antoniadi crater. The highest elevations of the Moon's surface, with the so-called Selenean summit at , 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 Imbrium, Serenitatis, Crisium, Smythii, 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-like offset of the ground surface in which one side of a fault has shifted vertically in relation to the other. The topographic expression of fault scarps results from the differential erosion of rocks of contrastin ...
cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. 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 does not have tectonic plates, its tectonic activity is slow, and cracks develop as it loses heat. Scientists have confirmed the presence of a cave on the Moon near the Sea of Tranquillity, not far from the 1969
Apollo 11 Apollo 11 was a spaceflight conducted from July 16 to 24, 1969, by the United States and launched by NASA. It marked the first time that humans Moon landing, landed on the Moon. Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin l ...
landing site. The cave, identified as an entry point to a collapsed lava tube, is roughly 45 meters wide and up to 80 m long. This discovery marks the first confirmed entry point to a lunar cave. The analysis was based on photos taken in 2010 by NASA's
Lunar Reconnaissance Orbiter The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric Polar orbit, polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic ...
. The cave's stable temperature of around could provide a hospitable environment for future astronauts, protecting them from extreme temperatures, solar radiation, and micrometeorites. However, challenges include accessibility and risks of avalanches and cave-ins. This discovery offers potential for future lunar bases or emergency shelters.


Volcanic features

The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called '' maria'' (singular ''mare'';
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
for "seas", as they were once believed to be filled with water) are vast solidified pools of ancient
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
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 Impact may refer to: * Impact (mechanics), a large force or mechanical shock over a short period of time * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Im ...
, though the Moon's largest expanse of basalt flooding,
Oceanus Procellarum Oceanus Procellarum ( ; from ) is a vast lunar mare on the western edge of the near side of the Moon. It is the only one of the lunar maria to be called an "Oceanus" (ocean), due to its size: Oceanus Procellarum is the largest of the maria ("s ...
, does not correspond to an obvious impact basin. Different episodes of lava flow in maria can often be recognized by variations in surface albedo and distinct flow margins. As the maria formed, cooling and contraction of the basaltic lava created
wrinkle ridge A wrinkle ridge is a type of feature commonly found on lunar maria, or basalt plains. These features are low, sinuous ridges formed on the mare surface that can extend for up to several hundred kilometers. Wrinkle ridges are tectonic features ...
s in some areas. These low, sinuous ridges can extend for hundreds of kilometers and often outline buried structures within the mare. Another result of maria formation is the creation of concentric depressions along the edges, known as arcuate rilles. These features occur as the mare basalts sink inward under their own weight, causing the edges to fracture and separate. In addition to the visible maria, the Moon has mare deposits covered by ejecta from impacts. Called cryptomares, these hidden mares are likely older than the exposed ones. Conversely, mare lava has obscured many impact melt sheets and pools. Impact melts are formed when intense shock pressures from collisions vaporize and melt zones around the impact site. Where still exposed, impact melt can be distinguished from mare lava by its distribution, albedo, and texture. Sinuous rilles, found in and around maria, are likely extinct lava channels or collapsed
lava tubes A lava tube, more rarely called a pyroduct, is a 'roofed conduit through which molten lava travels away from its vent'. If lava in the tube drains out, it will leave an empty cave. Lava tubes are common in low-viscosity volcanic systems. La ...
. They typically originate from volcanic vents, meandering and sometimes branching as they progress. The largest examples, such as Schroter's Valley and Rima Hadley, are significantly longer, wider, and deeper than terrestrial lava channels, sometimes featuring bends and sharp turns that again, are uncommon on Earth. Mare volcanism has altered impact craters in various ways, including filling them to varying degrees, and raising and fracturing their floors from uplift of mare material beneath their interiors. Examples of such craters include Taruntius and Gassendi. Some craters, such as
Hyginus Hyginus may refer to: People *Hyginus, the author of the '' Fabulae'', an important ancient Latin source for Greek mythology. *Hyginus, the author of the ''Astronomia'', a popular ancient Latin guide on astronomy, probably the same as the author ...
, are of wholly volcanic origin, forming as
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...
s or collapse pits. Such craters are relatively rare and tend to be smaller (typically a few kilometers wide), shallower, and more irregularly shaped than impact craters. They also lack the upturned rims characteristic of impact craters. Several
geologic province A geologic province or geological 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 ...
s containing
shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...
es and volcanic
domes A dome () is an architectural element similar to the hollow upper half of a sphere. There is significant overlap with the term cupola, which may also refer to a dome or a structure on top of a dome. The precise definition of a dome has been a m ...
are found within the near side maria. There are also some regions of pyroclastic deposits, scoria cones and non-basaltic domes made of particularly high viscosity lava. 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 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 mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years and as old as 4.2 billion years. In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. Moonquakes and releases of gas indicate 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 Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
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 Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an objec ...
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 a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...
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. Lunar craters exhibit a variety of forms, depending on their size. In order of increasing diameter, the basic types are simple craters with smooth bowl shaped interiors and upturned rims,
complex crater Complex craters are a type of large impact crater morphology. Complex craters are classified into two groups: ''central-peak craters'' and '' peak-ring craters''. Peak-ring craters have diameters that are larger in than central-peak craters and ...
s with flat floors, terraced walls and central peaks,
peak ring A peak ring crater is a type of complex crater, which is different from a multi-ringed basin or central-peak crater. A central peak is not seen; instead, a roughly circular ring or plateau, possibly discontinuous, surrounds the Impact crater, crat ...
basins, and multi-ring basins with two or more concentric rings of peaks. The vast majority of impact craters are circular, but some, like
Cantor A cantor or chanter is a person who leads people in singing or sometimes in prayer. Cantor as a profession generally refers to those leading a Jewish congregation, although it also applies to the lead singer or choir director in Christian contexts. ...
and Janssen, have more polygonal outlines, possibly guided by underlying faults and joints. Others, such as the
Messier Messier may refer to: People with the surname *Ashley Messier (born 2002), Canadian ice hockey player *Charles Messier (1730–1817), French astronomer * Doug Messier (born 1936), Canadian ice hockey player and coach * Éric Messier (born 1973), Ca ...
pair,
Schiller Johann Christoph Friedrich von Schiller (, short: ; 10 November 17599 May 1805) was a German playwright, poet, philosopher and historian. Schiller is considered by most Germans to be Germany's most important classical playwright. He was born i ...
, and
Daniell Daniell is a surname. Notable people with the surname include: * Alexander Daniell (1599–1668), Cornish landowner * Alfred Daniell (1853–1937) * Ave Daniell (1914–1999), American (gridiron) footballer * Charles Daniell (1827–1889), Majo ...
, are elongated. Such elongation can result from highly oblique impacts,
binary asteroid A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple a ...
impacts, fragmentation of impactors before surface strike, or closely spaced secondary impacts. The
lunar geologic timescale The lunar geological timescale (or selenological timescale) divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian ( Late and Early epochs), Nectarian, and Pre-Nectarian. The boundar ...
is based on the most prominent impact events, such as multi-ring formations like Nectaris, Imbrium, and Orientale that are between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon. The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. However care needs to be exercised with the
crater counting Crater counting is a method for estimating the age of a planet's surface based upon the assumptions that when a piece of planetary surface is new, then it has no impact craters; impact craters accumulate after that at a rate that is assumed known. ...
technique due to the potential presence of
secondary crater Secondary craters are impact craters formed by the ejecta that was thrown out of a larger crater. They sometimes form radial crater chains. In addition, secondary craters are often seen as clusters or rays surrounding primary craters. The study of ...
s. Ejecta from impacts can create secondary craters that often appear in clusters or chains but can also occur as isolated formations at a considerable distance from the impact. These can resemble primary craters, and may even dominate small crater populations, so their unidentified presence can distort age estimates. 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 The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized astronomical event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According ...
period of increased impacts. 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. This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.


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 Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
), and often have a sinuous shape. Their shape is often accentuated by low
albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
regions that wind between the bright swirls. They are located in places with enhanced surface
magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
s and many are located at the
antipodal point In mathematics, two points of a sphere (or n-sphere, including a circle) are called antipodal or diametrically opposite if they are the endpoints of a diameter, a straight line segment between two points on a sphere and passing through its cen ...
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 The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
, resulting in slower
space weathering Space weathering is the type of weathering that occurs to any object exposed to the harsh environment of outer space. Bodies without atmospheres (including the Moon, Mercury, the asteroids, comets, and most of the moons of other planets) take ...
.


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 Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...
and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting
comets A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, an ...
or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from
solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
, 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
lunar habitation Lunar habitation is any human habitation on the Moon. Lunar habitation is provided by surface habitats, possibly as part of a moonbase. History alt='' Eagle'', the first ever '' Eagle'', the first ever surface habitat and the first time p ...
as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive. In years since, signatures of water have been found to exist on the lunar surface. In 1994, the bistatic radar experiment located on the ''
Clementine A clementine (''Citrus × clementina'') is a tangor, a citrus fruit hybrid between a willowleaf mandarin orange ( ''C.'' × ''deliciosa'') and a sweet orange (''C. × sinensis''), named in honor of Clément Rodier, a French missionary who f ...
'' spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by
Arecibo Arecibo (; ) is a Arecibo barrio-pueblo, city and Municipalities of Puerto Rico, municipality on the northern coast of Puerto Rico, on the shores of the Atlantic Ocean, located north of Utuado, Puerto Rico, Utuado and Ciales, Puerto Rico, Ciale ...
, suggest these findings may rather be rocks ejected from young impact craters. In 1998, the neutron spectrometer on the ''Lunar Prospector'' spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. The 2008 '' Chandrayaan-1'' spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000  ppm. Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. In 2009, '' LCROSS'' sent a impactor into a 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 A melt inclusion is a small parcel or "blobs" of melt(s) that is entrapped by crystals growing in magma and eventually forming igneous rocks. In many respects it is analogous to a fluid inclusion within magmatic hydrothermal systems. Melt inclusi ...
in lunar sample 74220 was reported, the famous high-titanium "orange glass soil" of volcanic origin collected during the
Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...
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 The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...
. 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. The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun. In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the
Stratospheric Observatory for Infrared Astronomy The Stratospheric Observatory For Infrared Astronomy (SOFIA) was an 80/20 joint project of NASA and the German Aerospace Center (DLR) to construct and maintain an airborne observatory. NASA awarded the contract for the development of the aircra ...
(SOFIA).


