The Moon is
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
'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 often colloquially referred to as ''moons'' ...
. It is the
fifth largest satellite in the
Solar System
The Solar System Capitalization 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 "Solar ...
and the largest and most massive relative to its parent
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
, with a diameter about one-quarter that of Earth (comparable to the width of
Australia).
The Moon is a
planetary-mass object
A planetary-mass object (PMO), planemo, or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain ...
with a
differentiated rocky
''Rocky'' is a 1976 American sports drama film directed by John G. Avildsen and written by and starring Sylvester Stallone. It is the first installment in the ''Rocky'' franchise and stars Talia Shire, Burt Young, Carl Weathers, and Burge ...
body, making it a
satellite planet
A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal (sometimes spherical) in shape. Two moons in the Solar System are larger than the planet Mercury (though less massive): Ganymede ...
under the
geophysical definitions of the term and larger than all known
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
s of the Solar System.
It lacks any significant
atmosphere,
hydrosphere
The hydrosphere () is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This ...
, or
magnetic field. 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 experien ...
is about one-sixth of Earth's at , with
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
's moon
Io being the only satellite in the Solar System known to have a higher surface gravity and density.
The Moon orbits Earth at an
average distance of , or about 30 times Earth's diameter. Its gravitational influence 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 and
very slowly lengthens Earth's day. The
Moon's orbit
The Moon orbits Earth in the retrograde and prograde motion, prograde direction and completes one orbital period, revolution relative to the March Equinox, Vernal Equinox and the fixed stars, stars in about 27.32 days (a tropical month and sider ...
around Earth has a
sidereal 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, ...
of 27.3 days. During each
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, ...
of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in
lunar phases that form the basis for the months of a
lunar calendar
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 only directly on the solar year. The most commonly used calendar, t ...
. The Moon is
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 bo ...
to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (
the near side) to always face Earth, and the somewhat longer
lunar day
A lunar day is the period of time for Earth's Moon to complete one rotation on its axis with respect to the Sun. Due to tidal locking, it is the time the Moon takes to complete one orbit around Earth ( Earth rise to Earth set) plus about 2.2 mo ...
is the same as the synodic period. However, 59% of the total lunar surface can be seen from Earth through cyclical shifts in perspective known as
libration.
The most widely accepted
origin explanation posits that the Moon formed 4.51 billion years ago,
not long after Earth, out of the debris from a
giant impact between the planet and a hypothesized
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
-sized body called
Theia
In Greek mythology, Theia (; grc, Θεία, Theía, divine, also rendered Thea or Thia), also called Euryphaessa ( grc, Εὐρυφάεσσα) "wide-shining", is one of the twelve Titans, the children of the earth goddess Gaia and the Sky deity ...
. It then receded to a wider orbit because of
tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic
maria
Maria may refer to:
People
* Mary, mother of Jesus
* Maria (given name), a popular given name in many languages
Place names Extraterrestrial
* 170 Maria, a Main belt S-type asteroid discovered in 1877
* Lunar maria (plural of ''mare''), large, ...
("seas"), which fill the spaces between bright ancient crustal highlands and prominent
impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...
s. Most of the large
impact basin
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...
s and mare surfaces were in place by the end of the
Imbrian
The Imbrian is a lunar geologic period divided into two epochs, the Early and Late.
Early Imbrian
In the lunar geologic timescale, the Early Imbrian epoch occurred from 3,850 million years ago to about 3,800 million years ago. It overlaps the end ...
period, some three billion years ago. The lunar surface is fairly non-reflective, with 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
lunar soil
Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disinteg ...
being comparable to that of
asphalt
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...
. However, due to its large
angular diameter
The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...
, the
full moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This means ...
is the
brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total
solar eclipse.
Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology.
The first artificial object to reach the Moon was the
Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
's
Luna 2 uncrewed
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
in 1959; this was followed by the first successful soft landing by
Luna 9
Luna 9 (Луна-9), internal designation Ye-6 No.13, was an 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 survivable landing on a celestial body ...
in 1966. The only human lunar missions to date have been those of the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
'
Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned
lunar rocks
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 as ...
that have been used to develop a detailed
geological understanding of the
Moon's origins,
internal structure, and subsequent history.
Names and etymology
The usual
English
English usually refers to:
* English language
* English people
English may also refer to:
Peoples, culture, and language
* ''English'', an adjective for something of, from, or related to England
** English national ide ...
proper name for Earth's natural satellite is simply ''Moon'', with a capital M.
The noun ''moon'' is derived from
Old English
Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlers in the mid-5th c ...
''mōna'', which (like all its
Germanic cognates) stems from
Proto-Germanic
Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed proto-language of the Germanic branch of the Indo-European languages.
Proto-Germanic eventually developed from pre-Proto-Germanic into three Germanic bran ...
''*mēnōn'', which in turn comes from
Proto-Indo-European
Proto-Indo-European (PIE) is the reconstructed common ancestor of the Indo-European language family. Its proposed features have been derived by linguistic reconstruction from documented Indo-European languages. No direct record of Proto-Indo- ...
''*mēnsis'' "month" (from earlier ''*mēnōt'', genitive ''*mēneses'') which may be related to the verb "measure" (of time).
Occasionally, the name Luna is used in scientific writing and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. Cynthia is another poetic name, though rare, for the Moon personified as a goddess, while
Selene
In ancient Greek mythology and religion, Selene (; grc-gre, Σελήνη , meaning "Moon"''A Greek–English Lexicon's.v. σελήνη) is the goddess and the personification of the Moon. Also known as Mene, she is traditionally the daughter of ...
(literally "Moon") is the Greek goddess of the Moon.
The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, ''lūna''. The adjective ''selenian'' , derived from the Greek word for the Moon, ''selēnē'', and used to describe the Moon as a world rather than as an object in the sky, is rare, while its cognate ''selenic'' was originally a rare synonym but now nearly always refers to the chemical element
selenium
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
. The Greek word for the Moon does however provide us with 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. Historica ...
'', the study of the physical features of the Moon, as well as the element name ''selenium''.
