Moon

The **Moon** is Earth's only natural satellite. Together with Earth it forms the Earth-Moon satellite system. It is about one-quarter of Earth in diameter (comparable to the width of Australia). In the Solar System it is the fifth largest satellite, larger than any of the known dwarf planets and the largest (and most massive) satellite of a planet relative to the planet. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (0.1654 g). Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence very slowly lengthens Earth's day and is the main driver of Earth's tides. 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 phases 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 shifts in perspective due to 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-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 relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a 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.

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.

## 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 *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 usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, *lūna*. The adjective *selenian* /səliːniən/, derived from the Greek word for the Moon, σελήνη *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, 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, , for example in *M*☾ 'lunar mass' (also *ML*).