Earth–Moon system


Orbit

The Earth and the Moon form the Earth–Moon satellite system with a shared center of mass, or
barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
. This barycenter is (about a quarter of Earth's radius) beneath the Earth's surface. The Moon's orbit is slightly elliptical, with an
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...
of 0.055. The
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longe ...
of the geocentric lunar orbit, called the
lunar distance The instantaneous Earth–Moon distance, or distance to the Moon, is the distance from the center of Earth to the center of the Moon. In contrast, the Lunar distance (LD or \Delta_), or Earth–Moon characteristic distance, is a unit of measure ...
, is approximately 400,000 km (250,000 miles or 1.28 light-seconds), comparable to going around Earth 9.5 times. The Moon makes a complete orbit around Earth with respect to the fixed stars, its sidereal period, about once every 27.3 days. However, because the Earth–Moon system moves at the same time in its orbit around the Sun, it takes slightly longer, 29.5 days, to return to the same
lunar phase A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...
, completing a full cycle, as seen from Earth. This
synodic period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, ...
or synodic month is commonly known as the
lunar month In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Euro ...
and is equal to the length of the
solar day A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time. The synodic day is distinguished from the sidereal day, which is ...
on the Moon. Due to
tidal locking Tidal locking between a pair of co-orbiting astronomical body, astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where ...
, the Moon has a 1:1 spin–orbit resonance. This
rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
ratio makes the Moon's orbital periods around Earth equal to its corresponding
rotation period In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the '' sidereal rotation period'' (or ''sidereal day''), i.e., the time that the objec ...
s. This is the reason for only one side of the Moon, its so-called 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 In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see ...
, resulting in slightly changing perspectives, making over time and location on Earth about 59% of the Moon's surface visible from Earth. Unlike most satellites of other planets, the Moon's orbital plane is closer to the
ecliptic plane The ecliptic or ecliptic plane is the orbital plane of Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. From the perspec ...
than to the planet's
equatorial plane The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. By extension, it is also a plane of reference in the equatorial coordinate system. Due to Earth's axial tilt, the celestial e ...
. 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 18.61years, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.


Tidal effects

The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides closest to each other, resulting in
tidal forces The tidal force or tide-generating force is the difference in gravitational attraction between different points in a gravitational field, causing bodies to be pulled unevenly and as a result are being stretched towards the attraction. It is the d ...
. Ocean tides are the most widely experienced result of this, but tidal forces also considerably affect other mechanics of Earth, as well as the Moon and their system. The lunar solid crust experiences tides of around amplitude over 27 days, with three components: a fixed one due to Earth, because they are in
synchronous rotation Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
, 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 In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see ...
, 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). According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
. The cumulative effects of stress built up by these tidal forces produces
moonquakes A quake is the result when the surface of a planet, moon or star begins to shake, usually as the consequence of a sudden release of energy transmitted as seismic waves, and potentially with great violence. The types of quakes include earthquake, ...
. 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
seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...
s placed on the Moon by
Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...
astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...
s from 1969 through 1972. The most commonly known effect of tidal forces is elevated sea levels called ocean tides. While the Moon exerts most of the tidal forces, the Sun also exerts tidal forces and therefore contributes to the tides as much as 40% of the Moon's tidal force; producing in interplay the spring and neap tides. The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. The tide under the Moon is explained by the Moon's gravity being stronger on the water close to it. The tide on the opposite side can be explained either by the centrifugal force as the Earth orbits the
barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
or by the water's inertia as the Moon's gravity is stronger on the solid Earth close to it and it is pull away from the farther water. Thus, 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. 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 Inertia is the natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes the velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newto ...
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.


System evolution

Delays in the tidal peaks of both ocean and solid-body tides cause
torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
in opposition to the Earth's rotation. This "drains"
angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
and rotational
kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...
from Earth's rotation, slowing the Earth's rotation. That angular momentum, lost from the Earth, is transferred to the Moon in a process known as
tidal acceleration Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit (satel ...
, 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. Measurements from laser reflectors left during the Apollo missions ( lunar ranging experiments) have found that the Moon's distance increases by per year (roughly the rate at which human fingernails grow).
Atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s show that Earth's Day lengthens by about 17 
microsecond A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available. A microsecond is to one second, ...
s every year, slowly increasing the rate at which
UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...
is adjusted by
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...
s. 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, 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 A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...
, most likely engulfing the Earth–Moon system long before then. If the Earth–Moon system isn't engulfed by the enlarged Sun, the drag from the solar atmosphere can cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of , it will cross 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 force ...
, meaning that tidal interaction with Earth would break apart the Moon, turning it into a
ring system A ring system is a disc or torus orbiting an astronomical object that is composed of solid material such as dust, meteoroids, planetoids, moonlets, or stellar objects. Ring systems are best known as planetary rings, common components of sate ...
. Most of the orbiting rings will begin to decay, and the debris will impact Earth. Hence, even if the Sun does not swallow up Earth, the planet may be left moonless.


Position and appearance

The Moon's highest
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
at
culmination In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. These events are also known as meridian tran ...
varies by its
lunar phase A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...
, 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
North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating Direction (geometry), direction or geography. Etymology T ...
and
South Pole The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is the point in the Southern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True South Pole to distinguish ...
s the Moon is 24 hours above the horizon for two weeks every tropical month (about 27.3 days), comparable to the
polar day Midnight sun, also known as polar day, is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When midnight sun is s ...
of the
tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronom ...
.
Zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...
in the
Arctic The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...
use
moonlight Moonlight consists of mostly sunlight (with little earthlight) reflected from the parts of the Moon's surface where the Sun's light strikes. History The ancient Greek philosopher Anaxagoras was aware that "''the sun provides the moon with its ...
when the Sun is below the horizon for months on end. 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
northern hemisphere The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...
it appears upside down compared to the view from the southern hemisphere. 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 The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
. The distance between the Moon and Earth varies from around (
perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
) to (apogee), making the Moon's distance and apparent size fluctuate up to 14%. On average the Moon's
angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual an ...
is about 0.52°, roughly the same apparent size as the Sun (see ). In addition, a purely psychological effect, known as the
Moon illusion The Moon illusion is the optical illusion of the Moon appearing larger near the horizon than it does higher up in the sky. It has been known since ancient times and recorded by various cultures. The explanation of this illusion is still debate ...
, makes the Moon appear larger when close to the horizon.


Rotation

The tidally locked synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. The side of the Moon that faces Earth is called 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 In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse. ...
, the near side is dark. The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became
tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
in this orientation as a result of
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...
al effects associated with tidal deformations caused by Earth. With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998–99 NASA ''
Lunar Prospector ''Lunar Prospector'' was a spacecraft that orbited the Moon for 19 months in 1998-99. From a low polar orbit, it mapped surface composition including lunar hydrogen deposits, measured magnetic and gravity fields, and studied lunar outgassing e ...
'' 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.


Illumination and phases

The Moon rotates, as it orbits Earth, changing orientation toward the Sun, experiencing a
lunar day A lunar day is the time it takes for Earth's Moon to complete on its axis one synodic rotation, meaning with respect to the Sun. Informally, a lunar day and a lunar night is each approximately 14 Earth days. The formal lunar day is therefore t ...
. A lunar day is equal to one
lunar month In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Euro ...
(one synodic orbit around Earth) due to it being
tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
to Earth. Since the Moon is not tidally locked to the Sun, lunar daylight and night times both occur around the Moon. The changing position of the illumination of the Moon by the Sun during a lunar day is observable from Earth as the changing
lunar phase A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...
s, waxing crescent being the sunrise and the waning crescent the sunset phase of a day observed from afar. Lunar night is the darkest on the far side and during lunar eclipses on the near side (and darker than a moonless night on Earth). The near side is during its night illuminated by
Earthlight ''Earthlight'' is a science fiction novel by British writer Arthur C. Clarke, published in 1955. It is an expansion to novel length of a novella of the same name that he had published four years earlier. Overview ''Earthlight'' is a scie ...
, making the near side illuminated enough by the Earthlight to see lunar surface features from Earth where it is dark during its night phase due to Earthlight being reflected back to Earth. Earthshine makes the night on the near side about 43 times brighter, and sometimes even 55 times brighter than a night on Earth illuminated by the light of the full moon. In Earth's sky brightness and apparent size of the Moon changes also due to its elliptic orbit around Earth. At
perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
(closest), since the Moon is up to 14% closer to Earth than at
apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
(most distant), it subtends a
solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poin ...
which is up to 30% larger. Consequently, given the same phase, the Moon's brightness also varies by up to 30% between apogee and perigee. A full (or new) moon at such a position is called a supermoon.


Observational phenomena

There has been historical controversy over whether observed features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor
astronomical seeing In astronomy, seeing is the degradation of the real image, image of an astronomical object due to turbulence in the atmosphere of Earth that may become visible as blurring, twinkling or variable Distortion (optics), distortion. The origin of this ...
, or inadequate drawings. However,
outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (whic ...
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 diameter region of the lunar surface was modified by a gas release event about a million years ago.


Albedo and color

The Moon has an exceptionally low
albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
, giving it a
reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
that is slightly brighter than that of worn
asphalt Asphalt most often refers to: * Bitumen, also known as "liquid asphalt cement" or simply "asphalt", a viscous form of petroleum mainly used as a binder in asphalt concrete * Asphalt concrete, a mixture of bitumen with coarse and fine aggregates, u ...
. Despite this, it is the brightest object in the sky after the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
. This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon. Additionally,
color constancy Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple ...
in the
visual system The visual system is the physiological basis of visual perception (the ability to perception, detect and process light). The system detects, phototransduction, transduces and interprets information concerning light within the visible range to ...
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 Limb darkening is an optical effect seen in stars (including the Sun) and planets, where the central part of the disk appears brighter than the edge, or '' limb''. Its understanding offered early solar astronomers an opportunity to construct mode ...
, because of the reflective properties of
lunar soil Lunar regolith is the unconsolidated material found on the selenography, surface of the Moon and in the Lunar atmosphere, Moon's tenuous atmosphere. Sometimes referred to as Lunar soil, Lunar soil specifically refers to the component of regoli ...
, 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. At times, the Moon can appear red or blue. It may appear red during a
lunar eclipse A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, ...
, because of the red spectrum of the Sun's light being
refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
onto the Moon by Earth's atmosphere. Because of this red color, lunar eclipses are also sometimes called blood moons. The Moon can also seem red when it appears at low angles and through a thick atmosphere. The Moon may appear blue depending on the presence of certain particles in the air, such as volcanic particles, in which case it can be called a
blue moon A blue moon refers either to the presence of a second full moon in a calendar month, to the third full moon in a season containing four, or to a moon that appears blue due to atmospheric effects. The calendrical meaning of "blue moon" is unc ...
. Because the words "red moon" and "blue moon" can also be used to refer to specific full moons of the year, they do not always refer to the presence of red or blue
moonlight Moonlight consists of mostly sunlight (with little earthlight) reflected from the parts of the Moon's surface where the Sun's light strikes. History The ancient Greek philosopher Anaxagoras was aware that "''the sun provides the moon with its ...
.


