Moon
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The Moon
Adjective
lunar, selenic
Perigee
362,570 km (0.0024 AU)
(356,400–370,400 km)
Apogee
405,410 km (0.0027 AU)
(404,000–406,700 km)
Semi-major axis
384,399 km (0.00257 AU)
Eccentricity
0.0549
Orbital period
27.321582 d (27 d 7 h 43.1 min)
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
(between 18.29° and 28.58° to Earth's equator)
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
Mean radius
1,737.10 km (0.273 Earths)
Equatorial radius
1,738.14 km (0.273 Earths)
Polar radius
1,735.97 km (0.273 Earths)
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)
Mean density
3.3464 g/cm3
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)
Albedo
0.136
Surface temp.
equator
85°N
min
mean
max
100 K
220 K
390 K
70 K
130 K
230 K
Apparent magnitude
-2.5 to -12.9
-12.74 (mean full Moon)
Angular diameter
29.3 to 34.1 arcminutes
Surface pressure
10-7 Pa (day)
10-10 Pa (night)
Composition
Ar, He, Na, K, H, Rn
The Moon is Earth's only known natural satellite, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having a quarter the diameter of Earth and 1/81 its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter. It is in synchronous rotation with Earth, always showing the same face; the near side is marked with dark volcanic maria among the bright ancient crustal highlands and prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a similar reflectance to 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.The Moon is the only celestial body on which humans have landed. While 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—the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a detailed geological understanding of the Moon's origins (it is thought to have formed some 4.5 billion years ago in a giant impact event involving Earth), 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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System
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