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Moon

This article is about Earth's natural satellite. For moons in general, see Natural satellite. For other uses, see Moon (disambiguation).

Moon
Full Moon in the darkness of the night sky. It is patterned with a mix of light-tone regions and darker, irregular blotches, and scattered with varied circles surrounded by out-thrown rays of bright ejecta: impact craters.
Near side of the Moon, lunar north pole at top
Designations
Designation
Earth I
Adjectives
Symbol☾ or ☽
Orbital characteristics
Epoch J2000
Perigee362600 km
(356400–370400 km)
Apogee405400 km
(404000–406700 km)
384399 km  (1.28 ls, 0.00257 AU)[1]
Eccentricity0.0549[1]
27.321661 d
(27 d 7 h 43 min 11.5 s[1])
29.530589 d
(29 d 12 h 44 min 2.9 s)
1.022 km/s
Inclination5.145° to the ecliptic[2][a]
Regressing by one revolution in 18.61 years
Progressing by one
revolution in 8.85 years
Satellite ofEarth[b][3]
Physical characteristics
1737.4 km  
(0.2727 of Earth's)[1][4][5]
Equatorial radius
1738.1 km  
(0.2725 of Earth's)[4]
Polar radius
1736.0 km  
(0.2731 of Earth's)[4]
Flattening0.0012[4]
Circumference10921 km  (equatorial)
3.793×107 km2  
(0.074 of Earth's)
Volume2.1958×1010 km3  
(0.02 of Earth's)[4]
Mass7.346×1022 kg  
(0.0123 of Earth's)[1][4][6]
Mean density
3.344 g/cm3[1][4]
0.606 × Earth
1.622 m/s2 (5.32 ft/s2)[4]
0.1654 g0
0.3929±0.0009[7]
2.38 km/s
(8600 km/h; 5300 mph)
29.530589 d
(29 d 12 h 44 min 2.9 s; synodic; solar day) (spin-orbit locked)
27.321661 d  (spin-orbit locked)
Equatorial rotation velocity
4.627 m/s
North pole right ascension
  • 17h 47m 26s
  • 266.86°[10]
North pole declination
65.64°[10]
Albedo0.136[11]
Surface temp. min mean max
Equator 100 K[12] 250 K 390 K[12]
85°N  150 K 230 K[13]
Surface absorbed dose rate13.2 μGy/h
(during lunar daytime)[14]
Surface equivalent dose rate57.0 μSv/h
(during lunar daytime)[14]
0.2[15]
29.3 to 34.1 arcminutes[4][d]
Atmosphere[16]
Surface pressure
  • 10−7 Pa (1 picobar)  (day)
  • 10−10 Pa (1 femtobar)   
    (night)[e]
Composition by volume

The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period (lunar month) with its rotation period (lunar day) at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun's—are the main drivers of Earth's tides.

In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the United States from coast to coast). Within the Solar System, it is the largest and most massive satellite in relation to its parent planet, the fifth largest and most massive moon overall, and larger and more massive than all known dwarf planets.[17] Its surface gravity is about one sixth of Earth's, about half of that of Mars, and the second highest among all Solar System moons, after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia.

The lunar surface is covered in lunar dust and marked by mountains, impact craters, their ejecta, ray-like streaks, rilles and, mostly on the near side of the Moon, by dark maria ("seas"), which are plains of cooled magma. These maria were formed when molten lava flowed into ancient impact basins. The Moon is, except when passing through Earth's shadow during a lunar eclipse, always illuminated by the Sun, but from Earth the visible illumination shifts during its orbit, producing the lunar phases.[18] The Moon is the brightest celestial object in Earth's night sky. This is mainly due to its large angular diameter, while the reflectance of the lunar surface is comparable to that of asphalt. The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun completely during a total solar eclipse. From Earth about 59% of the lunar surface is visible over time due to cyclical shifts in perspective (libration), making parts of the far side of the Moon visible.

The Moon has been an important source of inspiration and knowledge for humans, having been crucial to cosmography, mythology, religion, art, time keeping, natural science, and spaceflight. In 1959, the first human-made objects to leave Earth and reach another body arrived at the Moon, with the flyby of the Soviet Union's Luna 1 and the intentional impact of Luna 2. In 1966, the Moon became the first extraterrestrial body with a soft landing by Luna 9 and a orbital insertion by Luna 10 were achieved. On July 20, 1969, humans for the first time landed on the Moon and any extraterrestrial body, at Mare Tranquillitatis with the lander Eagle of the United States' Apollo 11 mission. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the Apollo 17 crew. Since then, exploration of the Moon has continued robotically, and crewed missions are being planned to return beginning in the late 2020s.