Eclipses

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed " syzygy").
Solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season i ...
s occur at
new moon In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse. ...
, when the Moon is between the Sun and Earth. In contrast,
lunar eclipse A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, ...
s occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon, but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and
annular Annulus (or anulus) or annular indicates a ring- or donut-shaped area or structure. It may refer to: Human anatomy * ''Anulus fibrosus disci intervertebralis'', spinal structure * Annulus of Zinn, a.k.a. annular tendon or ''anulus tendineus com ...
(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 In astronomy, a corona (: coronas or coronae) is the outermost layer of a star's Stellar atmosphere, atmosphere. It is a hot but relatively luminosity, dim region of Plasma (physics), plasma populated by intermittent coronal structures such as so ...
becomes visible to the
naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...
. Because the distance between the Moon and Earth is very slowly increasing over time, the angular diameter of the Moon is decreasing. As it evolves toward becoming a
red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...
, 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. As the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years. Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky, the related phenomenon of
occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks f ...
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 Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...
of the lunar orbit, each year different stars are occulted.


History of exploration and human presence


Pre-telescopic observation (before 1609)

It is believed by some that the oldest
cave painting In archaeology, cave paintings are a type of parietal art (which category also includes petroglyphs, or engravings), found on the wall or ceilings of caves. The term usually implies prehistoric art, prehistoric origin. These paintings were often c ...
s from up to 40,000 BP of bulls and geometric shapes, or 20–30,000 year old
tally stick A tally stick (or simply a tally) was an ancient memory aid used to record and document numbers, quantities, and messages. Tally sticks first appear as animal bones carved with notches during the Upper Palaeolithic; a notable example is the Is ...
s were used to observe the phases of the Moon, keeping time using the waxing and waning of the Moon's phases. Aspects of the Moon were identified and aggregated in
lunar deities A lunar deity or moon deity is a deity who represents the Moon, or an aspect of it. These deities can have a variety of functions and traditions depending upon the culture, but they are often related. Lunar deities and Moon worship can be foun ...
from
prehistoric times Prehistory, also called pre-literary history, is the period of human history between the first known use of stone tools by hominins  million years ago and the beginning of recorded history with the invention of writing systems. The use o ...
and were eventually documented and put into symbols from the very first instances of
writing Writing is the act of creating a persistent representation of language. A writing system includes a particular set of symbols called a ''script'', as well as the rules by which they encode a particular spoken language. Every written language ...
in the
4th millennium BC File:4th millennium BC montage.jpg, 400x400px, From top left clockwise: The Temple of Ġgantija, one of the oldest freestanding structures in the world; Warka Vase; Bronocice pot with one of the earliest known depictions of a wheeled vehicle; Kish ...
. One of the earliest-discovered possible depictions of the Moon is a 3,000 BCE rock carving ''Orthostat 47'' at
Knowth Knowth (; ) is a prehistoric tomb overlooking the River Boyne in County Meath, Ireland. It comprises a large passage tomb surrounded by 17 smaller tombs, built during the Neolithic era around 3200 BC. It contains the largest assemblage of megali ...
, Ireland. The
crescent A crescent shape (, ) is a symbol or emblem used to represent the lunar phase (as it appears in the northern hemisphere) in the first quarter (the "sickle moon"), or by extension a symbol representing the Moon itself. In Hindu iconography, Hind ...
depicting the Moon as with the lunar deity Nanna/Sin have been found from the 3rd millennium BCE. The oldest named astronomer and poet
Enheduanna Enheduanna ( , also transliteration, transliterated as , , or variants; ) was the (high) priestess of the moon god Sin (mythology), Nanna (Sīn) in the Sumerian city-state of Ur in the reign of her father, Sargon of Akkad ( BCE). She was likely ...
, Akkadian high priestess to the lunar deity Nanna/Sin and pricess, daughter of
Sargon the Great Sargon of Akkad (; ; died 2279 BC), also known as Sargon the Great, was the first ruler of the Akkadian Empire, known for his conquests of the Sumerian city-states in the 24th to 23rd centuries BC.The date of the reign of Sargon is highly unc ...
( – BCE), had the Moon tracked in her chambers. The oldest found and identified depiction of the Moon in an astronomical relation to other astronomical features is the
Nebra sky disc The Nebra sky disc (, ) is a bronze disc of around diameter and a weight of , having a blue-green patina and inlaid with gold symbols. These symbols are interpreted generally as the Sun or full moon, a lunar crescent, and stars, including a clust ...
from , depicting features like the
Pleiades The Pleiades (), also known as Seven Sisters and Messier 45 (M45), is an Asterism (astronomy), asterism of an open cluster, open star cluster containing young Stellar classification#Class B, B-type stars in the northwest of the constellation Tau ...
next to the Moon. The
ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
philosopher
Anaxagoras Anaxagoras (; , ''Anaxagóras'', 'lord of the assembly'; ) was a Pre-Socratic Greek philosopher. Born in Clazomenae at a time when Asia Minor was under the control of the Persian Empire, Anaxagoras came to Athens. In later life he was charged ...
() 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 astronomers had recorded the 18-year Saros cycle of
lunar eclipse A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, ...
s, and Indian astronomers had described the Moon's monthly elongation. The Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses. In
Aristotle Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...
's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries.
Archimedes Archimedes of Syracuse ( ; ) was an Ancient Greece, Ancient Greek Greek mathematics, mathematician, physicist, engineer, astronomer, and Invention, inventor from the ancient city of Syracuse, Sicily, Syracuse in History of Greek and Hellenis ...
(287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. In the ,
Seleucus of Seleucia Seleucus of Seleucia ( ''Seleukos''; born c. 190 BC; fl. c. 150 BC) was a Hellenistic astronomer and philosopher. Coming from Seleucia on the Tigris, Mesopotamia, the capital of the Seleucid Empire, or, alternatively, Seleukia on the Erythraea ...
correctly thought that
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
s were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. The Chinese of the
Han dynasty The Han dynasty was an Dynasties of China, imperial dynasty of China (202 BC9 AD, 25–220 AD) established by Liu Bang and ruled by the House of Liu. The dynasty was preceded by the short-lived Qin dynasty (221–206 BC ...
believed the Moon to be energy equated to '' qi'' and their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun;
Jing Fang Jing Fang () (78–37 BC), born Li Fang (), courtesy name Junming (), was a Chinese music theorist, mathematician and astronomer born in present-day Puyang, Henan during the Han dynasty (202 BC – 220 AD). Although better known for his w ...
(78–37 BC) noted the sphericity of the Moon.
Ptolemy Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...
(90–168 AD) greatly improved on the numbers of Aristarchus, calculating a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters, close to the correct values of about 60 and 0.273 respectively. In the 2nd century AD,
Lucian Lucian of Samosata (Λουκιανὸς ὁ Σαμοσατεύς, 125 – after 180) was a Hellenized Syrian satirist, rhetorician and pamphleteer who is best known for his characteristic tongue-in-cheek style, with which he frequently ridi ...
wrote the novel ''
A True Story ''A True Story'' (, ''Alēthē diēgēmata''; or ), also translated as ''True History'', is a long novella or short novel written in the second century AD by the Syrian author Lucian of Samosata. The novel is a satire of outlandish tales that h ...
'', in which the heroes travel to the Moon and meet its inhabitants. In 510 AD, the Indian astronomer
Aryabhata Aryabhata ( ISO: ) or Aryabhata I (476–550 CE) was the first of the major mathematician-astronomers from the classical age of Indian mathematics and Indian astronomy. His works include the '' Āryabhaṭīya'' (which mentions that in 3600 ' ...
mentioned in his ''
Aryabhatiya ''Aryabhatiya'' (IAST: ') or ''Aryabhatiyam'' ('), a Indian astronomy, Sanskrit astronomical treatise, is the ''Masterpiece, magnum opus'' and only known surviving work of the 5th century Indian mathematics, Indian mathematician Aryabhata. Philos ...
'' that reflected sunlight is the cause of the shining of the Moon.Hayashi (2008), "Aryabhata I", ''Encyclopædia Britannica''. The astronomer and physicist
Ibn al-Haytham Ḥasan Ibn al-Haytham (Latinization of names, Latinized as Alhazen; ; full name ; ) was a medieval Mathematics in medieval Islam, mathematician, Astronomy in the medieval Islamic world, astronomer, and Physics in the medieval Islamic world, p ...
(965–1039) found that
sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions.
Shen Kuo Shen Kuo (; 1031–1095) or Shen Gua, courtesy name Cunzhong (存中) and Art name#China, pseudonym Mengqi (now usually given as Mengxi) Weng (夢溪翁),Yao (2003), 544. was a Chinese polymath, scientist, and statesman of the Song dynasty (960� ...
(1031–1095) of the
Song dynasty The Song dynasty ( ) was an Dynasties of China, imperial dynasty of China that ruled from 960 to 1279. The dynasty was founded by Emperor Taizu of Song, who usurped the throne of the Later Zhou dynasty and went on to conquer the rest of the Fiv ...
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. During the
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of global history. It began with the fall of the Western Roman Empire and ...
, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was "perfectly smooth".


Telescopic exploration (1609–1959)

The
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
was developed and reported on in 1608. The first record of telescopic astronomy and rough mapping of the Moon's features is from early summer 1609 by
Thomas Harriot Thomas Harriot (; – 2 July 1621), also spelled Harriott, Hariot or Heriot, was an English astronomer, mathematician, ethnographer and translator to whom the theory of refraction is attributed. Thomas Harriot was also recognized for his con ...
, but which he did not publish. At the same time
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
too started to use telescopes to observe the sky and the Moon, recording later that year more detailed observations and crucial conclusions, such as that the Moon was not smooth, featuring mountains and craters, which he published in 1610 in his ground-breaking and soon widely discussed book . Later in the 17th century, the efforts of
Giovanni Battista Riccioli Giovanni Battista Riccioli (17 April 1598 – 25 June 1671) was an Italian astronomer and a Catholic priest in the Jesuit order. He is known, among other things, for his experiments with pendulums and with falling bodies, for his discussion of ...
and
Francesco Maria Grimaldi Francesco Maria Grimaldi (2 April 1618 – 28 December 1663) was an Italian Jesuit priest, mathematician and physicist who taught at the Jesuit college in Bologna. He was born in Bologna to Paride Grimaldi and Anna Cattani. Work Between 164 ...
led to the system of naming of lunar features in use today. The more exact 1834–1836 of Wilhelm Beer and Johann Heinrich von Mädler, and their associated 1837 book , the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. Lunar craters, first noted by Galileo, were thought to be
volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...
until the 1870s proposal of Richard Proctor that they were formed by collisions. This view gained support in 1892 from the experimentation of geologist
Grove Karl Gilbert Grove Karl Gilbert (May 6, 1843 – May 1, 1918), known by the abbreviated name G. K. Gilbert in academic literature, was an American geologist. Biography Gilbert was born in Rochester, New York, and graduated from the University of Rochester. ...
, and from comparative studies from 1920 to the 1940s, leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of
astrogeology Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of celestial bodies such as planets and their moons, asteroids, comets, and meteorites. Although the geo- prefix t ...
.


