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 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 were formed when molten lava flowed into ancient impact basins. 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. Since then, exploration of the Moon has continued robotically with crewed missions being planned to return beginning in the late 2020s.
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 Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to restore hydrostatic equilibrium at its current orbital distance.
Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "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 (with the Moon appearing larger than the Sun) and 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.
Since pre-historic times people have taken note of the Moon's phases and its waxing and waning cycle, and used it to keep record of time. Tally 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 time periods of lunar cycles 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 mensis and Ancient Greek μείς (meis) or μήν (mēn), meaning "month") stem from the Proto-Indo-European (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).
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, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon.
Of particular significance has been the occasion of full moon, highlighted and celebrated in a range of calendars and cultures, an example being the Buddhist Vesak. The full moon around the southern or northern autumnal equinox is often called the harvest moon and is celebrated with festivities such as the Harvest Moon Festival of the Chinese lunar calendar, its second most important celebration after the Chinese lunisolar Lunar New Year.
Furthermore, association of time with the Moon can also be found in religion, such as the ancient Egyptian temporal and lunar deity Khonsu.
Since prehistoric and ancient times humans have drawn the Moon and have described a range of understandings of it, having prominent importance in different cosmologies, often exhibiting a spirit, being a deity or an aspect, particularly in astrology.
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 culturally impactful lunar landings. These new insights inspired cultural references, connecting romantic reflections about the Moon and speculative fiction such as science-fiction dealing with the Moon.
Contemporarily the Moon has been seen as a place for economic expansion into space, with missions prospecting for lunar resources. This has been accompanied with renewed public and critical reflection on humanity's cultural and 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 common.
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.
(Wikipedia)