Moon - The two sides of the Moon, Orbit and relationship to Earth, Origin and geologic evolution

The Earth's only natural satellite, lacking any atmosphere; about a quarter the size of the Earth, and treated as one of the family of terrestrial planets. It has the following characteristics: mass 0·073 × 10²⁷ g; radius 1738 km/1080 mi; mean density 3·34 g/cm³; equatorial gravity 162 cm/s; rotational period 27·3 days; orbital period 27·3 days; average distance from Earth 384 400 km/238 850 mi. Overall density is low, because of a major iron deficiency. Apollo seismic measurements indicate that the interior is solid to a depth of c.1000 km/600 mi; the nature of the core is still uncertain. There is no global magnetic field, but evidence of past magnetization is contained in individual rock samples. The equality of rotational and orbital rates is due to tidal despinning of the Moon into a stable synchronous period, so that the same hemisphere of the Moon always faces the Earth.

The brighter surface regions (highlands) represent the original lunar crustal material shaped by saturation bombardment of meteoritic material. The dark surface regions (mare regions), located mainly on the side observable from Earth, represent basaltic (volcanic) flooding of basins created by major asteroidal impacts. Apollo and Luna sample isotopic dating places mare basalts in the range of 3–4 thousand million years, in contrast to 4·2–4·5 thousand million for highland samples. Lunar evolution models based on Lunar Orbiter mapping of the Moon and on Apollo and Luna sample analyses suggest five principal episodes: accretion and large-scale melting; crustal separation and concurrent massive meteoritic bombardment; partial melting at depth; diminished bombardment with further melting at depth and emplacement of mare basalts; and cessation of volcanism and gradual internal cooling. The popular current theory for the creation of the Moon involves the impact of a Mars-sized object with the Earth, occasioning a catastrophic disruption of the Earth, and the accretion of the Moon in Earth orbit from debris torn from the Earth's mantle.

Moon

The Moon as seen from Earth
Orbital characteristics
Semi-major axis	384,399 km (0.00257 AU)
Perigee	363,104 km (0.0024 AU)
Apogee	405,696 km (0.0027 AU)
Orbital circumference	2,413,402 km (0.016 AU)
Eccentricity	0.0549
Revolution period (Sidereal period)	27.321582 d (27 d 7 h 43.1 min)
Synodic period	29.530588 d (29 d 12 h 44.0 min)
Avg. Orbital Speed	0.968 km/s
Inclination	between 18.29° and 28.58° to Earth equatorial plane; 5.145° to ecliptic
Longitude of the ascending node	regressing, 1 revolution in 18.6 years
Argument of perigee	progressing, 1 revolution in 8.85 years
Satellite of	Earth
Physical characteristics
Mean diameter Mean radius	3,474.128 km (0.273 Earths) 1,737.064 km (0.273 Earths)
Equatorial diameter Equatorial radius	3,476.278 km (0.273 Earths) 1,738.139 km (0.273 Earths)
Polar diameter Polar radius	3,471.944 km (0.273 Earths) 1,735.972 km (0.273 Earths)
Oblateness	0.00125
Surface area	3.793×107 km² (0.074 Earths)
Volume	2.1958×1010 km³ (0.020 Earths)
Mass	7.3477×1022 kg (0.0123 Earths)
Mean density	3,346.4 kg/m3
Equatorial gravity	1.622 m/s2 (0.1654 g)
Escape velocity	2.38 km/s
Rotation period	27.321582 d (synchronous)
Rotation velocity	16.655 km/h (at the equator)
Axial tilt	6.688° to orbit plane 1.543° to ecliptic
Albedo	0.12
Magnitude	-12.74
Angular size	from 29′to 33′
Surface temp.	min mean max 40 K 250 K 396 K
Bulk silicate composition (estimated wt%)
SiO2	44.4 %
Al2O3	6.14 %
FeO	10.9 %
MgO	32.7 %
CaO	4.6 %
Na2O	0.092 %
K2O	0.01 %
Cr2O3	0.61 %
MnO	0.15 %
TiO2	0.31 %
Atmospheric characteristics
Atmospheric pressure	10 (day) 10 (night)
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The Moon is Earth's only natural satellite. It has no formal English name other than "the Moon", although it is occasionally called Luna (Latin: moon), or Selene (Greek: moon), to distinguish it from the generic term "moon" (referring to any of the various natural satellites of other planets). The related adjective for the Moon is lunar (again from the Latin root), but this is not found in combination, the combining forms seleno-/-selene (again from the Greek) and -cynthion (from the Lunar deity Cynthia) being used in terms relating to the Moon in various other contexts (e.g.

