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.[17] 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.[18] 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 usual English proper name for Earth's natural satellite is simply Moon, with a capital M.[20][21] The noun moon is derived from Old English mna, which (like all its Germanic cognates) stems from Proto-Germanic *mnn,[22] which in turn comes from Proto-Indo-European *mnsis "month"[23] (from earlier *mnt, genitive *mneses) which may be related to the verb "measure" (of time).[24]
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Occasionally, the name Luna /lun/ is used in scientific writing[25] 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.[26] Cynthia /sni/ is another poetic name, though rare, for the Moon personified as a goddess,[27] while Selene /slini/ (literally "Moon") is the Greek goddess of the Moon.
The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizeable as one of its planetary-mass moons, making it a satellite planet under the geophysical definitions of the term.[18] It is smaller than Mercury and considerably larger than the largest dwarf planet of the Solar System, Pluto. While the minor-planet moon Charon of the Pluto-Charon system is larger relative to Pluto,[f][68] the Moon is the largest natural satellite of the Solar System relative to their primary planets.[g]
The Moon's mass is 1/81 of Earth's,[70] being the second densest among the planetary moons, and having the second highest surface gravity, after Io, at 0.1654 g and an escape velocity of 2.38 km/s (8600 km/h; 5300 mph).
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.[137][138] Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth.[139]
The Moon's highest altitude 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 and South Poles the Moon is 24 hours above the horizon for two weeks every tropical month (about 27.3 days), comparable to the polar day of the tropical year. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end.[187]
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 appears upside down compared to the view from the southern hemisphere.[188] Sometimes the "horns" of a crescent moon appear to be pointing more upwards than sideways. This phenomenon is called a wet moon and occurs more frequently in the tropics.[189]
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, 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 to new moon, the near side is dark.[193]
Brightness and apparent size of the Moon changes also due to its elliptic orbit around Earth. At perigee (closest), since the Moon is up to 14% closer to Earth than at apogee (most distant), it subtends a solid angle which is up to 30% larger. Consequently, given the same phase, the Moon's brightness also varies by up to 30% between apogee and perigee.[196] A full (or new) moon at such a position is called a supermoon.[190][191][197]
The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun.[70][j] 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.[200] 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 reflective properties of lunar soil, which 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.[201]
At times the Moon can appear red or blue.Red it may appear during a lunar eclipse, because of the red spectrum of the Sun's light being refracted onto the Moon by Earth's atmosphere. Lunar eclipses are because of this red colour sometimes called blood moons. The Moon furthermore appears red when it appears at low angles and through a thick atmosphere in general
Blue the Moon may appear depending on the particles in the air,[201] as in cases of volcanic particles,[202] and is therefore sometimes called a blue moon. Red and blue moon though does not necessarily refer to circumstances of red or blue moonlight, but may rather be names for particular full moons of a year.
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.[203] 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,[176] 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.[k] 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.[204]
Because the Moon's orbit around Earth is inclined by about 5.145 (5 9') to the 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.[205] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years.[206]
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.[299]
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.[300]
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.[316] 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.[316][317][318][319][320] e24fc04721
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