  1. ^ a b c d e f g Cite error: The named reference W06 was invoked but never defined (see the help page).
  2. ^ a b Cite error: The named reference Lang2011 was invoked but never defined (see the help page).
  3. ^ Cite error: The named reference Morais2002 was invoked but never defined (see the help page).
  4. ^ a b c d e f g h i j Cite error: The named reference NSSDC was invoked but never defined (see the help page).
  5. ^ Smith, David E.; Zuber, Maria T.; Neumann, Gregory A.; Lemoine, Frank G. (January 1, 1997). "Topography of the Moon from the Clementine lidar". Journal of Geophysical Research. 102 (E1): 1601. Bibcode:1997JGR...102.1591S. doi:10.1029/96JE02940. hdl:2060/19980018849. ISSN 0148-0227. S2CID 17475023.
  6. ^ Terry, Paul (2013). Top 10 of Everything. Octopus Publishing Group Ltd. p. 226. ISBN 978-0-600-62887-3.
  7. ^ Cite error: The named reference Williams1996 was invoked but never defined (see the help page).
  8. ^ Cite error: The named reference SolarViews was invoked but never defined (see the help page).
  9. ^ Makemson, Maud W. (1971). "Determination of selenographic positions". The Moon. 2 (3): 293–308. Bibcode:1971Moon....2..293M. doi:10.1007/BF00561882. S2CID 119603394.
  10. ^ a b Archinal, Brent A.; A'Hearn, Michael F.; Bowell, Edward G.; Conrad, Albert R.; Consolmagno, Guy J.; Courtin, Régis; Fukushima, Toshio; Hestroffer, Daniel; Hilton, James L.; Krasinsky, George A.; Neumann, Gregory A.; Oberst, Jürgen; Seidelmann, P. Kenneth; Stooke, Philip J.; Tholen, David J.; Thomas, Paul C.; Williams, Iwan P. (2010). "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009" (PDF). Celestial Mechanics and Dynamical Astronomy. 109 (2): 101–135. Bibcode:2011CeMDA.109..101A. doi:10.1007/s10569-010-9320-4. S2CID 189842666. Archived from the original (PDF) on March 4, 2016. Retrieved September 24, 2018. also available "via usgs.gov" (PDF). Archived (PDF) from the original on April 27, 2019. Retrieved September 26, 2018.
  11. ^ Cite error: The named reference Saari was invoked but never defined (see the help page).
  12. ^ a b Bugby, D. C.; Farmer, J. T.; O’Connor, B. F.; Wirzburger, M. J.; C. J. Stouffer, E. D. Abel (January 2010). Two-Phase Thermal Switching System for a Small, Extended Duration Lunar Surface Science Platform. AIP Conference Proceedings. Vol. 1208. pp. 76–83. Bibcode:2010AIPC.1208...76B. doi:10.1063/1.3326291. hdl:2060/20100009810.
  13. ^ Cite error: The named reference Vasavada1999 was invoked but never defined (see the help page).
  14. ^ a b Zhang S, Wimmer-Schweingruber RF, Yu J, Wang C, Fu Q, Zou Y, et al. (2020). "First measurements of the radiation dose on the lunar surface". Science Advances. 6 (39). Bibcode:2020SciA....6.1334Z. doi:10.1126/sciadv.aaz1334. PMC 7518862. PMID 32978156. We measured an average total absorbed dose rate in silicon of 13.2 ± 1 μGy/hour ... LND measured an average dose equivalent of 1369 μSv/day on the surface of the Moon
  15. ^ "Encyclopedia - the brightest bodies". IMCCE. Archived from the original on March 21, 2023. Retrieved June 1, 2023.
  16. ^ Cite error: The named reference L06 was invoked but never defined (see the help page).
  17. ^ Cite error: The named reference Metzger2021 was invoked but never defined (see the help page).
  18. ^ "Is the 'full moon' merely a fallacy?". NBC News. February 28, 2004. Archived from the original on June 1, 2023. Retrieved May 30, 2023.


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