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The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
). The Moon is a planetary-mass object with a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term and larger than all known dwarf planets of the Solar System. It lacks any significant
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, hydrosphere, or
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
. Its surface gravity is about one-sixth of Earth's at , with Jupiter'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 In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the lon ...
of , or about 30 times Earth's diameter. Its gravitational influence is the main driver of Earth's tides and very slowly lengthens Earth's day. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in
lunar phase Concerning the lunar month of ~29.53 days as viewed from Earth, the lunar phase or Moon phase is the shape of the Moon's directly sunlit portion, which can be expressed quantitatively using areas or angles, or described qualitatively using the t ...
s that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side ( the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. However, 59% of the total lunar surface can be seen from Earth through cyclical shifts in perspective known as
libration In lunar astronomy, libration is the wagging or wavering of the Moon perceived by Earth-bound observers and caused by changes in their perspective. It permits an observer to see slightly different hemispheres of the surface at different tim ...
. 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 The giant-impact hypothesis, sometimes called the Big Splash, or the Theia (planet), Theia Impact, suggests that the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planet, approximately 4.5 billion years ago ...
between the planet and a hypothesized Mars-sized body called Theia. 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 ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is fairly non-reflective, with the reflectance of lunar soil being comparable to that of asphalt. However, due to its large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total
solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six month ...
. Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's
Luna 2 ''Luna 2'' ( rus, Луна 2}), originally named the Second Soviet Cosmic Rocket and nicknamed Lunik 2 in contemporaneous media, was the sixth of the Soviet Union's Luna programme spacecraft launched to the Moon, E-1 No.7. It was the first spac ...
uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.


Names and etymology

The usual English 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 settlement of Britain, Anglo ...
''mōna'', which (like all its Germanic cognates) stems from Proto-Germanic ''*mēnōn'', which in turn comes from Proto-Indo-European ''*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 (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. The Greek word for the Moon does however provide us with the prefix ''seleno-'', as in '' selenography'', the study of the physical features of the Moon, as well as the element name ''selenium''. The Greek goddess of the wilderness and the hunt, Artemis, equated with the Roman
Diana Diana most commonly refers to: * Diana (name), a given name (including a list of people with the name) * Diana (mythology), ancient Roman goddess of the hunt and wild animals; later associated with the Moon * Diana, Princess of Wales (1961–1997) ...
, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia, from her legendary birthplace on
Mount Cynthus Mount Cynthus (, ''Kýnthos'') is located on the isle of Delos, part of the Greek Cyclades. Mythology In Greek mythology, Leto gave birth to Apollo and Artemis on this island, having been shunned by Zeus' wife Hera who was extremely jealous of ...
. These names – Luna, Cynthia and Selene – are reflected in technical terms for
lunar orbit In astronomy, lunar orbit (also known as a selenocentric orbit) is the orbit of an object around the Moon. As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by spacecraft around the Moon. The ...
s such as ''apolune'', ''pericynthion'' and ''selenocentric''. The astronomical symbol for the Moon is a crescent, , for example in ''M'' 'lunar mass' (also ''ML'').


Natural history


Lunar geologic timescale


Formation

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the
origin of the Solar System The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
. 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 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 to dissipate the energy of the passing Moon. A co-formation of Earth and the Moon together in the
primordial Primordial may refer to: * Primordial era, an era after the Big Bang. See Chronology of the universe * Primordial sea (a.k.a. primordial ocean, ooze or soup). See Abiogenesis * Primordial nuclide, nuclides, a few radioactive, that formed before ...
accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other fo ...
does not explain the depletion of metals in the Moon. None of these hypotheses can account for the high angular momentum of the Earth–Moon system. The prevailing theory is that the Earth–Moon system formed after a
giant impact The giant-impact hypothesis, sometimes called the Big Splash, or the Theia (planet), Theia Impact, suggests that the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planet, approximately 4.5 billion years ago ...