First missions to the Moon (1959–1976)

After
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
the first
launch system A launch vehicle is typically a rocket-powered vehicle designed to carry a payload (a crewed spacecraft or satellites) from Earth's surface or lower atmosphere to outer space. The most common form is the ballistic missile-shaped multist ...
s were developed and by the end of the 1950s they reached capabilities that allowed the
Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
and the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
to launch
spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
into space. The
Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...
fueled a closely followed development of launch systems by the two states, resulting in the so-called
Space Race The Space Race (, ) was a 20th-century competition between the Cold War rivals, the United States and the Soviet Union, to achieve superior spaceflight capability. It had its origins in the ballistic missile-based nuclear arms race between t ...
and its later phase the Moon Race, accelerating efforts and interest in
exploration of the Moon The physical exploration of the Moon began when ''Luna 2'', a space probe launched by the Soviet Union, made a deliberate impact on the surface of the Moon on 14 September, 1959. Prior to that the only available means of lunar exploration had ...
. After the first spaceflight of
Sputnik 1 Sputnik 1 (, , ''Satellite 1''), sometimes referred to as simply Sputnik, was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program ...
in 1957 during
International Geophysical Year The International Geophysical Year (IGY; ), also referred to as the third International Polar Year, was an international scientific project that lasted from 1 July 1957 to 31 December 1958. It marked the end of a long period during the Cold War w ...
the spacecraft of the Soviet Union's ''Luna'' program were the first to accomplish a number of goals. Following three unnamed failed missions in 1958, the first human-made object ''
Luna 1 ''Luna 1'', also known as ''Mechta'' ( , ''Literal translation, lit.'': ''Dream''), ''E-1 No.4'' and ''First Lunar Rover'', was the first spacecraft to reach the vicinity of Earth's Moon, the first spacecraft to leave Earth's orbit, and the fi ...
'' escaped Earth's gravity and passed near the Moon in 1959. Later that year the first human-made object ''
Luna 2 ''Luna 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 spacecraft Moon landi ...
'' reached the Moon's surface by intentionally impacting. By the end of the year ''
Luna 3 Luna 3, or E-2A No.1 (), was a Soviet spacecraft launched in 1959 as part of the Luna programme. It was the first mission to photograph the far side of the Moon and the third Soviet space probe to be sent to the neighborhood of the Moon. The hi ...
'' reached as the first human-made object the normally occluded
far side of the Moon The far side of the Moon is the hemisphere of the Moon that is facing away from Earth, the opposite hemisphere is the near side. It always has the same surface oriented away from Earth because of synchronous rotation in the Moon's orbit. C ...
, taking the first photographs of it. The first spacecraft to perform a successful lunar
soft landing A soft landing is any type of aircraft, rocket or spacecraft landing that does not result in significant damage to or destruction of the vehicle or its payload, as opposed to a hard landing. The average vertical speed in a soft landing should b ...
was ''
Luna 9 Luna 9 (Луна-9), internal designation Ye-6 No.13, was an uncrewed space mission of the Soviet Union's Luna programme. On 3 February 1966, the Luna 9 spacecraft became the first spacecraft to achieve a soft landing on the Moon and return ima ...
'' and the first vehicle to orbit the Moon was ''
Luna 10 Luna 10 (or Lunik 10) was a 1966 Soviet lunar robotic spacecraft mission in the Luna program. It was the first artificial satellite of the Moon, and any other body other than Earth and the Sun (in heliocentric orbit). Luna 10 conducted extens ...
'', both in 1966. Following President
John F. Kennedy John Fitzgerald Kennedy (May 29, 1917 – November 22, 1963), also known as JFK, was the 35th president of the United States, serving from 1961 until his assassination in 1963. He was the first Roman Catholic and youngest person elected p ...
's 1961 commitment to a crewed Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center (FFRDC) in La Cañada Flintridge, California, Crescenta Valley, United States. Founded in 1936 by Cali ...
's
Ranger program The Ranger program was a series of uncrewed space missions by the United States in the 1960s whose objective was to obtain the first close-up images of the surface of the Moon. The Ranger spacecraft were designed to take images of the lunar su ...
, the
Lunar Orbiter program The Lunar Orbiter program was a series of five uncrewed lunar orbiter missions launched by the United States in 1966 and 1967. Intended to help select Apollo landing sites by mapping the Moon's surface, they provided the first photographs from ...
and the
Surveyor program The Surveyor program was a NASA program that, from June 1966 through January 1968, sent seven robotic spacecraft to the surface of the Moon. Its primary goal was to demonstrate the feasibility of Soft landing (rocketry), soft landings on the Moo ...
. The crewed
Apollo program The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...
was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968
Apollo 8 Apollo 8 (December 21–27, 1968) was the first crewed spacecraft to leave Sphere of influence (astrodynamics), Earth's gravitational sphere of influence, and the first human spaceflight to reach the Moon. The crew orbited the Moon ten times ...
made the first human mission to lunar orbit (the first Earthlings, two tortoises, had circled the Moon three months earlier on the Soviet Union's
Zond 5 Zond 5 () was a spacecraft of the Soviet Zond program. In September 1968 it became the first spaceship to travel to and circle the Moon in a circumlunar trajectory, the first Moon mission to include animals, and the first to return safely to Ea ...
, followed by turtles on
Zond 6 Zond 6 was a formal member of the Soviet Zond program, and an unpiloted version of the Soyuz 7K-L1 crewed Moon-flyby spacecraft. It was launched on a lunar flyby mission on November 10, 1968, from a parent satellite (68-101B) in Earth parking o ...
). The first time a person landed on the Moon and any extraterrestrial body was when
Neil Armstrong Neil Alden Armstrong (August 5, 1930 – August 25, 2012) was an American astronaut and aerospace engineering, aeronautical engineer who, in 1969, became the Apollo 11#Lunar surface operations, first person to walk on the Moon. He was al ...
, the commander of the American mission
Apollo 11 Apollo 11 was a spaceflight conducted from July 16 to 24, 1969, by the United States and launched by NASA. It marked the first time that humans Moon landing, landed on the Moon. Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin l ...
, set foot on the Moon at 02:56 UTC on July 21, 1969. Considered the culmination of the
Space Race The Space Race (, ) was a 20th-century competition between the Cold War rivals, the United States and the Soviet Union, to achieve superior spaceflight capability. It had its origins in the ballistic missile-based nuclear arms race between t ...
, 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. While at the same time another mission, the robotic sample return mission
Luna 15 Luna 15 was a robotic space mission of the Soviet Luna programme, that was in lunar orbit together with the Apollo 11 Command module ''Columbia''. On 21 July 1969, while Apollo 11 astronauts finished the first human moonwalk, Luna 15, a robotic ...
by the Soviet Union had been in orbit around the Moon, becoming together with Apollo 11 the first ever case of two extraterrestrial missions being conducted at the same time. The Apollo missions 11 to 17 (except
Apollo 13 Apollo 13 (April 1117, 1970) was the seventh crewed mission in the Apollo program, Apollo space program and would have been the third Moon landing. The craft was launched from Kennedy Space Center on April 11, 1970, but the landing was abort ...
, which aborted its planned lunar landing) removed of lunar rock and soil in 2,196 separate samples. Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes,
seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...
s, and
magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...
s, were installed at the
Apollo 12 Apollo 12 (November 14–24, 1969) was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969, by NASA from the Kennedy Space Center in Florida. Commander Charles ...
, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used.
Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...
in 1972 remains the last crewed mission to the Moon. Explorer 49 in 1973 was the last dedicated U.S. probe to the Moon until the 1990s. The Soviet Union continued sending robotic missions to the Moon until 1976, deploying in 1970 with Luna 17 the first remote controlled
rover Rover may refer to: People Name * Constance Rover (1910–2005), English historian * Jolanda de Rover (born 1963), Dutch swimmer * Rover Thomas (c. 1920–1998), Indigenous Australian artist Stage name * Rover (musician), French singer-songw ...
Lunokhod 1 ''Lunokhod 1'' (Russian language, Russian: Луноход-1 "Moonwalker 1"), also known as Аппарат 8ЕЛ № 203 ("Device 8EL No. 203") was the first rover (space exploration), robotic rover lunar rover, on the Moon and the first to freel ...
on an extraterrestrial surface, and collecting and returning 0.3 kg of rock and soil samples with three ''Luna''
sample return mission A sample-return mission is a spacecraft mission to collect and return samples from an extraterrestrial location to Earth for analysis. Sample-return missions may bring back merely atoms and molecules or a deposit of complex compounds such as lo ...
s (''
Luna 16 ''Luna 16'' was an uncrewed 1970 space mission, part of the Soviet Luna program. It was the first robotic probe to land on the Moon and return a sample of lunar soil to Earth. The 101 grams (3.56 ounces) sample was returned from Mare Fecundit ...
'' in 1970, ''
Luna 20 ''Luna 20'' was the second of three successful Soviet lunar sample return missions. It was flown as part of the Luna program as a robotic competitor to the six successful Apollo lunar sample return missions. ''Luna 20'' was placed in an int ...
'' in 1972, and ''
Luna 24 ''Luna 24'' was a robotic probe of the Soviet Union's Luna programme. The 24th mission of the Luna series of spacecraft, the mission of the ''Luna 24'' probe was the third Soviet mission to return lunar soil samples from the Moon (the first t ...
'' in 1976).


