Moon

Moon

The Moon as seen from Earth. This is a nearly full moon
Designations
Alternative names	Luna
Adjectives	lunar, selenic
Orbital characteristics
Perigee	363,104 km  (0.002 4 AU)
Apogee	405,696 km  (0.002 7 AU)
Semi-major axis	384,399 km  (0.002 57 AU[1])
Eccentricity	0.054 9[1]
Orbital period	27.321 582 d  (27 d 7 h 43.1 min[1])
Synodic period	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)
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][2]
Flattening	0.001 25
Circumference	10,921 km (equatorial)
Surface area	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]
Surface gravity	1.622 m/s² (0.165 4 g)
Escape velocity	2.38 km/s
Sidereal rotation period	27.321 582 d (synchronous)
Equatorial rotation velocity	4.627 m/s
Axial tilt	1.542 4° (to ecliptic) 6.687° (to orbit plane)
Albedo	0.12
Surface temp. min mean max equator 100 K 220 K 85°N[3] 70 K 130 K 230 K
Apparent magnitude	−2.5 to −12.9[4] −12.74 (mean full moon)[2]
Angular diameter	29.3 to 34.1 arcminutes[2][5]
Surface pressure	2.25  × 10-12 torr [6]

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.

Our moon is about one-fourth of the width of the Earth.^[8] 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.^[9] 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.

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5 1.6} Wieczorek, M. A. (2006). "The constitution and structure of the lunar interior". Reviews in Mineralogy and Geochemistry. 60 (1): 221–364. Bibcode:2006RvMG...60..221W. doi:10.2138/rmg.2006.60.3. ISSN 1529-6466.
2. ↑ ^{2.0 2.1 2.2} Williams, Dr. David R. (February 2, 2006). "Moon Fact Sheet". NASA (National Space Science Data Center). Retrieved 2008-12-31.
3. ↑ A.R. Vasavada; D.A. Paige; S.E. Wood (1999). "Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits". Icarus. 141 (2): 179. Bibcode:1999Icar..141..179V. doi:10.1006/icar.1999.6175.
4. ↑ The maximum value is given based on scaling of the brightness from the value of -12.74 given for an equator to Moon-centre distance of 378 000 km in the NASA factsheet reference to the minimum Earth-Moon distance given there, after the latter is corrected for the Earth's equatorial radius of 6 378 km, giving 350 600 km. The minimum value (for a distant new moon) is based on a similar scaling using the maximum Earth-Moon distance of 407 000 km (given in the factsheet) and by calculating the brightness of the earthshine onto such a new moon. The brightness of the earthshine is [ Earth albedo × (Earth radius / Radius of Moon's orbit)² ] relative to the direct solar illumination that occurs for a full moon. ({{{1}}}; {{{1}}} radius × equatorial {{{1}}}).
5. ↑ The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378 000 km, the angular size is 1896 arcseconds. The same fact sheet gives extreme Earth-Moon distances of 407 000 km and 357 000 km. For the maximum angular size, the minimum distance has to be corrected for the Earth's equatorial radius of 6 378 km, giving 350 600 km.
6. ↑ "Encyclopædia Britannica". Encyclopædia Britannica Online. September 10, 2008. Retrieved July 14, 2009.
7. ↑ Lucey, Paul; Korotev, Randy L.; et al. (2006). "Understanding the lunar surface and space-Moon interactions". Reviews in Mineralogy and Geochemistry. 60 (1): 83–219. Bibcode:2006RvMG...60...83L. doi:10.2138/rmg.2006.60.2.
8. ↑ Dyches, Preston (28 July 2021). "Five Things to Know about the Moon - NASA Science". NASA. Retrieved 21 November 2023.
9. ↑ The acceleration due to gravity on the Moon is 1.62 m/s². This is approximately 1/6 of the acceleration due to gravity on Earth, which is 9.81 m/s².