Moon – Wikipedia, the free encyclopedia

Moon Designations Adjective lunar, selenic Orbital characteristics Perigee 363,295 km (0.0024 AU) Apogee 405,503 km (0.0027 AU) Semi-major axis 384,399 km (0.00257AU)[1] Eccentricity 0.0549[1] Orbital period 27.321582d (27 d 7 h 43.1 min[1]) Synodic period 29.530589d (29 d 12 h 44 min 2.9 s) Average orbital speed 1.022km/s Inclination 5.145 to the ecliptic[2] (between 18.29 and 28.58 to Earth's equator)[1] Longitude of ascending node regressing by one revolution in 18.6years Argument of perigee progressing by one revolution in 8.85years Satellite of Earth Physical characteristics Mean radius 1,737.10km (0.273 Earths)[1][3] Equatorial radius 1,738.14km (0.273Earths)[3] Polar radius 1,735.97km (0.273 Earths)[3] Flattening 0.00125 Circumference 10,921 km (equatorial) Surface area 3.793107 km2 (0.074 Earths) Volume 2.19581010 km3 (0.020 Earths) Mass 7.34771022 kg (0.012300 Earths[1]) Mean density 3.3464 g/cm3[1] Equatorial surface gravity 1.622 m/s2 (0.165 4 g) Escape velocity 2.38 km/s Sidereal rotation period 27.321582 d (synchronous) Equatorial rotation velocity 4.627 m/s Axial tilt 1.5424 (to ecliptic) 6.687 (to orbit plane)[2] Albedo 0.136[4] Surface temp. min mean max equator 100 K 220 K 390 K 85N[5] 70 K 130 K 230 K Apparent magnitude 2.5 to 12.9[a] 12.74 (mean full moon)[3] Angular diameter 29.3 to 34.1 arcminutes[3][b] Atmosphere[6] Surface pressure 107Pa (day) 1010 Pa (night)[c] Composition Ar, He, Na, K, H, Rn

The Moon is the only natural satellite of the Earth[d][7] and the fifth largest moon in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary,[e] having 27% the diameter and 60% the density of Earth, resulting in 181 its mass. Among satellites with known densities, the Moon is the second densest, after Io, a satellite of Jupiter.

The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. This matching of apparent visual size is a coincidence. The Moon's linear distance from the Earth is currently increasing at a rate of 3.820.07cm per year, but this rate is not constant.[8]

The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body.

The Moon is the only celestial body other than Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history.

After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft. Of these, orbital missions have dominated: Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters, which have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has also seen two rover missions: the final Soviet Lunokhod mission in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu rover on 14 December 2013.

Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.

The English proper name for Earth's natural satellite is "the Moon".[9][10] The noun moon derives from moone (around 1380), which developed from mone (1135), which derives from Old English mna (dating from before 725), which, like all Germanic language cognates, ultimately stems from Proto-Germanic *mnn.[11]

The principal modern English adjective pertaining to the Moon is lunar, derived from the Latin Luna. Another less common adjective is selenic, derived from the Ancient Greek Selene (), from which the prefix "seleno-" (as in selenography) is derived.[12]

Several mechanisms have been proposed for the Moon's formation 4.527 0.010 billion years ago,[f] some 3050million years after the origin of the Solar System.[13] Recent research presented by Rick Carlson indicates a slightly younger age of between 4.4 and 4.45 billion years.[14][15] These mechanisms included the fission of the Moon from the Earth's crust through centrifugal force[16] (which would require too great an initial spin of the Earth),[17] the gravitational capture of a pre-formed Moon[18] (which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon),[17] and the co-formation of the Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metallic iron in the Moon).[17] These hypotheses also cannot account for the high angular momentum of the EarthMoon system.[19]

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Moon - Wikipedia, the free encyclopedia