The Greek goddess of the wilderness and the hunt,
Artemis
In ancient Greek mythology and religion, Artemis (; grc-gre, Ἄρτεμις) is the goddess of the hunt, the wilderness, wild animals, nature, vegetation, childbirth, care of children, and chastity. She was heavily identified wit ...
, equated with the Roman
Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called
Cynthia
Cynthia is a feminine given name of Greek origin: , , "from Mount Cynthus" on Delos island. The name has been in use in the Anglosphere since the 1600s. There are various spellings for this name, and it can be abbreviated to Cindy, Cyndi, Cyndy, ...
, from her legendary birthplace on
Mount Cynthus.
These names – Luna, Cynthia and Selene – are reflected in technical terms for
lunar orbits such as ''apolune'', ''pericynthion'' and ''selenocentric''.
The
astronomical symbol
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 antiq ...
for the Moon is a crescent,
![Moon decrescent symbol (bold)](https://upload.wikimedia.org/wikipedia/commons/b/bd/Moon_decrescent_symbol_%28bold%29.svg)
, for example in ''M''
☾ 'lunar mass' (also ''M
L'').
Natural history
Lunar geologic timescale
Formation
Isotope dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...
of lunar samples suggests the Moon formed around 50 million years after the
origin of the Solar System.
Historically, several formation mechanisms have been proposed, but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through
centrifugal force
In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is parall ...
would require too great an initial rotation rate of Earth.
Gravitational capture of a pre-formed Moon
depends on an unfeasibly extended
atmosphere of Earth
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
to
dissipate
In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy (internal, bulk flow kinetic, or system potential) transforms from an initial form to ...
the energy of the passing Moon.
A co-formation of Earth and the Moon together in the
primordial accretion disk does not explain the depletion of metals in the Moon.
None of these hypotheses can account for the high
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
of the Earth–Moon system.
The prevailing theory is that the Earth–Moon system formed after a
giant impact of a
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
-sized body (named ''
Theia
In Greek mythology, Theia (; grc, Θεία, Theía, divine, also rendered Thea or Thia), also called Euryphaessa ( grc, Εὐρυφάεσσα) "wide-shining", is one of the twelve Titans, the children of the earth goddess Gaia and the Sky deity ...
'') with the
proto-Earth. The 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 forc ...
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, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and
Vesta have, according to meteorites from them, very different oxygen and tungsten
isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two,
although this is debated.
The impact 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.
Similarly, the newly formed Moon would have had its own
lunar 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.Above a high resolution threshold for simulations, a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's
Roche limit
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 forc ...
. 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.
Natural development
![Archean](https://upload.wikimedia.org/wikipedia/commons/f/f2/Archean.png)
After the Moon's formation the Moon settled in orbit around Earth much closer than today, making both bodies appear much larger in each's sky and causing on both more frequent and stronger
eclipses and
tidal effects.
Since then, 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 away f ...
, the Moon's orbit around Earth has become significantly larger as well as longer,
tidally locking the so-called
lunar near side, always facing Earth with this same side.
The post formation cooled
lunar surface
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to " lunar science") is quite different from that of Earth. The Moon lacks a true atmosphere, which eliminates erosion due to weather. It does ...
has been shaped by large and many small
impact events, retaining a broadly
cratered landscape of all ages, as well as by
volcanic activity
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
, producing the prominent
lunar maria
The lunar maria (; singular: mare ) are large, dark, basaltic plains on Earth's Moon, formed by ancient asteroid impacts on the far side on the Moon that triggered volcanic activity on the opposite (near) side. They were dubbed , Latin for 'seas' ...
. Volcanically active until 1.2 billion years ago, 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 some as old as 4.2 billion years.
The causes for the eruption of mare basalts, particularly their uneven occurrence on mainly the near-side, like the
lunar highlands
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to " lunar science") is quite different from that of Earth. The Moon lacks a true atmosphere, which eliminates erosion due to weather. It does ...
on the
far side, has been an unresolved issue due to differing explanations. One explanation suggests that large
meteorites were hitting the Moon in its early history leaving large craters which then were filled with
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
. Other explanations suggest processes of lunar volcanism.
Physical characteristics
The Moon is a very slightly
scalene ellipsoid
An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation.
An ellipsoid is a quadric surface; that is, a surface that may be defined as the z ...
due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit.
Size and mass
![Moons of solar system v7](https://upload.wikimedia.org/wikipedia/commons/4/4f/Moons_of_solar_system_v7.jpg)
The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizeable as one of its
planetary-mass moon
A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal (sometimes spherical) in shape. Two moons in the Solar System are larger than the planet Mercury (though less massive): Ganymed ...
s, making it a
satellite planet
A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal (sometimes spherical) in shape. Two moons in the Solar System are larger than the planet Mercury (though less massive): Ganymede ...
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 of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
of the Solar System,
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
. While the
minor-planet moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are importan ...
Charon of the
Pluto-Charon system
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 S ...
is larger relative to Pluto, the Moon is the largest natural satellite of the Solar System relative to their
primary planets.
The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of
Australia.
The whole surface area of the Moon is about 38 million square kilometers, slightly less than the area of the
Americas
The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World.
Along with th ...
(
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 or geography.
Etymology
The word ''north ...
and
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
).
The Moon's mass is 1/81 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 experien ...
, after
Io, at and an escape velocity of .
Structure
![Return of the moon diagram](https://upload.wikimedia.org/wikipedia/commons/8/80/Return_of_the_moon_diagram.svg)
The Moon is a
differentiated body that was initially in
hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, 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. In the planetary ...
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 (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centra ...
. 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 incl ...
mantle from the
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
and sinking of the minerals
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
,
clinopyroxene
The pyroxenes (commonly abbreviated to ''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 calcium (Ca), sodium (Na), iron (Fe I ...
, and
orthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-density
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
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 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 minerals most ...
.
The
Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.
The crust is on average about thick.
The Moon is the second-densest satellite in the Solar System, after
Io.
However, the inner core of the Moon is small, with a radius of about or less,
around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten. The pressure at the lunar core is estimated to be .
Magnetic and gravitational fields
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 currently have a global
dipolar
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating.