Moon Treaty and explorational absence (1976–1990)

Following the last Soviet mission to the Moon of 1976, there was little further lunar exploration for fourteen years. Astronautics had shifted its focus towards the exploration of the inner (e.g.
Venera program The Venera (, 'Venus') program was a series of space probes developed by the Soviet Union between 1961 and 1984 to gather information about the planet Venus. Thirteen probes successfully entered the Venusian atmosphere, including the two ...
) and outer (e.g.
Pioneer 10 ''Pioneer 10'' (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. ''Pioneer 10'' became the first of five artificial objects to achieve the escape velocity needed ...
, 1972)
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
s, but also towards Earth orbit, developing and continuously operating, beside
communication satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. ...
s,
Earth observation satellite An Earth observation satellite or Earth remote sensing satellite is a satellite used or designed for Earth observation (EO) from orbit, including spy satellites and similar ones intended for non-military uses such as environmental monitoring, me ...
s (e.g.
Landsat program The Landsat program is the longest-running enterprise for acquisition of satellite imagery of Earth. It is a joint National Aeronautics and Space Administration, NASA / United States Geological Survey, USGS program. On 23 July 1972, the Landsa ...
, 1972),
space telescope A space telescope (also known as space observatory) is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO ...
s and particularly
space station A space station (or orbital station) is a spacecraft which remains orbital spaceflight, in orbit and human spaceflight, hosts humans for extended periods of time. It therefore is an artificial satellite featuring space habitat (facility), habitat ...
s (e.g.
Salyut program The ''Salyut'' programme (, , meaning "salute" or "fireworks") was the first space station programme, undertaken by the Soviet Union. It involved a series of four crewed scientific research space stations and two crewed military reconnaissa ...
, 1971). Negotiation in 1979 of
Moon treaty The Agreement Governing the Activities of States on the Moon and Other Celestial Bodies,Hiten'' – ''Hagoromo'', the first dedicated lunar mission since 1976, reached the Moon. Sent by
Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
, it became the first mission that was not a Soviet Union or U.S. mission to the Moon. In 1994, the U.S. dedicated a mission to fly a spacecraft (''
Clementine A clementine (''Citrus × clementina'') is a tangor, a citrus fruit hybrid between a willowleaf mandarin orange ( ''C.'' × ''deliciosa'') and a sweet orange (''C. × sinensis''), named in honor of Clément Rodier, a French missionary who f ...
'') to the Moon again for the first time since 1973. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. In 1998, this was followed by the ''
Lunar Prospector ''Lunar Prospector'' was a spacecraft that orbited the Moon for 19 months in 1998-99. From a low polar orbit, it mapped surface composition including lunar hydrogen deposits, measured magnetic and gravity fields, and studied lunar outgassing e ...
'' 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 next years saw a row of first missions to the Moon by a new group of states actively exploring the Moon. Between 2004 and 2006 the first spacecraft by the
European Space Agency The European Space Agency (ESA) is a 23-member International organization, international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 ...
(ESA) (''
SMART-1 SMART-1 was a European Space Agency satellite that orbited the Moon. It was launched on 27 September 2003 at 23:14 Coordinated Universal Time, UTC from the Guiana Space Centre in Kourou, French Guiana. "SMART-1" stands for Small Missions for Ad ...
'') reached the Moon, recording the first detailed survey of chemical elements on the lunar surface. The
Chinese Lunar Exploration Program The Chinese Lunar Exploration Program (CLEP; ), also known as the Chang'e Project () after the Chinese Moon goddess Chang'e, is an ongoing series of robotic Moon missions by the China National Space Administration (CNSA). Engineering Program ...
reached the Moon for the first time with the orbiter ''
Chang'e 1 Chang'e 1 (; ) was an uncrewed Chinese lunar-orbiting spacecraft, part of the first phase of the Chinese Lunar Exploration Program. The spacecraft was named after the Chinese Moon goddess, Chang'e. Chang'e 1 was launched on 24 October 2007 a ...
'' (2007–2009), obtaining a full image map of the Moon.
India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
reached, orbited and impacted the Moon in 2008 for the first time with its '' Chandrayaan-1'' and Moon Impact Probe, becoming the fifth and sixth state to do so, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil. The U.S. launched the ''Lunar Reconnaissance Orbiter'' (LRO) and the '' LCROSS'' impactor on June 18, 2009. ''LCROSS'' completed its mission by making a planned and widely observed impact in the crater Cabeus on October 9, 2009, whereas ''LRO'' is currently in operation, obtaining precise lunar
altimetry An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. T ...
and high-resolution imagery. China continued its lunar program in 2010 with ''
Chang'e 2 Chang'e 2 (; ) is a Chinese uncrewed lunar probe that was launched on 1 October 2010. It was a follow-up to the Chang'e 1 lunar probe, which was launched in 2007. Chang'e 2 was part of the first phase of the Chinese Lunar Exploration Program, ...
'', mapping the surface at a higher resolution over an eight-month period, and in 2013 with ''
Chang'e 3 Chang'e 3 (; ) is a robotic lunar exploration mission operated by the China National Space Administration (CNSA), incorporating a robotic lander and China's first lunar rover. It was launched in December 2013 as part of the second phase of ...
'', a lunar lander along with a
lunar rover A lunar rover or Moon rover is a space exploration Rover (space exploration), vehicle designed to move across the surface of the Moon. The Apollo program's Lunar Roving Vehicle was driven on the Moon by members of three American crews, Apollo 15, ...
named '' Yutu'' (). This was the first lunar rover mission since ''
Lunokhod 2 ''Lunokhod 2'' ( ("Moonwalker 2"), also known as Аппарат 8ЕЛ № 204 ("Device 8EL No. 204")) was the second of two uncrewed space mission, uncrewed lunar rover (space exploration), rovers that landed on the Moon by the Soviet Union as ...
'' in 1973 and the first lunar
soft landing A soft landing is any type of aircraft, rocket or spacecraft landing that does not result in significant damage to or destruction of the vehicle or its payload, as opposed to a hard landing. The average vertical speed in a soft landing should b ...
since ''
Luna 24 ''Luna 24'' was a robotic probe of the Soviet Union's Luna programme. The 24th mission of the Luna series of spacecraft, the mission of the ''Luna 24'' probe was the third Soviet mission to return lunar soil samples from the Moon (the first t ...
'' in 1976, making China the third country to achieve this. In 2014 the first privately funded probe, the Manfred Memorial Moon Mission, reached the Moon. Another Chinese rover mission, ''
Chang'e 4 Chang'e 4 (; ) is a robotic spacecraft mission in the Chinese Lunar Exploration Program of the CNSA. It made a soft landing on the far side of the Moon, the first spacecraft to do so, on 3 January 2019. A communication relay satellite, , w ...
'', achieved the first landing on the Moon's far side in early 2019. Also in 2019, India successfully sent its second probe, '' Chandrayaan-2'' to the Moon. In 2020, China carried out its first robotic
sample return mission A sample-return mission is a spacecraft mission to collect and return samples from an extraterrestrial location to Earth for analysis. Sample-return missions may bring back merely atoms and molecules or a deposit of complex compounds such as lo ...
(''
Chang'e 5 Chang'e 5 () was the fifth lunar exploration mission in the Chinese Lunar Exploration Program of CNSA, and China's first lunar sample-return mission. Like its predecessors, the spacecraft is named after the Chinese moon goddess, Chang'e. ...
''), bringing back 1,731 grams of lunar material to Earth. The U.S. developed plans for returning to the Moon beginning in 2004, and with the signing of the U.S.-led
Artemis Accords The Artemis Accords are a series of non-binding multilateral arrangements between the United States government and other world governments that elaborates on the norms expected to be followed in outer space. The Accords are related to the Artem ...
in 2020, the
Artemis program The Artemis program is a Exploration of the Moon, Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA), formally established in 2017 via Space Policy Directive 1. The program's stated long-ter ...
aims to return the astronauts to the Moon in the 2020s. The Accords have been joined by a growing number of countries. The introduction of the Artemis Accords has fueled a renewed discussion about the international framework and cooperation of lunar activity, building on the
Moon Treaty The Agreement Governing the Activities of States on the Moon and Other Celestial Bodies,
in the 2010s. Notably, Japan's spacecraft, the Smart Lander for Investigating Moon, survived 3 lunar nights. The
IM-1 IM-1 was a robotic Moon landing mission conducted by Intuitive Machines (IM) in February 2024 using a Intuitive Machines Nova-C, Nova-C lunar lander. After contact with the lunar surface on February 22 the lander tipped to an unplanned 30 de ...
lander became the first commercially built lander to land on the Moon in 2024. China launched the
Chang'e 6 Chang'e 6 () was the sixth robotic lunar exploration mission by the China National Space Administration (CNSA) and the second CNSA lunar sample-return mission. Like its predecessors in the Chinese Lunar Exploration Program, the spacecraft is ...
on May 3, 2024, which conducted another lunar sample return from the
far side of the Moon The far side of the Moon is the hemisphere of the Moon that is facing away from Earth, the opposite hemisphere is the near side. It always has the same surface oriented away from Earth because of synchronous rotation in the Moon's orbit. C ...
. It also carried a Chinese rover to conduct
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functio ...
of lunar surface.
Pakistan Pakistan, officially the Islamic Republic of Pakistan, is a country in South Asia. It is the List of countries and dependencies by population, fifth-most populous country, with a population of over 241.5 million, having the Islam by country# ...
sent a lunar orbiter called ICUBE-Q along with Chang'e 6. Nova-C 2, iSpace Lander and Blue Ghost were all launched to the Moon in 2024.


Future

Beside the progressing
Artemis program The Artemis program is a Exploration of the Moon, Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA), formally established in 2017 via Space Policy Directive 1. The program's stated long-ter ...
and supporting Commercial Lunar Payload Services, leading an international and commercial crewed opening up of the Moon and sending the first woman, person of color and non-US citizen to the Moon in the 2020s, China is continuing its ambitious Chang'e program, having announced with Russia's struggling ''
Luna-Glob Luna-Glob (, meaning ''Lunar sphere'') is a Moon exploration programme by Roscosmos meant to progress toward the creation of a fully robotic lunar base. When completed, the programme is intended to continue with crewed lunar missions, starting ...
'' program joint missions. Both the Chinese and US lunar programs have the goal to establish in the 2030s a
lunar base A moonbase (or lunar base) is a human outpost on or below the surface of the Moon. More than a mere site of activity or temporary camp, moonbases are extraterrestrial bases, supporting robotic or human activity, by providing surface infrastructu ...
with their international partners, though the US and its partners will first establish an orbital
Lunar Gateway The Lunar Gateway, or simply Gateway, is a planned space station which is to be assembled in orbit around the Moon. The Gateway is intended to serve as a communication hub, science laboratory, and habitation module for astronauts as part ...
station in the 2020s, from which Artemis missions will land the
Human Landing System A Human Landing System (HLS) is a spacecraft in the U.S. National Aeronautics and Space Administration Administrator, National Aeronautics and Space Administration's (NASA) Artemis program that is expected to land humans on the Moon. These are being ...
to set up temporary surface camps. While the Apollo missions were explorational in nature, the Artemis program plans to establish a more permanent presence. To this end, NASA is partnering with industry leaders to establish key elements such as modern communication infrastructure. A 4G connectivity demonstration is to be launched aboard an Intuitive Machines Nova-C lander in 2024. Another focus is on
in situ resource utilization In space exploration, in situ resource utilization (ISRU) is the practice of collection, processing, storing and use of materials found or manufactured on other astronomical objects (the Moon, Mars, asteroids, etc.) that replace materials th ...
, which is a key part of the DARPA lunar programs.
DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adva ...
has requested that industry partners develop a 10–year lunar architecture plan to enable the beginning of a lunar economy.


