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Moon

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

Moon Moon symbol
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 varying sizes of impact craters, circles surrounded by out-thrown rays of bright ejecta.
The near side of the Moon as viewed from Earth, fully illuminated by the Sun (north is at top)
Designations
Designation
Earth I
Adjectives
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
Mean radius
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)
Surface area
3.793×107 km2  
(0.074 of Earth's)
Volume2.1958×1010 km3  
(0.02 of Earth's)[4]
Mass7.342×1022 kg  
(0.0123 of Earth's)[1][4][6]
Mean density
3.344 g/cm3[1][4]
0.606 × Earth
1.622 m/s2  (0.1654 g; 5.318 ft/s2)[4]
Moment of inertia factor
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[14]
Surface equivalent dose rate57.0 μSv/h[14]
29.3 to 34.1 arcminutes[4][d]
Atmosphere[15]
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. At about one-quarter the diameter of Earth (comparable to the width of Australia),[16] it is the largest natural satellite in the Solar System relative to the size of its planet,[f] the fifth largest satellite in the Solar System overall, and is larger than any known dwarf planet. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under geophysical definitions of the term.[17] It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (0.1654 g); Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

Orbiting Earth at an average distance of 384,400 km (238,900 mi),[18] or about 30 times Earth's diameter, its gravitational influence slightly lengthens Earth's day and is the main driver of Earth's tides. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration.[19]

The most widely accepted origin explanation posits that the Moon formed about 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

Both the Moon's prominence in the earthly sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.

Name and etymology

See also: Moon § Mythology and art

The usual English proper name for Earth's natural satellite is simply the Moon, with a capital M.[20][21] The noun moon is derived from Old English mōna, which (like all its Germanic cognates) stems from Proto-Germanic *mēnōn,[22] which in turn comes from Proto-Indo-European *mēnsis "month"[23] (from earlier *mēnōt, genitive *mēneses) which may be related to the verb "measure" (of time).[24]

Occasionally, the name Luna /ˈluːnə/ is used in scientific writing[25] and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon.[26] Cynthia /ˈsɪnθiə/ is another poetic name, though rare, for the Moon personified as a goddess,[27] while Selene /səˈliːniː/ (literally "Moon") is the Greek goddess of the Moon.

The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. The adjective selenian /səliːniən/,[28] derived from the Greek word for the Moon, σελήνη selēnē, and used to describe the Moon as a world rather than as an object in the sky, is rare,[29] while its cognate selenic was originally a rare synonym[30] but now nearly always refers to the chemical element selenium.[31] The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography, the study of the physical features of the Moon, as well as the element name selenium.[32][33]

The Greek goddess of the wilderness and the hunt, Artemis, equated with the Roman Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia, from her legendary birthplace on Mount Cynthus.[34] These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits such as apolune, pericynthion and selenocentric.

Formation

Main articles: Origin of the Moon, Giant-impact hypothesis, and Circumplanetary disk

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System.[35][36] Historically, several formation mechanisms have been proposed,[37] but none satisfactorily explained the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force[38] would require too great an initial rotation rate of Earth.[39] Gravitational capture of a pre-formed Moon[40] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon.[39] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon.[39] None of these hypotheses can account for the high angular momentum of the Earth–Moon system.[41]

The evolution of the Moon and a tour of the Moon

The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The impact blasted material into orbit about the Earth and then the material accreted and formed the Moon[42][43] just beyond the Earth's Roche limit of ~2.56 R