However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today.
This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized.
Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the
antipodes of the giant impact basins.
On average the Moon's surface gravity is
(; ), about half of the surface gravity of
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
and about a sixth of Earth's. The
Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the
Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are
mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. 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.
Atmosphere
![Apollo 17 twilight ray sketch](https://upload.wikimedia.org/wikipedia/commons/7/72/Apollo_17_twilight_ray_sketch.jpg)
The Moon has an
atmosphere so tenuous as to be nearly
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
, 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 (which ...
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 ca ...
, a product of the bombardment of lunar soil by solar wind ions.
Elements that have been detected include
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
and
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...
, 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 consis ...
and
neon from the solar wind; and
argon-40,
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 3.8 days. It is transient in the decay chain of primordial uranium-238 and is the immediate decay product of ...
, and
polonium-210, outgassed after their creation by
radioactive decay within the crust and mantle.
The absence of such neutral species (atoms or molecules) as
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
,
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
and
magnesium
Magnesium is a chemical element with the 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 of the periodic ta ...
, 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 terrestr ...
, is not understood.
Water vapor has been detected by ''
Chandrayaan-1
Chandrayaan-1 (, ) was the first Indian lunar probe under the Chandrayaan program. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impact ...
'' 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, grc, Ἀπόλλωνος, Apóllōnos, label=genitive , ; , grc-dor, Ἀπέλλων, Apéllōn, ; grc, Ἀπείλων, Apeílōn, label= Arcadocypriot Greek, ; grc-aeo, Ἄπλουν, Áploun, la, Apollō, la, Apollinis, label ...
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 and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
. 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's Lunar Dust EXperiment (LDEX) found particle counts peaked during the
Geminid,
Quadrantid
The Quadrantids (QUA) are a meteor shower that peaks in early January and whose radiant lies in the constellation Boötes. The zenithal hourly rate (ZHR) of this shower can be as high as that of two other reliably rich meteor showers, the Persei ...
,
Northern Taurid, and
Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being more dense near the boundary between the Moon's dayside and nightside.
Surface conditions
Ionizing radiation from
cosmic ray
Cosmic rays 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 Solar System in our own ...
s, the Sun and the 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 isotopes— ...
produce radiation levels on average of 1,369
microsievert
The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing radi ...
s per day, which is about 2-3 times more than on the
International Space Station
The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...
at about 400 km above Earth in orbit,
5-10 times more than during a trans-Atlantic flight,
200 times more than on Earth's surface.
For further comparison radiation on a flight to Mars is about 1.84 millisieverts per day and on Mars 0.64 millisieverts per day.
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 orb ...
with respect to the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...
is only 1.5427°,
much less than the 23.44° of Earth. Because of this small tilt, 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 tilted orbit around the Sun. In temperate and ...
than on Earth and it allows for the existence of some
peaks of eternal light at the
Moon's north pole, at the rim of the crater
Peary.
The surface is exposed to drastic temperature differences ranging from to depending on the
solar irradiance.
Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow, 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
Charles Hermite () FRS FRSE MIAS (24 December 1822 – 14 January 1901) was a French mathematician who did research concerning number theory, quadratic forms, invariant theory, orthogonal polynomials, elliptic functions, and algebra.
Hermi ...
. 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 bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...
.
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 terrestr ...
, formed by impact processes. The finer regolith, the
lunar soil
Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disinteg ...
of
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
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, carbon (in the form of charcoal) and potassium nitrate (saltpeter). Th ...
. 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 for example are considered making it unlikely for spacecrafts to harbor bacterial spores at the Moon longer than just one lunar orbit.
Surface features
![Apollo 17 AS17-140-21497](https://upload.wikimedia.org/wikipedia/commons/f/f3/Apollo_17_AS17-140-21497.jpg)
The
topography of the Moon has been measured with
laser altimetry and
stereo image analysis. Its most extensive
topographic feature
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary scien ...
is the giant far-side
South Pole–Aitken basin
The South Pole–Aitken basin (SPA Basin, ) is an immense impact crater on the far side of the Moon. At roughly in diameter and between deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest, and dee ...
, some in diameter, the largest crater on the Moon and the second-largest confirmed impact
crater in the Solar System.
At deep, its floor is the lowest point on the surface of the Moon.
The highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. Other large impact basins such as
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 or offset on the ground surface where one side of a fault has moved vertically with respect to the other. It is the topographic expression of faulting attributed to the displacement of the land surface by movement a ...
cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. Similar shrinkage features exist on
Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat.
Volcanic features
![Moon names](https://upload.wikimedia.org/wikipedia/commons/6/61/Moon_names.svg)
The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called ''
maria
Maria may refer to:
People
* Mary, mother of Jesus
* Maria (given name), a popular given name in many languages
Place names Extraterrestrial
* 170 Maria, a Main belt S-type asteroid discovered in 1877
* Lunar maria (plural of ''mare''), large, ...
'' (singular ''mare'';
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
for "seas", as they were once believed to be filled with water) are vast solidified pools of ancient
basalt
Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...
ic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. The majority of these lava deposits erupted or flowed into the depressions associated with
impact basins
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...
. Several
geologic province
A geologic province is a spatial entity with common geologic attributes. A province may include a single dominant structural element such as a basin or a fold belt, or a number of contiguous related elements. Adjoining provinces may be simila ...
s containing
shield volcanoes 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".
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.
Moonquake
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:
Earthqua ...
s and releases of gas indicate continued lunar activity.
Evidence of recent lunar volcanism has been identified at 70
irregular mare patch
An irregular mare patch also known as an IMP, is a smooth, rounded, slightly mounded area, generally about 500 meters wide, occurring in the lunar maria.Wood, Charles. "Strange Little IMPs." ''Sky and Telescope,'' February 2015 issue.
Discovery
...
es, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin.