Human presence

In 1959 the first extraterrestrial probes reached the Moon (
Luna program The Luna programme (from the Russian word " Luna" meaning "Moon"), occasionally called ''Lunik'' by western media, was a series of robotic spacecraft missions sent to the Moon by the Soviet Union between 1959 and 1976. The programme accompli ...
), just a year into the
space age The Space Age is a period encompassing the activities related to the space race, space exploration, space technology, and the cultural developments influenced by these events, beginning with the launch of Sputnik 1 on October 4, 1957, and co ...
, after the first ever orbital flight. Since then, humans have sent a range of probes and people to the Moon. The first stay of people on the Moon was conducted in 1969, in a series of crewed exploration missions (the
Apollo Program The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...
), the last having taken place in 1972. Uninterrupted presence has been the case through the remains of impactors, landings and lunar orbiters. Some landings and orbiters have maintained a small lunar infrastructure, providing continuous observation and communication at the Moon. Increasing human activity in cislunar space as well as on the Moon's surface, particularly missions at the far side of the Moon or the lunar north and south polar regions, are in need for a lunar infrastructure. For that purpose, orbiters in orbits around the Moon or the Earth–Moon Lagrange points, have since 2006 been operated. With highly eccentric orbits providing continuous communication, as with the orbit of Queqiao and Queqiao-2 relay satellite or the planned first extraterrestrial space station, the
Lunar Gateway The Lunar Gateway, or simply Gateway, is a planned space station which is to be assembled in orbit around the Moon. The Gateway is intended to serve as a communication hub, science laboratory, and habitation module for astronauts as part ...
.


Human impact

While the Moon has the lowest planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed. If there is astronomy performed from the Moon, it will need to be free from any physical and radio pollution. While the Moon has no significant atmosphere, traffic and impacts on the Moon causes clouds of dust that can spread far and possibly contaminate the original state of the Moon and its special scientific content. Scholar Alice Gorman asserts that, although the Moon is inhospitable, it is not dead, and that sustainable human activity would require treating the Moon's ecology as a co-participant. Note: see Val Plumwood which Alice Gorman cites regarding co-participation. The so-called " Tardigrade affair" of the 2019 crashed
Beresheet ''Beresheet'' (, ''Bərēšīṯ'', "In the beginning"; Book of Genesis) was a demonstrator of a small robotic lunar lander and lunar probe operated by SpaceIL and Israel Aerospace Industries. Its aims included inspiring youth and promoting c ...
lander and its carrying of
tardigrade Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them . In 1776, th ...
s has been discussed as an example for lacking measures and lacking international regulation for
planetary protection Planetary protection is a guiding principle in the design of an interplanetary mission, aiming to prevent biological contamination of both the target celestial body and the Earth in the case of sample-return missions. Planetary protection refle ...
.
Space debris Space debris (also known as space junk, space pollution, space waste, space trash, space garbage, or cosmic debris) are defunct human-made objects in spaceprincipally in Earth orbitwhich no longer serve a useful function. These include dere ...
beyond Earth around the Moon has been considered as a future challenge with increasing numbers of missions to the Moon, particularly as a danger for such missions. As such lunar waste management has been raised as an issue which future lunar missions, particularly on the surface, need to tackle. Human remains have been transported to the Moon, including by private companies such as Celestis and Elysium Space. Because the Moon has been sacred or significant to many cultures, the practice of space burials have attracted criticism from
indigenous peoples There is no generally accepted definition of Indigenous peoples, although in the 21st century the focus has been on self-identification, cultural difference from other groups in a state, a special relationship with their traditional territ ...
leaders. For example, then
Navajo Nation The Navajo Nation (), also known as Navajoland, is an Indian reservation of Navajo people in the United States. It occupies portions of northeastern Arizona, northwestern New Mexico, and southeastern Utah. The seat of government is located in ...
president Albert Hale criticized NASA for sending the
cremated Cremation is a method of Disposal of human corpses, final disposition of a corpse through Combustion, burning. Cremation may serve as a funeral or post-funeral rite and as an alternative to burial. In some countries, including India, Nepal, and ...
ashes of scientist
Eugene Shoemaker Eugene Merle Shoemaker (April 28, 1928 – July 18, 1997) was an American geologist. He co-discovered Comet Shoemaker–Levy 9 with his wife Carolyn S. Shoemaker and David H. Levy. This comet hit Jupiter in July 1994: the impact was telev ...
to the Moon in 1998. 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 ''Fallen Astronaut'' is a aluminum sculpture created by Belgian artist Paul Van Hoeydonck. It is a stylized figure of an astronaut in a spacesuit, intended to commemorate the astronauts and Astronaut#Cosmonaut, cosmonauts who have died in t ...
'' memorial, and other artifacts. Longterm missions continuing to be active are some orbiters such as the 2009-launched
Lunar Reconnaissance Orbiter The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric Polar orbit, polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic ...
surveilling the Moon for future missions, as well as some Landers such as the 2013-launched
Chang'e 3 Chang'e 3 (; ) is a robotic lunar exploration mission operated by the China National Space Administration (CNSA), incorporating a robotic lander and China's first lunar rover. It was launched in December 2013 as part of the second phase of ...
with its Lunar Ultraviolet Telescope still operational. Five retroreflectors have been installed on the Moon since the 1970s and since used for accurate measurements of the physical
libration In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see ...
s through laser ranging to the Moon. There are several missions by different agencies and companies planned to establish a long-term human presence on the Moon, with the
Lunar Gateway The Lunar Gateway, or simply Gateway, is a planned space station which is to be assembled in orbit around the Moon. The Gateway is intended to serve as a communication hub, science laboratory, and habitation module for astronauts as part ...
as the currently most advanced project as part of the
Artemis program The Artemis program is a Exploration of the Moon, Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA), formally established in 2017 via Space Policy Directive 1. The program's stated long-ter ...
.


Astronomy from the Moon

The Moon has been used as a site for astronomical and
Earth observation Earth observation (EO) is the gathering of information about the physical, chemical, and biosphere, biological systems of the planet Earth. It can be performed via remote sensing, remote-sensing technologies (Earth observation satellites) or throu ...
s. The Earth appears in the Moon's sky with an
apparent size The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual a ...
of 1° 48 to 2°, three to four times the size of the Moon or Sun in Earth's sky, or about the apparent width of two little fingers at an arm's length away. Observations from the Moon started as early as 1966 with the first images of Earth from the Moon, taken by
Lunar Orbiter 1 The 1966 Lunar Orbiter 1 robotic spacecraft mission, part of NASA's Lunar Orbiter program, was the first American spacecraft to orbit the Moon. It was designed primarily to photograph smooth areas of the lunar surface for selection and verific ...
. Of particular cultural significance is the 1968 photograph called ''
Earthrise ''Earthrise'' is a photograph of Earth and part of the Moon's surface that was taken from lunar orbit by astronaut William Anders on December 24, 1968, during the Apollo 8 mission. Nature photographer Galen Rowell described it as "the most in ...
'', taken by
Bill Anders William Alison Anders (17 October 1933 – 7 June 2024) was an American United States Air Force (USAF) major general, electrical engineer, nuclear engineer, NASA astronaut, and businessman. In December 1968, he was a member of the crew of ...
of
Apollo 8 Apollo 8 (December 21–27, 1968) was the first crewed spacecraft to leave Sphere of influence (astrodynamics), Earth's gravitational sphere of influence, and the first human spaceflight to reach the Moon. The crew orbited the Moon ten times ...
in 1968. In April 1972 the
Apollo 16 Apollo 16 (April 1627, 1972) was the tenth human spaceflight, crewed mission in the United States Apollo program, Apollo space program, administered by NASA, and the fifth and penultimate to Moon landing, land on the Moon. It was the second o ...
mission set up the first dedicated telescope, the Far Ultraviolet Camera/Spectrograph, recording various astronomical photos and spectra. The Moon is recognized as an excellent site for telescopes. It is relatively nearby; certain craters near the poles are permanently dark and cold and especially useful for
infrared telescope An infrared telescope is a telescope that uses infrared light to detect celestial bodies. Infrared light is one of several types of radiation present in the electromagnetic spectrum. All celestial objects with a temperature above absolute zero ...
s; and
radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...
s on the far side would be shielded from the radio chatter of Earth. The
lunar soil Lunar regolith is the unconsolidated material found on the selenography, surface of the Moon and in the Lunar atmosphere, Moon's tenuous atmosphere. Sometimes referred to as Lunar soil, Lunar soil specifically refers to the component of regoli ...
, although it poses a problem for any moving parts of
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
s, can be mixed with
carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range ( nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''S ...
s and epoxies and employed in the construction of mirrors up to 50 meters in diameter. A lunar
zenith telescope A zenith telescope is a type of telescope that is designed to point straight up at or near the zenith. They are used for precision measurement of star positions, to simplify telescope construction, or both. A classic zenith telescope, also known ...
can be made cheaply with an
ionic liquid An ionic liquid (IL) is a salt (chemistry), salt in the liquid state at ambient conditions. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as wate ...
.


Living on the Moon

The only instances of humans living on the Moon have taken place in an
Apollo Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed sp ...
for several days at a time (for example, during the
Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...
mission). One challenge to astronauts during their stay on the surface is that lunar dust sticks to their suits and is carried into their quarters. Astronauts could taste and smell the dust, which smells like gunpowder and was called the "Apollo aroma". This fine lunar dust can cause health issues. In 2019, at least one plant seed sprouted in an experiment on the Chang'e 4 lander. It was carried from Earth along with other small life in its ''Lunar Micro Ecosystem''.


