Moon
Earth's only natural satellite / From Wikipedia, the free encyclopedia
The Moon is Earth's only natural satellite. We usually see it in the night sky and also during the day. Some other planets also have moons or natural satellites.
 The Moon as seen from Earth. This is a nearly full moon | |||||||||||||
| Designations | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Luna | |||||||||||||
| Adjectives | lunar, selenic | ||||||||||||
| Orbital characteristics | |||||||||||||
| Perigee | 363,104 km (0.002 4 AU) | ||||||||||||
| Apogee | 405,696 km (0.002 7 AU) | ||||||||||||
| 384,399 km (0.002 57 AU[1]) | |||||||||||||
| Eccentricity | 0.054 9[1] | ||||||||||||
| 27.321 582 d (27 d 7 h 43.1 min[1]) | |||||||||||||
| 29.530 589 d (29 d 12 h 44 min 2.9 s) | |||||||||||||
Average orbital speed | 1.022 km/s | ||||||||||||
| Inclination | 5.145° to the ecliptic[1] (between 18.29° and 28.58° to Earth's equator) | ||||||||||||
| regressing by one revolution in 18.6 years | |||||||||||||
| progressing by one revolution in 8.85 years | |||||||||||||
| Satellite of | Earth | ||||||||||||
| Physical characteristics | |||||||||||||
Mean radius | 1,737.10 km (0.273 Earths)[1][2] | ||||||||||||
| Flattening | 0.001 25 | ||||||||||||
| Circumference | 10,921 km (equatorial) | ||||||||||||
| 3.793 × 107 km² (0.074 Earths) | |||||||||||||
| Volume | 2.195 8 × 1010 km³ (0.020 Earths) | ||||||||||||
| Mass | 7.347 7 × 1022 kg (0.012 3 Earths[1]) | ||||||||||||
Mean density | 3,346.4 kg/m³[1] | ||||||||||||
| 1.622 m/s² (0.165 4 g) | |||||||||||||
| 2.38 km/s | |||||||||||||
Sidereal rotation period | 27.321 582 d (synchronous) | ||||||||||||
Equatorial rotation velocity | 4.627 m/s | ||||||||||||
| 1.542 4° (to ecliptic) 6.687° (to orbit plane) | |||||||||||||
| Albedo | 0.12 | ||||||||||||
| |||||||||||||
| −2.5 to −12.9[4] −12.74 (mean full moon)[2] | |||||||||||||
| 29.3 to 34.1 arcminutes[2][5] | |||||||||||||
Surface pressure | 2.25 × 10-12 torr [6] | ||||||||||||
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Our moon is about one-fourth of the width of the Earth. Because it is far away it looks small, about half a degree wide. The gravity on the moon is one-sixth of the Earth's gravity.[7] It means that something will be one-sixth as heavy on the Moon than on Earth. The Moon is a rocky and dusty place. It moves slowly away from the Earth at a rate of 3.8 cm per year, due to the effect of tidal dissipation.
For the origin of the Moon, see the giant impact hypothesis.