The lighter-colored regions of the Moon are called ''terrae'', or more commonly ''highlands'', because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
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
![Daedalus crater AS11-41-6151](https://upload.wikimedia.org/wikipedia/commons/e/ed/Daedalus_crater_AS11-41-6151.jpg)
A major geologic process that has affected the Moon's surface is
impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...
ing, with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than on the Moon's near side. The
lunar geologic timescale is based on the most prominent impact events, including
Nectaris,
Imbrium, and
Orientale; structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional
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
A multi-ringed basin (also a multi-ring impact basin) is not a simple bowl-shaped crater, or a peak ring crater, but one containing multiple concentric topographic rings; a multi-ringed basin could be described as a massive impact crater, surrou ...
have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.
The radiometric ages of impact-melted rocks collected during the
Apollo missions
The Apollo program was a United States human spaceflight program carried out from 1961 to 1972 by the National Aeronautics and Space Administration (NASA), which landed the first astronauts on the Moon. The program used the Saturn IB and Saturn ...
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 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 to the hypot ...
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
![Reiner-gamma-clem1](https://upload.wikimedia.org/wikipedia/commons/c/cf/Reiner-gamma-clem1.jpg)
Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and often have a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface
magnetic fields and many are located at the
antipodal point
In mathematics, antipodal points of a sphere are those diametrically opposite to each other (the specific qualities of such a definition are that a line drawn from the one to the other passes through the center of the sphere so forms a true d ...
of major impacts. Well known swirls include the
Reiner Gamma feature and
Mare Ingenii
Mare Ingenii (Latin ''ingeniī'', the "Sea of Cleverness") is one of the few lunar mare features on the far side of the Moon. The mare sits in the Ingenii basin, which is of the Pre-Nectarian epoch, which lies in turn in the outer part of the ol ...
. 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 upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
, 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 on ...
.
Presence of water
![Chandrayaan1 Spacecraft Discovery Moon Water](https://upload.wikimedia.org/wikipedia/commons/8/89/Chandrayaan1_Spacecraft_Discovery_Moon_Water.jpg)
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as
photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting
comets
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
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 upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
, 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 social and material structure and activity on the Moon making human habitation on the Moon.
Therefore, it is more than the particular surface space habitats and more specific than the moonbases which can consist of a ...
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 fir ...
'' spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by
Arecibo
Arecibo (; ) is a city and municipality on the northern coast of Puerto Rico, on the shores of the Atlantic Ocean, located north of Utuado and Ciales; east of Hatillo; and west of Barceloneta and Florida. It is about west of San Juan, th ...
, 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
Chandrayaan-1 (, ) was the first Indian lunar probe under the Chandrayaan program. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impact ...
'' spacecraft has since confirmed the existence of surface water ice, using the on-board
Moon Mineralogy Mapper
The Moon Mineralogy Mapper (M3) is one of two instruments that NASA contributed to India's first mission to the Moon, Chandrayaan-1, launched October 22, 2008. It is an imaging spectrometer, and the team is led by Principal investigator Carle Piet ...
. 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 hydro ...
, 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 inclusio ...
in lunar sample 74220 was reported,
the famous high-titanium "orange glass soil" of volcanic origin collected during the
Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's
upper mantle
The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...
. 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 (SOFIA).
Earth–Moon system
Orbit
![Dscovrepicmoontransitfull](https://upload.wikimedia.org/wikipedia/commons/5/50/Dscovrepicmoontransitfull.gif)
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 stays located at all times (about a quarter of Earth's radius) beneath the Earth's surface, making the Moon seemingly orbit the Earth.
The
orbital eccentricity, giving ovalness of the orbit, is 0.055.
The
Lunar distance, or the
semi-major axis of the geocentric lunar orbit, is approximately 400,000 km, which is a quarter of a million miles or 1.28 light-seconds, and a unit of measure in astronomy. This is not to be confused with the instantaneous Earth–Moon distance, or distance to the Moon, the momentanous distance from the center of Earth to the center of the Moon.
The Moon makes a complete orbit around Earth with respect to the fixed stars, its
sidereal 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, ...
, 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 at the same
lunar phase, 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 Eur ...
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 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 ...
, the Moon has a 1:1
spin–orbit resonance. This
rotation–
orbit
In celestial mechanics, an orbit 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 object or position in space such as ...
ratio makes the Moon's orbital periods around Earth equal to its corresponding
rotation periods. This is the reason for only one side of the Moon, its so-called
near side
The near side of the Moon is the lunar hemisphere that always faces towards Earth, opposite to the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth—a ...
, being visible from Earth. That said, while the movement of the Moon is in resonance, it still is not without nuances such as
libration, resulting in slightly changing perspectives, making over time and location on Earth about 59% of the Moon's surface visible from Earth.
Unlike most satellites of other planets, the Moon's orbital plane is closer to the
ecliptic plane 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. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projecti ...
. 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 Cassini's laws provide a compact description of the motion of the Moon. They were established in 1693 by Giovanni Domenico Cassini, a prominent scientist of his time.For the original statement of the laws, see
Refinements of these laws to include ...
.
Tidal effects
![Tide animation](https://upload.wikimedia.org/wikipedia/commons/7/77/Tide_animation.gif)
The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides of closest to each other, resulting in
tidal forces
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
.
Ocean tides are the most widely experienced result of this, but tidal forces considerably affect also other mechanics of Earth, as well as the Moon and their system.
The lunar solid crust experiences tides of around 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 bo ...
, 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, 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 Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic ...
.
The cumulative effects of stress built up by these tidal forces produces
moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from
seismometer
A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...
s placed on the Moon by
Apollo
Apollo, grc, Ἀπόλλωνος, Apóllōnos, label=genitive , ; , grc-dor, Ἀπέλλων, Apéllōn, ; grc, Ἀπείλων, Apeílōn, label= Arcadocypriot Greek, ; grc-aeo, Ἄπλουν, Áploun, la, Apollō, la, Apollinis, label ...
astronauts from 1969 through 1972.
The most commonly known effect of tidal forces are 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. As a result, there are two high tides, and two low tides in about 24 hours.
Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth.
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 idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
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.
Delays in the tidal peaks of both ocean and solid-body tides cause
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
in opposition to the Earth's rotation. This "drains"
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
and rotational
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...
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 away f ...
, 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 betwe ...