Legal status

Although ''
Luna Luna commonly refers to: * Earth's Moon, named "Luna" in Latin, Spanish and other languages * Luna (goddess) In Sabine and ancient Roman religion and myth, Luna is the divine embodiment of the Moon (Latin ''Lūna'' ). She is often presented as t ...
'' landers scattered pennants of the
Soviet Union The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. Likewise no private ownership of parts of the Moon, or as a whole, is considered credible. The 1967
Outer Space Treaty The Outer Space Treaty, formally the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, is a Multilateralism, multilateral treaty that forms the bas ...
defines the Moon and all outer space as the " province of all mankind". It restricts the use of the Moon to peaceful purposes, explicitly banning military installations and
weapons of mass destruction A weapon of mass destruction (WMD) is a Biological agent, biological, chemical weapon, chemical, Radiological weapon, radiological, nuclear weapon, nuclear, or any other weapon that can kill or significantly harm many people or cause great dam ...
. A majority of countries are parties of this treaty. The 1979 Moon Agreement was created to elaborate, and restrict the exploitation of the Moon's resources by any single nation, leaving it to a yet unspecified international regulatory regime. As of January 2020, it has been signed and ratified by 18 nations, none of which have
human spaceflight Human spaceflight (also referred to as manned spaceflight or crewed spaceflight) is spaceflight with a crew or passengers aboard a spacecraft, often with the spacecraft being operated directly by the onboard human crew. Spacecraft can also be ...
capabilities. Since 2020, countries have joined the U.S. in their
Artemis Accords The Artemis Accords are a series of non-binding multilateral arrangements between the United States government and other world governments that elaborates on the norms expected to be followed in outer space. The Accords are related to the Artem ...
, which are challenging the treaty. The U.S. has furthermore emphasized in a presidential
executive order In the United States, an executive order is a directive by the president of the United States that manages operations of the federal government. The legal or constitutional basis for executive orders has multiple sources. Article Two of the ...
("Encouraging International Support for the Recovery and Use of Space Resources.") that "the United States does not view outer space as a 'global commons and calls the Moon Agreement "a failed attempt at constraining free enterprise." With Australia signing and ratifying both the Moon Treaty in 1986 as well as the Artemis Accords in 2020, there has been a discussion if they can be harmonized. In this light an ''Implementation Agreement'' for the Moon Treaty has been advocated for, as a way to compensate for the shortcomings of the Moon Treaty and to harmonize it with other laws and agreements such as the Artemis Accords, allowing it to be more widely accepted. In the face of such increasing commercial and national interest, particularly prospecting territories, U.S. lawmakers have introduced in late 2020 specific regulation for the conservation of historic landing sites and interest groups have argued for making such sites
World Heritage Site World Heritage Sites are landmarks and areas with legal protection under an treaty, international treaty administered by UNESCO for having cultural, historical, or scientific significance. The sites are judged to contain "cultural and natural ...
s and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon. In 2021, the ''Declaration of the Rights of the Moon'' was created by a group of "lawyers, space archaeologists and concerned citizens", drawing on precedents in the Rights of Nature movement and the concept of legal personality for non-human entities in space.


Coordination and regulation

Increasing human activity at the Moon has raised the need for coordination to safeguard international and commercial lunar activity. Issues from cooperation to mere coordination, through for example the development of a shared Lunar time, have been raised. In particular the establishment of an international or
United Nations The United Nations (UN) is the Earth, global intergovernmental organization established by the signing of the Charter of the United Nations, UN Charter on 26 June 1945 with the stated purpose of maintaining international peace and internationa ...
regulatory regime for lunar human activity has been called for by the
Moon Treaty The Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, but remains contentious. Current lunar programs are multilateral, with the US-led
Artemis program The Artemis program is a Exploration of the Moon, Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA), formally established in 2017 via Space Policy Directive 1. The program's stated long-ter ...
and the China-led International Lunar Research Station. For broader international cooperation and coordination, the International Lunar Exploration Working Group (ILEWG), the Moon Village Association (MVA) and more generally the International Space Exploration Coordination Group (ISECG) has been established.


In culture and life


Timekeeping

Since pre-historic times people have taken note of the Moon's phases and its waxing and waning cycle and used it to keep record of time.
Tally stick A tally stick (or simply a tally) was an ancient memory aid used to record and document numbers, quantities, and messages. Tally sticks first appear as animal bones carved with notches during the Upper Palaeolithic; a notable example is the Is ...
s, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. The counting of the days between the Moon's phases eventually gave rise to generalized
time period The categorization of the past into discrete, quantified named blocks of time is called periodization.Adam Rabinowitz. And king It’s about time: historical periodization and Linked Ancient World Data''. Study of the Ancient universe Papers, 2 ...
s of lunar cycles as
month A month is a unit of time, used with calendars, that is approximately as long as a natural phase cycle of the Moon; the words ''month'' and ''Moon'' are cognates. The traditional concept of months arose with the cycle of Moon phases; such lunar mo ...
s, and possibly of its phases as
week A week is a unit of time equal to seven days. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of worship. Weeks are ofte ...
s. The words for the month in a range of different languages carry this relation between the period of the month and the Moon etymologically. The English ''month'' as well as ''moon'', and its cognates in other Indo-European languages (e.g. the
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
and
Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
(''meis'') or (mēn), meaning "month") stem from the
Proto-Indo-European Proto-Indo-European (PIE) is the reconstructed common ancestor of the Indo-European language family. No direct record of Proto-Indo-European exists; its proposed features have been derived by linguistic reconstruction from documented Indo-Euro ...
(PIE) root of ''moon'', *''méh1nōt'', derived from the PIE verbal root *''meh1''-, "to measure", "indicat nga functional conception of the Moon, i.e. marker of the month" (
cf. The abbreviation cf. (short for either Latin or , both meaning 'compare') is generally used in writing to refer the reader to other material to make a comparison with the topic being discussed. However some sources offer differing or even contr ...
the English words ''measure'' and ''menstrual''). To give another example from a different
language family A language family is a group of languages related through descent from a common ancestor, called the proto-language of that family. The term ''family'' is a metaphor borrowed from biology, with the tree model used in historical linguistics ...
, the
Chinese language Chinese ( or ) is a group of languages spoken natively by the ethnic Han Chinese majority and List of ethnic groups in China, many minority ethnic groups in China, as well as by various communities of the Chinese diaspora. Approximately 1.39& ...
uses the same word () for ''moon'' as for ''month'', which furthermore can be found in the symbols for the word ''week'' (). This lunar timekeeping gave rise to the historically dominant, but varied,
lunisolar calendar A lunisolar calendar is a calendar in many cultures, that combines monthly lunar cycles with the solar year. As with all calendars which divide the year into months, there is an additional requirement that the year have a whole number of mont ...
s. The 7th-century
Islamic calendar The Hijri calendar (), also known in English as the Islamic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the Ramad ...
is an example of a purely
lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based on the solar year, and lunisolar calendars, whose lunar months are br ...
, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon. Of particular significance has been the occasion of full moon, highlighted and celebrated in a range of calendars and cultures, an example being the Buddhist
Vesak Vesak (; Sanskrit: '), also known as Buddha Jayanti, Buddha Purnima, Visak Bochea and Buddha Day, is a holiday traditionally observed by Buddhism, Buddhists in South Asia and Southeast Asia, as well as in Tibet and Mongolia. It is among the ...
. The full moon around the southern or northern autumnal equinox is often called the harvest moon and is celebrated with festivities such as the Harvest Moon Festival of the Chinese lunar calendar, its second most important celebration after the Chinese lunisolar
Lunar New Year Lunar New Year is the beginning of a new year based on lunar calendars or, informally, lunisolar calendars. Lunar calendar years begin with a new moon and have a fixed number of lunar months, usually twelve, in contrast to lunisolar calendar ye ...
. Furthermore, association of time with the Moon can also be found in religion, such as the ancient Egyptian temporal and lunar deity
Khonsu Khonsu (; also transliterated Chonsu, Khensu, Khons, Chons, Khonshu, or Konshu; ) is an ancient Egyptian god of lunar deity, the Moon. His name means 'traveller', and this may relate to the perceived nightly travel of the Moon across the sky. Al ...
.


Cultural representation

Humans have not only observed the Moon since
prehistoric times Prehistory, also called pre-literary history, is the period of human history between the first known use of stone tools by hominins  million years ago and the beginning of recorded history with the invention of writing systems. The use o ...
, but have also developed intricate perceptions of the Moon. Over time the Moon has been characterized and associated in many different ways, from having a spirit or being a deity, and an aspect thereof or an aspect in astrology, being made an important part of many cosmologies. This rich history of humans viewing the Moon has been evidenced starting with depictions from 40,000 BP and in written form from the
4th millennium BCE File:4th millennium BC montage.jpg, 400x400px, From top left clockwise: The Temple of Ġgantija, one of the oldest freestanding structures in the world; Warka Vase; Bronocice pot with one of the earliest known depictions of a wheeled vehicle; K ...
in the earliest cases of
writing Writing is the act of creating a persistent representation of language. A writing system includes a particular set of symbols called a ''script'', as well as the rules by which they encode a particular spoken language. Every written language ...
. The oldest named astronomer and poet
Enheduanna Enheduanna ( , also transliteration, transliterated as , , or variants; ) was the (high) priestess of the moon god Sin (mythology), Nanna (Sīn) in the Sumerian city-state of Ur in the reign of her father, Sargon of Akkad ( BCE). She was likely ...
, Akkadian high priestess to the lunar deity Nanna/Sin and pricess, daughter of
Sargon the Great Sargon of Akkad (; ; died 2279 BC), also known as Sargon the Great, was the first ruler of the Akkadian Empire, known for his conquests of the Sumerian city-states in the 24th to 23rd centuries BC.The date of the reign of Sargon is highly unc ...
( – BCE), tracked the Moon and wrote poems about her divine Moon.