The average distance from the Moon to the Earth is 384,401 kilometres (238,857 mi). The Moon is the Solar System's fifth largest moon, both by diameter and mass, ranking behind Ganymede, Titan, Callisto, and Io.

The Soviet Union's (USSR) Luna program was the first to reach the Moon with unmanned vehicles, or space probes: The first man-made object to escape Earth's gravity and pass near the Moon was Luna 1 in 1959.

The United States' Apollo program achieved the first (and only) manned missions to the Moon: The first manned mission to orbit the Moon was Apollo 8 in 1968, and the first people to land and walk on the Moon came aboard Apollo 11 in 1969.

The two sides of the Moon

The Moon is in synchronous rotation, meaning that it keeps nearly the same face turned toward Earth at all times. The side of the Moon that faces Earth is called the near side, and the opposite side is called the far side. The far side should not be confused with the dark side, as the unilluminated hemisphere only corresponds to the far side during full moon.

90° W	Near side
90° E	Far side

Orbit and relationship to Earth

See also: Libration

The Moon makes a complete orbit about the Earth approximately once every 27.3 days.

The Earth and Moon have many physical effects upon one another, including the tides. Most of the tidal effects seen on the Earth are caused by the Moon's gravitational pull, with a smaller contribution from the Sun.

The surface of the Moon is 1/4 the size of Earth's land area, about as large as Russia, Canada, and the United States combined.

In 1997 the asteroid 3753 Cruithne was found to have an unusual Earth-associated horseshoe orbit, and has been dubbed by some to be a second moon of Earth.

Origin and geologic evolution

Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal forces, leaving an ocean basin (presumed to be the Pacific Ocean) behind as a scar. Others speculated that the Moon formed elsewhere and was captured into Earth's orbit, but the conditions required for this to have occured are not too probable.

The coformation or condensation hypothesis posits that the Earth and the Moon formed together at the same time from the primordial accretion disk.

The high angular momentum of the Earth-Moon system makes it implausible that the Moon formed along with the Earth or was captured later.

The formation of the Moon is believed to have occurred at 4.527 ± 0.01 billion years, which would imply that it formed only 30 to 50 million years after the origin of the solar system.

Most of the Moon's mare basalts erupted during the Imbrian period, around 3 to 3.5 billion years ago.

Physical characteristics

Composition

It is held that more than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. The Moon has a mean density of 3,346.2 kg/m³, making it the second densest moon in the solar system, after Io.

A complete global mapping of the Moon for the abundance of major and minor elements has not yet been performed.

Landscape

When observed with Earth based telescopes, the Moon can be seen to have some 30,000 craters having a diameter of at least 1 km, but close up observation from lunar orbit reveals a multitude of ever smaller craters.

Blanketed atop the Moon's crust is a highly comminuted and "impact gardened" surficial layer called regolith.

Using images taken by the Clementine mission, it appears that four mountainous regions on the rim of the 73 km-wide Peary crater at the Moon's north pole remain illuminated for the entire lunar day.

Gravity field

See also: Mascon

The gravitational field of the Moon has been determined by the tracking of radio signals emitted by orbiting spacecraft. Unforntunately, because of the Moon's synchronous rotation it is not possible to track spacecraft much over the limbs of the Moon.

The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant impact basins.

Magnetic field

Compared to that of Earth, the Moon has only a very weak external magnetic field.

Presence of water

Over time, comets and meteoroids continuously bombard the Moon. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive.

The equatorial Moon rock collected by Apollo astronauts contained no traces of water.

Atmosphere

The Moon has a relatively insignificant and tenuous atmosphere.

Eclipses

See also: Occultation

Eclipses happen only if Sun, Earth, and Moon are all in a straight line. Solar eclipses can only occur near a new moon, whereas lunar eclipses can only occur near a full moon. The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible.

Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing.

Observation of the Moon

See also: Lunar phase, New moon, Full moon, Earthshine, Moonlight, and Halo (optical phenomenon)

During the brightest full moons, the Moon can have an apparent magnitude of about −12.6. When the Moon is in a quarter phase, its brightness is not one half of a full Moon.

The Moon appears larger when close to the horizon. The angular diameter of the Moon from Earth is about one half of one degree, and is actually about 1.5% smaller when the Moon is near the horizon than when it is high in the sky (because it is farther away by up to 1 Earth radius).

Another quirk of the visual system causes us to see the Moon as almost pure white, when in fact it reflects only about 7% of the light falling on it (about as dark as a lump of coal). however, there is nothing next to the Moon to reflect the light falling on the Moon, therefore it is perceived as the brightest object visible.

Various lighter and darker colored areas (primarily maria) create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, amongst others.