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 equal to 1000 ...
s every year, slowly increasing the rate at which
UTC 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 observ ...
s.
This tidal drag makes the rotation of 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 already the case with the Moon. Eventually, after 50 billion years,
also the Earth would be made to always face the Moon with the same side, though the Sun will become a
red giant engulfing the Earth-Moon system long before the latter occurs.
Position and appearance
![Lunar libration with phase Oct 2007 (continuous loop)](https://upload.wikimedia.org/wikipedia/commons/c/ca/Lunar_libration_with_phase_Oct_2007_%28continuous_loop%29.gif)
The Moon's highest
altitude
Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
at
culmination varies by its
lunar phase, or more correctly its orbital position, and time of the year, or more correctly the position of the Earth's axis. The full moon is highest in the sky during winter and lowest during summer (for each hemisphere respectively), with its altitude changing towards dark moon to the opposite.
At the
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 or geography.
Etymology
The word ''north ...
and
South Pole
The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on Earth and lies antipod ...
s the Moon is 24 hours above the horizon for two weeks every
tropical 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 ...
(about 27.3 days), comparable to the
polar day 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 of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time f ...
.
Zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
in the
Arctic
The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenland), Finland, Iceland, N ...
use
moonlight 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 it is seen upside down compared to the view in 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
''Wet Moon'' is a series of graphic novels by Sophie Campbell and published by Oni Press. Primarily set in the fictional southern American college town of Wet Moon, the series stars a large cast of characters, most in their late teens and earl ...
and occurs more frequently in the
tropics
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referr ...
.
The
distance between the Moon and Earth varies from around to at
perigee
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ell ...
(closest) and apogee (farthest), respectively, making the Moon's apparent size fluctuate. On average the Moon's
angular diameter
The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...
is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see ). Additionally when close to the horizon a purely psychological effect, known as the
Moon illusion, makes the Moon appear larger.
Despite the Moon's tidal locking, the effect of
libration makes about 59% of the Moon's surface visible from Earth over the course of one month.
Rotation
![Tidal locking of the Moon with the Earth](https://upload.wikimedia.org/wikipedia/commons/5/56/Tidal_locking_of_the_Moon_with_the_Earth.gif)
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
The near side of the Moon is the lunar hemisphere that always faces towards Earth, opposite to the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth—a ...
, 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
The term dark moon describes the last visible crescent of a waning Moon. The duration of a dark moon varies between 1.5 and 3.5 days, depending on its ecliptic latitude. In current astronomical usage, the new moon occurs in the middle of this ...
to
new moon, the near side is dark.
The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became
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 bo ...
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. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
al effects associated with
tidal
Tidal is the adjectival form of tide.
Tidal may also refer to:
* ''Tidal'' (album), a 1996 album by Fiona Apple
* Tidal (king), a king involved in the Battle of the Vale of Siddim
* TidalCycles, a live coding environment for music
* Tidal (servic ...
deformations caused by Earth. With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA ''
Lunar Prospector'' mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth.
Illumination and phases
Half of the Moon's surface is always illuminated by the Sun (except during a
lunar eclipse). Earth also reflects light onto the Moon, observable at times as
Earthlight
''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 ...
when it is again reflected back to Earth from areas of the
near side of the Moon
The near side of the Moon is the lunar hemisphere that always faces towards Earth, opposite to the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth—a ...
that are not illuminated by the Sun.
With the different positions of the Moon, different areas of it are illuminated by the Sun. This illumination of different lunar areas, as viewed from Earth, produces the different
lunar phases during the
synodic 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 ...
. A phase is equal to the area of the visible lunar sphere that is illuminated by the Sun. This area or ''degree of illumination'' is given by
, where
is the
elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).
![Moon phases en](https://upload.wikimedia.org/wikipedia/commons/4/46/Moon_phases_en.jpg)
On 14 November 2016, the Moon was at full phase closer to Earth than it had been since 1948. It was 14% closer and larger than its farthest position in apogee. This closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because of its increased apparent diameter, which made it a particularly notable example of a "
supermoon
A supermoon is a full moon or a new moon that nearly coincides with perigee—the closest that the Moon comes to the Earth in its elliptic orbit—resulting in a slightly larger-than-usual Angular diameter, apparent size of the lunar disk as v ...
".
At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:
:
When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase.
Albedo and color
![Two Lunar Phases](https://upload.wikimedia.org/wikipedia/commons/5/59/Two_Lunar_Phases.jpg)
The Moon has an exceptionally low albedo, giving it a diffuse reflection, reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun.
This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at
full moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This means ...
.
Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the Lambert's cosine law#Lambertian scatterers, reflective properties of
lunar soil
Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disinteg ...
, which retroreflector, 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.
Viewed from Earth the air filters the reflected light, at times giving it a red color depending on the angle of the Moon in the sky and thickness of the atmosphere, or a blue tinge depending on the particles in the air,
as in cases of volcanic particles. The terms Lunar eclipse#Blood moon, blood moon and blue moon do not necessarily refer to circumstances of red or blue
moonlight, but are rather particular cultural references such as particular
full moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This means ...
s of a year.
There has been historical controversy over whether observed features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However,
outgassing
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 (which ...
does occasionally occur and could be responsible for a minor percentage of the reported transient lunar phenomenon, lunar transient phenomena. Recently, it has been suggested that a roughly diameter region of the lunar surface was modified by a gas release event about a million years ago.
Eclipses
Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "Syzygy (astronomy), syzygy"). Solar eclipses occur at
new moon, when the Moon is between the Sun and Earth. In contrast,
lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total eclipse, total (with the Moon appearing larger than the Sun) and annular eclipse, annular (with the Moon appearing smaller than the Sun) solar eclipses. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time,
the angular diameter of the Moon is decreasing. As it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur.
Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the ecliptic, orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.
The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the Saros (astronomy), saros, which has a period of approximately 18 years.
Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky, the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.
History of exploration and human presence
Pre-telescopic observation (before 1609)
It is believed by some that 20–30,000 year old tally sticks, were used to observe the phases of the Moon, keeping time using the waxing and waning of Lunar phases, the Moon's phases.