Crescent

For the representation of the Moon, especially its
lunar phase A lunar phase or Moon phase is the apparent shape of the Moon's directly sunlit portion as viewed from the Earth. Because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth. In common usage, the four maj ...
s, the
crescent A crescent shape (, ) is a symbol or emblem used to represent the lunar phase (as it appears in the northern hemisphere) in the first quarter (the "sickle moon"), or by extension a symbol representing the Moon itself. In Hindu iconography, Hind ...
(🌙) has been a recurring symbol in a range of cultures since at least 3,000 BCE or possibly earlier with bull horns dating to the earliest
cave painting In archaeology, cave paintings are a type of parietal art (which category also includes petroglyphs, or engravings), found on the wall or ceilings of caves. The term usually implies prehistoric art, prehistoric origin. These paintings were often c ...
s at 40,000 BP. In
writing system A writing system comprises a set of symbols, called a ''script'', as well as the rules by which the script represents a particular language. The earliest writing appeared during the late 4th millennium BC. Throughout history, each independen ...
s such as Chinese the crescent has developed into the symbol , the word for Moon, and in ancient Egyptian it was the symbol , meaning Moon and spelled like the ancient Egyptian lunar deity '' Iah'', which the other ancient Egyptian lunar deities
Khonsu Khonsu (; also transliterated Chonsu, Khensu, Khons, Chons, Khonshu, or Konshu; ) is an ancient Egyptian god of lunar deity, the Moon. His name means 'traveller', and this may relate to the perceived nightly travel of the Moon across the sky. Al ...
and
Thoth Thoth (from , borrowed from , , the reflex of " eis like the ibis") is an ancient Egyptian deity. In art, he was often depicted as a man with the head of an African sacred ibis, ibis or a baboon, animals sacred to him. His feminine count ...
were associated with. Iconographically the crescent was used in
Mesopotamia Mesopotamia is a historical region of West Asia situated within the Tigris–Euphrates river system, in the northern part of the Fertile Crescent. Today, Mesopotamia is known as present-day Iraq and forms the eastern geographic boundary of ...
as the primary symbol of Nanna/Sîn, the ancient
Sumeria Sumer () is the earliest known civilization, located in the historical region of southern Mesopotamia (now south-central Iraq), emerging during the Chalcolithic and early Bronze Ages between the sixth and fifth millennium BC. Like nearby Elam ...
n lunar deity, who was the father of Inanna/Ishtar, the goddess of the planet
Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
(symbolized as the eight pointed Star of Ishtar), and Utu/Shamash, the god of the Sun ( symbolized as a disc, optionally with eight rays), all three often depicted next to each other. Nanna/Sîn is, like some other lunar deities, for example Iah and Khonsu of ancient Egypt,
Mene The moonfish of the genus ''Mene'', the sole extant genus of the family Menidae, are disk-shaped fish which bear a vague resemblance to gourami, thanks to their thread-like pelvic fins. Today, the genus is represented only by ''Mene maculata'' ...
/
Selene In ancient Greek mythology and Ancient Greek religion, religion, Selene (; , meaning "Moon")''A Greek–English Lexicon's.v. σελήνη is the goddess and personification of the Moon. Also known as Mene (), she is traditionally the daughter ...
of ancient Greece and
Luna Luna commonly refers to: * Earth's Moon, named "Luna" in Latin, Spanish and other languages * Luna (goddess) In Sabine and ancient Roman religion and myth, Luna is the divine embodiment of the Moon (Latin ''Lūna'' ). She is often presented as t ...
of ancient Rome, depicted as a
horned deity Deities depicted with Horn (anatomy), horns or Antler, antlers are found in numerous religions across the world. Horned animals, such as bulls, goats, and rams, may be worshiped as deities or serve as inspiration for a deity's appearance in reli ...
, featuring crescent shaped headgears or crowns. The particular arrangement of the crescent with a star known as the
star and crescent The conjoined representation of a star and a crescent is used in various historical contexts, including as a prominent symbol of the Ottoman Empire, and in contemporary times, as a national symbol by some countries, and by some Muslims as a sym ...
(☪️) goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and the planet Venus, in combination. It came to represent the selene goddess
Artemis In ancient Greek religion and Greek mythology, mythology, Artemis (; ) is the goddess of the hunting, hunt, the wilderness, wild animals, transitions, nature, vegetation, childbirth, Kourotrophos, care of children, and chastity. In later tim ...
, and via the patronage of
Hecate Hecate ( ; ) is a goddess in ancient Greek religion and mythology, most often shown holding a pair of torches, a key, or snakes, or accompanied by dogs, and in later periods depicted as three-formed or triple-bodied. She is variously associat ...
, which as
triple deity A triple deity is a deity with three apparent forms that function as a singular whole. Such deities may sometimes be referred to as threefold, tripled, triplicate, tripartite, triune, triadic, or as a trinity. The number three has a long history ...
under the
epithet An epithet (, ), also a byname, is a descriptive term (word or phrase) commonly accompanying or occurring in place of the name of a real or fictitious person, place, or thing. It is usually literally descriptive, as in Alfred the Great, Suleima ...
''trimorphos''/''trivia'' included aspects of Artemis/ Diana, came to be used as a symbol of Byzantium, with
Virgin Mary Mary was a first-century Jewish woman of Nazareth, the wife of Saint Joseph, Joseph and the mother of Jesus. She is an important figure of Christianity, venerated under titles of Mary, mother of Jesus, various titles such as Perpetual virginity ...
(
Queen of Heaven Queen of Heaven () is a title given by the Catholic Church and Eastern Orthodoxy, to Mary, mother of Jesus, and, to a lesser extent, in Anglicanism and Lutheranism. The title has long been a tradition, included in prayers and devotional literat ...
) later taking her place, becoming depicted in Marian veneration on a crescent and adorned with stars. Since then the heraldric use of the star and crescent proliferated, Byzantium's symbolism possibly influencing the development of the Ottoman flag, specifically the combination of the Turkish crescent with a star, and becoming a popular symbol for Islam (as the '' hilal'' of the
Islamic calendar The Hijri calendar (), also known in English as the Islamic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the Ramad ...
) and for a range of nations.


Other association

The features of the Moon, the contrasting brighter highlands and darker maria, have been seen by different cultures forming abstract shapes. Such shapes are among others the
Man in the Moon In many cultures, several pareidolic images of a human face, head or body are recognized in the disc of the full moon; they are generally known as the Man in the Moon. The images are based on the appearance of the dark areas (known as lunar m ...
(e.g. Coyolxāuhqui) or the
Moon Rabbit The Moon rabbit, Moon hare or Jade rabbit is a mythical figure in both East Asian and indigenous American folklore, based on interpretations that identify the lunar mare, dark markings on the near side of the Moon as a rabbit or hare. In East A ...
(e.g. the Chinese Tu'er Ye or in Indigenous American mythologies the aspect of the Mayan Moon goddess, from which possibly
Awilix Awilix () (also spelled Ahuilix, Auilix and Avilix) was a goddess (or possibly a god) of the Mesoamerican chronology, Postclassic Kʼicheʼ people, Kʼicheʼ Maya civilization, Maya, who had a large Kʼicheʼ Kingdom of Qʼumarkaj, kingdom in the ...
is derived, or of Metztli/
Tēcciztēcatl In Aztec mythology, Tecciztecatl ( , "person from Tēcciztlān," a place name meaning "Place of the Conch," from ''tēcciztli'' or "conch"; also Tecuciztecatl, Teucciztecatl, from the variant form ''tēucciztli'') was a lunar deity, representing t ...
). Occasionally some lunar deities have been also depicted driving a chariot across the sky, such as the Hindu Chandra/Soma, the Greek Artemis, which is associated with Selene, or Luna, Selene's ancient Roman equivalent. Color and material wise the Moon has been associated in Western
alchemy Alchemy (from the Arabic word , ) is an ancient branch of natural philosophy, a philosophical and protoscientific tradition that was historically practised in China, India, the Muslim world, and Europe. In its Western form, alchemy is first ...
with
silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
, while gold is associated with the Sun. Through a miracle, the so-called splitting of the Moon () in
Islam Islam is an Abrahamic religions, Abrahamic monotheistic religion based on the Quran, and the teachings of Muhammad. Adherents of Islam are called Muslims, who are estimated to number Islam by country, 2 billion worldwide and are the world ...
, association with the Moon applies also to
Muhammad Muhammad (8 June 632 CE) was an Arab religious and political leader and the founder of Islam. Muhammad in Islam, According to Islam, he was a prophet who was divinely inspired to preach and confirm the tawhid, monotheistic teachings of A ...
."Muhammad." ''
Encyclopædia Britannica The is a general knowledge, general-knowledge English-language encyclopaedia. It has been published by Encyclopædia Britannica, Inc. since 1768, although the company has changed ownership seven times. The 2010 version of the 15th edition, ...
''. 2007. Encyclopædia Britannica Online, p.13


Representation in modern culture

The perception of the Moon in the
modern era The modern era or the modern period is considered the current historical period of human history. It was originally applied to the history of Europe and Western history for events that came after the Middle Ages, often from around the year 1500 ...
has been informed by
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
-enabled modern astronomy and later by
spaceflight Spaceflight (or space flight) is an application of astronautics to fly objects, usually spacecraft, into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such ...
which enabled actual human activity at the Moon, particularly the culturally impactful lunar landings. These new insights inspired cultural references, connecting romantic reflections about the Moon and speculative fiction such as science-fiction dealing with the Moon. Contemporarily the Moon has been seen as a place for economic expansion into space, with missions prospecting for
lunar resources An artificially colored mosaic constructed from a series of 53 images taken through three spectral filters by ''Galileo's'' imaging system as the spacecraft flew over the northern regions of the Moon on 7 December 1992. The colors indicate d ...
. This has been accompanied with renewed public and critical reflection on humanity's cultural and legal relation to the celestial body, especially regarding
colonialism Colonialism is the control of another territory, natural resources and people by a foreign group. Colonizers control the political and tribal power of the colonised territory. While frequently an Imperialism, imperialist project, colonialism c ...
, as in the 1970 poem " Whitey on the Moon". In this light the Moon's nature has been invoked, particularly for lunar conservation and as a
common Common may refer to: As an Irish surname, it is anglicised from Irish Gaelic surname Ó Comáin. Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common, a central public park in Boston, Massachusetts * Cambridge Com ...
. In 2021 20 July, the date of the first crewed Moon landing, became the annual ''International Moon Day''.


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 ''Lunatic'' is a term referring to a person who is seen as Mental disorder, mentally ill, Risk, dangerous, Foolishness, foolish, or crazy—conditions once attributed to "lunacy". The word derives from ''lunaticus'' meaning "of the moon" or "moo ...
'' are derived from the Latin name for the Moon, ''Luna''. Philosophers
Aristotle Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...
and
Pliny the Elder Gaius Plinius Secundus (AD 23/24 79), known in English as Pliny the Elder ( ), was a Roman Empire, Roman author, Natural history, naturalist, and naval and army commander of the early Roman Empire, and a friend of the Roman emperor, emperor Vesp ...
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.


See also

*
List of natural satellites Of the Solar System's eight planets and its list of possible dwarf planets, nine most likely dwarf planets, six planets and seven dwarf planets are known to be orbited by at least 430 natural satellites, or moons. At least List of gravitational ...
*
Selenography Selenography is the study of the surface and physical features of the Moon (also known as geography of the Moon, or selenodesy). Like geography and areography, selenography is a subdiscipline within the field of planetary science. Historically, ...
(geography of the Moon) * Timekeeping on the Moon


Explanatory notes


References


Further reading

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


External links


NASA images and videos about the Moon
*


Cartographic resources


Unified Geologic Map of the Moon
United States Geological Survey The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...

Moon Trek – An integrated map browser of datasets and maps for the Moon

Consolidated Lunar Atlas
* 3D zoomable globes: *
The Moon
on
Google Maps Google Maps is a web mapping platform and consumer application offered by Google. It offers satellite imagery, aerial photography, street maps, 360° interactive panorama, interactive panoramic views of streets (Google Street View, Street View ...
, a 3-D rendition of the Moon akin to Google Earth *
World Wind Central on Moon



earthly mission

Large image of the Moon's north pole area
{{Portal bar, Solar System, Astronomy, Stars, Spaceflight, Outer space * Articles containing video clips Astronomical objects known since antiquity Planetary satellite systems Planetary-mass satellites Moons with a prograde orbit Solar System