The highest altitude of the Moon on a day varies and has the same limits as the Sun. This means that in the winter the Moon is highest in the sky when it is full, and the full moon is highest in winter.

Like the Sun, the Moon can also give rise to the atmospheric effects including a 22 degree halo ring and the smaller coronal rings seen more often through thin clouds. For more information on how the Moon appears in Earth's sky, see Lunar phase.

Exploration of the Moon

See also: Project Apollo, Robotic exploration of the Moon, and Future lunar missions

The first leap in lunar observation was caused by the invention of the telescope.

The Cold War-inspired space race between the Soviet Union and the United States of America led to an acceleration of interest in the Moon. Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on July 21, 1969. The last person (as of 2006) to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972. Moon samples have been brought back to Earth by three Luna missions (Luna 16, 20, and 24) and the Apollo missions 11 through 17 (excepting Apollo 13, which aborted its planned lunar landing).

From the mid-1960s to the mid-1970s, there were a total of 65 Moon landings (both manned and robotic, with 10 in 1971 alone), but after Luna 24 in 1976 they stopped. In 1990 Japan orbited the Moon with the Hiten spacecraft, becoming the third country to place a spacecraft into lunar orbit.

In 1994, the US finally returned to the Moon, robotically at least, sending the Joint Defense Department/NASA spacecraft Clementine. This mission obtained the first near global topographic map of the Moon, as well as the first global multispectral images of the lunar surface. The People's Republic of China has expressed ambitious plans for exploring the Moon and has started the Chang'e program for lunar exploration.

Human understanding of the Moon

See also: Moon in art and literature and Lunar effect

The Moon has been the subject of many works of art and literature and the inspiration for countless others.

In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated today. Among the first in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras, who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former.

By the Middle Ages, before the invention of the telescope, more and more people began to recognize the Moon as a sphere, though they believed that it was "perfectly smooth".

In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters.

On maps, the dark parts of the Moon's surface were called maria (singular mare) or seas, and the light parts were called terrae or continents.

In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon.

There remained some controversy over whether features on the Moon could undergo changes.

During the Nazi era in Germany, the Welteislehre theory, which claimed the Moon was made of solid ice, was promoted by Nazi leaders.

The far side of the Moon remained completely unknown until the Luna 3 probe was launched in 1959, and was extensively mapped by the Lunar Orbiter program in the 1960s.

Legal status

See also: Space law

Though several flags of the Soviet Union and the United States have been symbolically planted on the Moon, the Russian and U.S. governments make no claims to any part of the Moon's surface. A second treaty, the Moon Treaty, was proposed to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations.

Several individuals have made claims to the Moon in whole or in part, though none of these claims are generally considered credible (see Extraterrestrial real estate).

Moon phases

Full Moon Names: U.S. Naval Observatory: phase of the Moon for any date and time 1800-2199 A.D. Current Moon Phase Display current moon phase as wallpaper in Windows

Space missions

The Apollo Lunar Surface Journal (NASA) — Definitive history of Apollo lunar exploration programme. Assembled Panoramas from the Apollo Missions Digital Lunar Orbiter Photographic Atlas of the Moon The Project Apollo Archive Clementine Lunar Image Browser

Scientific

The Moon – by Rosanna and Calvin Hamilton The Moon – by Bill Arnett Inconstant Moon – by Kevin Clarke

The Moon Society (non-profit educational site) Origin of the Moon - computer model of accretion subsequent to computer model of collision

Myth and folklore

Do things get crazy when the moon is full? by Cecil Adams Once in a Blue Moon – What is a blue moon? by Ann-Marie Imbornoni

The Moon In Folklore – by Virginia Marin The Rabbit in the Moon – by John Hardy

Others

USGS Planetary GIS webserver – the Moon Why does the Moon appear bigger near the horizon? The Lunar Navigator: Interactive Maps Of The Moon features free, interactive online access to maps of the Moon's surface A comprehensive guide to the Earth's Moon – moonpeople.com (Includes a discussion forum) Distance from the Earth to the Moon, illustrated 3D VRML Moon globe 3D maps of Moon in NASA World Wind Google Moon A view of the moon, including a reference to the myth that the moon is made of cheese.

The Moon Earths nearest celestial neighbor presented by Utah Skies The Two Sides of the Moon An ABC Science online feature: Geoscientific debate about the origins of the Moon Lunar Photo of the Day Lunar scientist Charles A. Wood's lunar counterpart to the Astronomy Picture of the Day Corkscrew Asteroids (PhysOrg.com), Asteroid 2003 YN107 as Earth's "second moon" Space.com: All About the Moon Moon Reference and News Geody Moon World's search engine that supports NASA World Wind, Celestia, and other applications.