[
One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving ''Orthostat 47'' at Knowth, Ireland.]
The ancient Greece, ancient Greek philosopher Anaxagoras () reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.[ Elsewhere in the to , Babylonian astronomy, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, and Indian astronomy, Indian astronomers had described the Moon's monthly elongation.] The Chinese astronomy, Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses.[
In Aristotle's (384–322 BC) On the Heavens, description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether (classical element), aether, an Aristotelian physics, influential philosophy that would dominate for centuries. Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. In the , Seleucus of Seleucia correctly theorized 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. In the same century, Aristarchus of Samos, Aristarchus Aristarchus On the Sizes and Distances, computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the Earth radius, radius of Earth for the distance.
Although the Chinese of the Han Dynasty believed the Moon to be energy equated to ''qi'', their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78–37 BC) noted the sphericity of the Moon.[ Ptolemy (90–168 AD) greatly improved on the numbers of Aristarchus, calculating the values of a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively. In the 2nd century AD, Lucian wrote the novel ''True History, A True Story'', in which the heroes travel to the Moon and meet its inhabitants. In 499 AD, the Indian astronomer Aryabhata mentioned in his ''Aryabhatiya'' that reflected sunlight is the cause of the shining of the Moon. The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions. Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.][
During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".
]
Telescopic exploration (1609-1959)
In 1609, Galileo Galilei used an early telescope to make drawings of the Moon for his book , and deduced that it was not smooth but had mountains and craters. Thomas Harriot had made, but not published such drawings a few months earlier.
Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–1836 of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book , the first trigonometry, 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 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, and from comparative studies from 1920 to the 1940s, leading to the development of lunar geologic timescale, lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology.
First missions to the Moon (1959–1990)
After World War II the first launch systems were developed and by the end of the 1950s they reached capabilities that allowed the Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
and the United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
to launch spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
s into space. The Cold War fueled a closely followed development of launch systems by the two states, resulting in the so-called Space Race and its later phase the Moon Race, accelerating efforts and interest in exploration of the Moon.
After the first spaceflight of Sputnik 1 in 1957 during International Geophysical Year the spacecrafts of the Soviet Union's Luna programme, ''Luna'' program were the first to accomplish a number of goals. Following three unnamed failed missions in 1958, the first human-made object ''Luna 1'' escaped Earth's gravity and passed near the Moon in 1959. Later that year the first human-made object '' Luna 2'' reached the Moon's surface by Lander (spacecraft)#Impactors, intentionally impacting. By the end of the year ''Luna 3'' reached as the first human-made object the normally occluded far side of the Moon, taking the first photographs of it.
The first spacecraft to perform a successful lunar lander (spacecraft), soft landing was ''Luna 9
Luna 9 (Луна-9), internal designation Ye-6 No.13, was an 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 survivable landing on a celestial body ...
'' and the first vehicle to orbit the Moon was ''Luna 10'', both in 1966.
Following President John F. Kennedy's 1961 commitment to a manned Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program, the Lunar Orbiter program and the Surveyor program. The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet human lunar programs, Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race.
Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) removed of lunar rock and soil in 2,196 moon rock, separate samples.
Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived Apollo Lunar Surface Experiments Package, instrument stations, including heat flow probes, seismometer
A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...
s, and magnetometers, were installed at the Apollo 12, Apollo 14, 14, Apollo 15, 15, Apollo 16, 16, and Apollo 17, 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, but as the stations' Lunar laser ranging experiment, lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used.
Apollo 17 in 1972 remains the last crewed mission to the Moon. Explorer 49 in 1973 was the last dedicated U.S. probe to the Moon until 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 (space exploration), rover Lunokhod 1 on an extraterrestrial surface, and collecting and returning 0.3 kg of rock and soil samples with three ''Luna'' sample return missions (''Luna 16'' in 1970, ''Luna 20'' in 1972, and ''Luna 24'' in 1976).
Moon Treaty and explorational absence (1976–1990)
A near lunar quietude of fourteen years followed the Luna 24, last Soviet mission to the Moon of 1976. Astronautics had shifted its focus towards the exploration of the inner Solar System, inner (e.g. Venera program) and outer Solar System, outer (e.g. Pioneer 10, 1972) Solar System
The Solar System Capitalization 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 "Solar ...
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s, but also towards Earth orbit, developing and continuously operating, beside communication satellites, Earth observation satellites (e.g. Landsat program, 1972) space telescopes and particularly space stations (e.g. Salyut program, 1971).
The until 1979 negotiated Moon treaty, with its ratification in 1984 by its few signatories was about the only major activity regarding the Moon until 1990.
Renewed exploration (1990-present)
In 1990 ''Hiten (spacecraft), Hiten''-''Hagoromo'', the first dedicated lunar mission since 1976, reached the Moon. Sent by Japan, it became the first mission that was not a Soviet Union or U.S. mission to the Moon.
In 1994, the U.S. dedicated a mission to fly a spacecraft (''Clementine
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 fir ...
'') 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 Multi-spectral image, multispectral images of the lunar surface. In 1998 this was followed by the '' Lunar Prospector'' mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.
The 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 (ESA) (''SMART-1'') reached the Moon, recording the first detailed survey of chemical elements on the lunar surface.
The Chinese Lunar Exploration Program began with ''Chang'e 1'' between 2007 and 2009, obtaining a full image map of the Moon.
Indian Space Research Organisation, India reached the Moon in 2008 for the first time with its ''Chandrayaan-1
Chandrayaan-1 (, ) was the first Indian lunar probe under the Chandrayaan program. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impact ...
'', creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of Lunar water, water molecules in lunar soil.
The U.S. launched the ''Lunar Reconnaissance Orbiter'' (LRO) and the '' LCROSS'' impactor on 18 June 2009. ''LCROSS'' completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, whereas ''LRO'' is currently in operation, obtaining precise lunar altimetry and high-resolution imagery.
