Moon

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

The **Moon** is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times Earth's diameter. The Moon always presents the same side to Earth, because gravitational pull has locked its rotation to the planet. This results in the lunar day of 29.5 Earth days matching the lunar month. The Moon's gravitational pull – and to a lesser extent the Sun's – are the main drivers of the tides.

## The Moon

The near side of the Moon (north at top) as seen from Earth

## Designations

### Adjectives

Lunar Selenian (poetic) Cynthian (poetic) Moonly (poetic)

## Atmosphere

### Surface pressure

10−7 Pa (1 picobar) (day)10−10 Pa (1 femtobar) (night)

### Composition by volume

He Ar Ne Na K H Rn

The Moon is in geophysical terms a planetary-mass object or satellite planet. It has a mass that amounts to 1.2% of Earth's, and a diameter that is roughly one-quarter of Earth's or with 3,474 km (2,159 mi) about as wide as Australia. Within the Solar System it is the most massive and largest satellite in relation to its parent planet, the fifth most massive and largest moon overall, and more massive and larger than all known dwarf planets. Its surface gravity is about one sixth of Earth's, about half of that of Mars, and after Jupiter's moon Io the second highest among all Solar System moons. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia.

The lunar surface is covered in lunar dust and marked by mountains, impact craters, their ejecta, ray-like streaks and, mostly on the near side of the Moon, by dark maria ("seas"), which are plains of cooled magma. These maria formed when large impacts on the far side of the Moon heated up low lying layers of its crust on the near side. The Moon is, beside when passing through Earth's shadow during a lunar eclipse, always illuminated by the Sun, but from Earth the visible illumination shifts during its orbit, producing the lunar phases. The Moon is the brightest celestial object in Earth's night sky. This is mainly due to its large angular diameter, while the reflectance of the lunar surface is comparable to that of asphalt. The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse. From Earth about 59% of the lunar surface is visible over time due to cyclical shifts in perspective (libration), making parts of the far side of the Moon visible.

For humans the Moon has been an important source of inspiration and knowledge, having been crucial to cosmography, mythology, religion, art, time keeping, natural science, and spaceflight. On September 13, 1959, the first human-made object to reach an extraterrestrial body arrived on the Moon, the Soviet Union's Luna 2 impactor. In 1966 the Moon became the first extraterrestrial body where soft landings and orbital insertions were achieved. On July 20, 1969, humans for the first time landed on the Moon and any extraterrestrial body, at Mare Tranquillitatis with the lander *Eagle* of the United States' Apollo 11 mission. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the Apollo 17 crew. Thereafter exploration of the Moon has continued robotically, with crewed missions being planned to return starting in the late 2020s.

## Names and etymology

See also: Moon § Cultural representation

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 *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** /ˈluːnə/ 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** /ˈsɪnθiə/ is another poetic name, though rare, for the Moon personified as a goddess, while **Selene** /səˈliːniː/ (literally "Moon") is the Greek goddess of the Moon.

The English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, *lūna*. *Selenian* /səliːniən/ is an adjective used to describe the Moon as a world, rather than as a celestial object, but its use is rare. It is derived from σελήνη *selēnē*, the Greek word for the Moon, and its cognate *selenic* was originally a rare synonym but now nearly always refers to the chemical element selenium. The element name *selenium* and the prefix *seleno-* (as in *selenography*, the study of the physical features of the Moon) come from this Greek word.

The Greek goddess of the wilderness and the hunt, Artemis, 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, 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 for the Moon is a crescent, https://upload.wikimedia.org/wikipedia/commons/thumb/b/bd/Moon\_decrescent\_symbol\_%28bold%29.svg/24px-Moon\_decrescent\_symbol\_%28bold%29.svg.png, for example in *M*☾ 'lunar mass' (also *ML*).

## Natural history

### Lunar geologic timescale

Main article: Lunar geologic timescale

The lunar geological periods are named after their characteristic features, from most impact craters outside the dark mare, to the mare and later craters, and finally the young, still bright and therefore readily visible craters with ray systems like Copernicus or Tycho.

### Formation

Main articles: Origin of the Moon, Giant-impact hypothesis, and Circumplanetary disk

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. Historically, several formation mechanisms have been proposed, but none satisfactorily 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 accretion disk 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 of a Mars-sized body (named *Theia*) with the proto-Earth. The oblique impact blasted material into orbit about the Earth and the material accreted and formed the Moon just beyond the Earth's Roche limit of ~2.56 *R*🜨.

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth. However, models from 2007 and later suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated.

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

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. Models that have the Moon acquiring a significant amount of the proto-earth are more difficult to reconcile with geochemical data for the isotopes of zirconium, oxygen, silicon, and other elements. Above a high resolution threshold for simulations,[*clarify*] a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.

On 1 November 2023, scientists reported that, according to computer simulations, remnants of a protoplanet, named Theia, could be inside the Earth, left over from a collision with the Earth in ancient times, and afterwards becoming the Moon.