This article is about Earth's Moon. For moons in general, see Natural satellite. For other uses, see Moon (disambiguation).
 Full moon as seen from Earth's northern hemisphere |
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| Designations | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Adjective | lunar, selenic | |||||||||
| Orbital characteristics | ||||||||||
| Perigee | 362,570 km (0.0024 AU) (356,400–370,400 km) |
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| Apogee | 405,410 km (0.0027 AU) (404,000–406,700 km) |
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| Semi-major axis | 384,399 km (0.00257 AU)[1] | |||||||||
| Eccentricity | 0.0549[1] | |||||||||
| Orbital period | 27.321582 d (27 d 7 h 43.1 min[1]) | |||||||||
| Synodic period | 29.530589 d (29 d 12 h 44 min 2.9 s) | |||||||||
| Average orbital speed | 1.022 km/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 18.6 years | |||||||||
| Argument of perigee | progressing by one revolution in 8.85 years | |||||||||
| Satellite of | Earth | |||||||||
| Physical characteristics | ||||||||||
| Mean radius | 1,737.10 km (0.273 Earths)[1][3] | |||||||||
| Equatorial radius | 1,738.14 km (0.273 Earths)[3] | |||||||||
| Polar radius | 1,735.97 km (0.273 Earths)[3] | |||||||||
| Flattening | 0.00125 | |||||||||
| Circumference | 10,921 km (equatorial) | |||||||||
| Surface area | 3.793 × 107 km2 (0.074 Earths) | |||||||||
| Volume | 2.1958 × 1010 km3 (0.020 Earths) | |||||||||
| Mass | 7.3477 × 1022 kg (0.0123 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] |
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| Albedo | 0.136[4] | |||||||||
| Surface temp. equator 85°N[5] |
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| Apparent magnitude | −2.5 to −12.9[a] −12.74 (mean full moon)[3] |
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| Angular diameter | 29.3 to 34.1 arcminutes[3][b] | |||||||||
| Atmosphere[6] | ||||||||||
| Surface pressure | 10−7 Pa (day) 10−10 Pa (night)[c] |
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| Composition | Ar, He, Na, K, H, Rn | |||||||||
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 very dark, with a reflectance similar to that of coal. 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. Earlier in Earth's history, the Moon was closer to Earth, and had an apparent visual size greater than that of the Sun.
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 380 kg 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, notably by the final Soviet Lunokhod rover. Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters. These spacecraft have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. 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.
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