China continued its lunar program in 2010 with ''Chang'e 2'', mapping the surface at a higher resolution over an eight-month period, and in 2013 with ''Chang'e 3'', a lunar lander (spacecraft), lander along with a lunar rover named ''Yutu (rover), Yutu'' (Chinese: 玉兔; literally "Jade Rabbit"). This was the first lunar rover mission since ''Lunokhod 2'' in 1973 and the first lunar Soft landing (rocketry), soft landing since ''Luna 24'' in 1976.
In 2014 the first privately funded probe, the Manfred Memorial Moon Mission, reached the Moon.
Another Chinese rover mission, ''Chang'e 4'', achieved the first landing on Far side of the Moon, 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 (''Chang'e 5''), bringing back 1,731 grams of lunar material to Earth.
With the signing of the U.S.-led Artemis Accords in 2020, the Artemis program aims to return in 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 and the ESA-led Moon Village concept. The U.S. Vision for Space Exploration, developed plans for returning to the Moon beginning in 2004, which resulted in several programs. The Artemis program has advanced the farthest, and includes plans to send the first woman to the Moon as well as build an international lunar space station called Lunar Gateway.
Future
Upcoming lunar missions include the Artemis program missions and Russia's first lunar mission, ''Luna-Glob'': an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian ''Fobos-Grunt'' mission.
In 2021, China announced a plan with Russia to develop and construct an International Lunar Research Station in the 2030s. In 2006, India and other countries expressed a hope to send people to the Moon by 2020.
Human presence
Humans have been active around and on the Moon for more than half a century, having send a wide range of missions to the Moon, having stayed robotically and with people, leaving many traces and having set up temporary moonbases.
Lunar orbit has seen uninterrupted List of extraterrestrial orbiters#Moon, presence of orbiters since 2006, performing mainly lunar observation, but also providing relayed communication for missions on the lunar surface. Lunar orbits and List of objects at Lagrange points#Earth–Moon Lagrange points, orbits around Earth–Moon Lagrange points, have been gaining attention for being used to establish a near-lunar infrastructure, and with it enable increasing human activity in this so-called cislunar space as well as on the Moon's surface. Particularly missions at the far side of the Moon or the Lunar north pole, lunar north and Lunar south pole, south polar regions have been and will be in need for oebitees with special orbits, such as the Queqiao relay satellite or the planed first extraterrestrial space station, the Lunar Gateway.
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 from the Moon, astronomy performed from the Moon, it will need to be free from any physical and radio spectrum pollution, 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 lander and its carrying of tardigrades has been discussed as an example for lacking measures and lacking international regulation for planetary protection.
Space debris beyond Earth around the Moon has been considered as a future challenge with increasing numbers of missions to the Moon, particularly as a danger for such missions. As such lunar waste management has been raised as an issue which future lunar missions, particularly on the surface, need to tackle.
Beside the remains of human activity on the Moon, there have been some intended permanent installations like the ''Moon Museum'' art piece, Apollo 11 goodwill messages, six lunar plaques, the ''Fallen Astronaut'' memorial, and other artifacts.
Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang'e 3 with its Lunar Ultraviolet Telescope still operational.
Five retroreflectors have been installed on the Moon since the 1970s and since used for accurate measurements of the physical librations through Lunar Laser Ranging experiment, laser ranging to the Moon.
There are List of missions to the Moon#Future missions, several missions by different agencies and companies planned to establish a longterm human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.
Astronomy from the Moon
For many years, the Moon has been recognized as an excellent site for telescopes. It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth. The lunar soil
Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disinteg ...
, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and Epoxy, epoxies and employed in the construction of mirrors up to 50 meters in diameter. A lunar zenith telescope can be made cheaply with an ionic liquid.
In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph.
The Moon has been also a sight of Earth observation, particularly culturally as in the imagery called Earthrise.
Living on the Moon
The only instances of Space habitat (facility)#Extraterrestrial surface habitat, humans living on the Moon have taken place in an Apollo Lunar Module for several days at a time (for example, during the Apollo 17 mission). One challenge to astronauts during their stay on the surface is that #Dust, lunar dust sticks to their suits and is carried into their quarters. Astronauts could taste and smell the dust, calling it the "Apollo aroma". This fine lunar dust can Lunar dust#Harmful effects of lunar dust, cause health issues.
In 2019 at least one plant seed sprouted in an experiment on the Change 4#Lunar lander, Chang'e 4 lander. It was carried from Earth along with other small life in its ''Lunar Micro Ecosystem''.
Legal status
Although ''Luna program, Luna'' landers scattered pennants of the Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...
on the Moon, and Lunar Flag Assembly, U.S. flags were symbolically planted at their landing sites by the List of Apollo astronauts, Apollo astronauts, no nation claims ownership of any part of the Moon's surface. Likewise no Extraterrestrial real estate, private ownership of parts of the Moon, or as a whole, is considered credible.
The 1967 Outer Space Treaty defines the Moon and all outer space as the "common heritage of mankind, province of all mankind". It restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. A majority of countries are parties of this treaty.
The 1979 Moon Treaty, Moon Agreement was created to elaborate, and restrict the exploitation of the Lunar resources, 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 capabilities.
Since 2020 countries have joined the U.S. in their Artemis Accords, which are challenging the treaty. The U.S. has furthermore emphasized in an presidential executive order ("Encouraging International Support for the Recovery and Use of Space Resources.") that "the United States does not view outer space as a 'global commons and calls the Moon Agreement "a failed attempt at constraining free enterprise."
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, 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 Sites 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
In light of future development on the Moon some international and multi-space agency organizations have been created:
* International Lunar Exploration Working Group (ILEWG)
* Moonbase#Moon Village, Moon Village Association (MVA)
* International Space Exploration Coordination Group (ISECG)
In culture and life
Calendar
Since pre-historic times people have taken note of Lunar phases, the Moon's phases, its waxing and waning, and used it to keep record of time. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. The counting of the days between the Moon's phases gave eventually rise to generalized Unit of time, time periods of the full lunar cycle as months, and possibly of its phases as weeks.
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 and Ancient 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. Its proposed features have been derived by linguistic reconstruction from documented Indo-European languages. No direct record of Proto-Indo- ...
