A moon just past full as seen from Earth's northern hemisphere
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Name | Moon | Adjective | lunar | Periapsis | 362,570 km (0.0024 AU) (356,400-370,400 km) | Apoapsis | 405,410 km (0.0027 AU) (404,000-406,700 km) | Semi-major axis | 384,399 km ( ) | Eccentricity | | Orbital period | (27 d 7 h 43.1 min) | Synodic period | (29 d 12 h 44 min 2.9 s) | Avg 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 peri | 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 | | Circumference | 10,921 km (equatorial) | Surface area | 3.793 (0.074 Earths) | Volume | 2.1958 (0.020 Earths) | Mass | 7.3477 (0.0123 Earths) | 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 | Apparent magnitude | −2.5 to −12.9 −12.74 (mean full Moon) | Angular diameter | 29.3 to 34.1 arcminutes | Pressure | 10−7 Pa (day) 10−10 Pa (night) | Composition | Ar, He, Na, K, H, Rn |
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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 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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