(PIE) root of ''moon'', *''méh1nōt'', derived from the PIE verbal root *''meh1''-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker of the month" (cf. the English words ''measure'' and ''menstrual''). To give another example from a different language family, the Chinese language uses the same word () for ''moon'' as well 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 calendars. The 7th-century Islamic calendar is an example of a purely lunar calendar
A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly on the solar year. The most commonly used calendar, t ...
, 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
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This means ...
, highlighted and celebrated in a range of calenders and cultures. Around Autumnal equinox (disambiguation), autumnal equinox, the Full Moon is called the Harvest moon, Harvest Moon and is celebrated with festivities such as the Harvest Moon Festival of the Chinese Lunar Calendar, its second most important celebration after Chinese New Year.
Furthermore, association of time with the Moon can also be found in religion, such as the ancient Egyptian temporal and lunar deity Khonsu.
Cultural representation
Since prehistoric and ancient times humans have depicted and interpreted the Moon, Moon (astrology), particularly for astrology and religion, as lunar deity.
For the representation of the Moon, especially its lunar phases, the crescent symbol (🌙) has been particularly used by many cultures. In writing systems such as Chinese the crescent has developed into the symbol , the word for Moon, and in ancient Egyptian it was the symbol , which is spelled like the ancient Egyptian lunar deity ''Iah'', meaning Moon.
Iconographically the crescent was used in Mesopotamia as the primary symbol of Sin (mythology), Nanna/Sîn,[ the ancient Sumerian lunar deity,] who was the father of Innana, Innana/Ishtar, the goddess of the planet Venus (symbolized as the Octagram, eight pointed Star of Ishtar),[ and Utu, Utu/Shamash, the god of the Sun (Solar symbol, symbolized as a disc, optionally with eight rays),][ all three often depicted next to each other. Nanna was later known as Sîn,] and was particularly associated with magic and sorcery.
The crescent was further used as an element of lunar Horned deity, deities wearing headgears or crowns in an arrangement reminiscent of horns, as in the case of the ancient Greek Selene
In ancient Greek mythology and religion, Selene (; grc-gre, Σελήνη , meaning "Moon"''A Greek–English Lexicon's.v. σελήνη) is the goddess and the personification of the Moon. Also known as Mene, she is traditionally the daughter of ...
or the ancient Egyptian Khonsu. Selene is associated with Artemis
In ancient Greek mythology and religion, Artemis (; grc-gre, Ἄρτεμις) is the goddess of the hunt, the wilderness, wild animals, nature, vegetation, childbirth, care of children, and chastity. She was heavily identified wit ...
and paralleled by the Roman Luna (goddess), Luna, which both are occasionally depicted Luna (goddess)#Chariot of the moon, driving a chariot, like the Hindu lunar deity Chandra. The different or sharing Aspect (religion), aspects of deities within Pantheon (religion), pantheons has been observed in many cultures, especially by later or contemporary culture, particularly forming triple deity, triple deities. The Moon in Roman mythology for example has been associated with Juno (mythology), Juno and Diana (goddess), Diana, while Luna being identified as their byname and as part of a triplet (''diva triformis'') with Diana and Proserpina, Hecate being identified as their binding manifestation as ''trimorphos''.
The star and crescent (☪️) arrangement goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a Byzantine flags and insignia, symbol of Byzantium, possibly influencing the development of the Flags of the Ottoman Empire, Ottoman flag, specifically the combination of the Turkish crescent with a star.
Since then the heraldry, heraldric use of the star and crescent proliferated becoming a popular Symbols of Islam, symbol for Islam (as the ''wikt:hilal#Noun, hilal'' of the Islamic calendar) and Star and crescent#Contemporary use, for a range of nations.
In Roman Catholic Marian veneration, the Virgin Mary (Queen of Heaven) has been depicted since the late middle ages on a crescent and adorned with stars. In Islam Muhammad is particularly attributed with the Moon through the so-called splitting of the Moon ( ar, انشقاق القمر) miracle.["Muhammad." ''Encyclopædia Britannica''. 2007. Encyclopædia Britannica Online, p.13]
The contrast between the brighter highlands and the darker maria have been seen by different cultures forming abstract shapes, which are among others the Man in the Moon or the Moon Rabbit (e.g. the Chinese Tu'er Ye or in Mythologies of the indigenous peoples of the Americas, Indigenous American mythologies, as with the aspect of the Maya moon goddess, Mayan Moon goddess).
In Western alchemy silver is associated with the Moon, and gold with the Sun.
Modern culture representation
The perception of the Moon in modern times has been informed by telescope enabled modern astronomy and later by spaceflight enabled actual human activity at the Moon, particularly the Apollo program#Cultural impact, 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 commercialization of space, economic expansion into space, with missions prospecting for lunar resources. This has been accompanied with renewed public and critical reflection on humanity's cultural and #Legal status, legal relation to the celestial body, especially regarding colonialism, 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 commons, common.
Lunar effect
The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. The Moon has long been associated with insanity and irrationality; the words ''lunacy'' and ''lunatic'' are derived from the Latin name for the Moon, ''Luna''. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims.
See also
*List of natural satellites
*Selenography (Geography of the Moon)
Explanatory notes
References
Further reading
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External links
NASA images and videos about the Moon
* Albums of images and high-resolution overflight videos by Seán Doran, based on Lunar Reconnaissance Orbiter Camera, LROC data, o
Flickr
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YouTube
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Cartographic resources
Unified Geologic Map of the Moon
– United States Geological Survey
Moon Trek – An integrated map browser of datasets and maps for the Moon
The Moon on Google Maps
a 3-D rendition of the Moon akin to Google Earth
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Gazetteer of Planetary Nomenclature (USGS)
List of feature names.
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* 3D zoomable globes:
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* Maps and panoramas at Apollo landing sites
Selene, Kaguya (Selene) images
Lunar Earthside chart (4497 x 3150px)
Large image of the Moon's north pole area
Observation tools
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* See when the next new crescent moon is visible for any location.
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Moon,
Moons, *
Articles containing video clips
Astronomical objects known since antiquity
Planetary satellite systems
Planetary-mass satellites
Moons with a prograde orbit