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This page refers to Earth's moon. For other moons in the solar system, please see natural satellite. For other things named Moon see Moon (disambiguation).

Click image for description
Orbital characteristics
Mean radius 384,400 km
Eccentricity 0.0549
Revolution period 27 day 7 h 43.7 min
Inclination 5.1454°
Is a satellite of Earth
Physical characteristics
Equatorial diameter 3,474.8 km
Surface area 38 million km2
Mass 7.347673 × 1022 kg
Mean density 3.344 g/cm3
Equatorial gravity 1.622 m/s2,
or 0.165gee
Rotation period 27 day 7 h 43.7 min
Axial tilt 1.5424°
Albedo 0.12
Surface temp
min mean max
40 K 250 K 396 K
Atmospheric characteristics
Atmospheric pressure 3 × 10-13kPa
Helium 25%
Neon 25%
Hydrogen 23%
Argon 20%
Carbon dioxide
Crust composition
Oxygen 43%
Silicon 21%
Aluminium 10%
Calcium 9%
Iron 9%
Magnesium 5%
Titanium 2%
Nickel 0.6%
Sodium 0.3%
Chromium 0.2%
Potassium 0.1%
Manganese 0.1%
Sulfur 0.1%
Phosphorus 500 ppm
Carbon 100 ppm
Nitrogen 100 ppm
Hydrogen 50 ppm
Helium 20 ppm

The Moon is the only natural satellite of Earth. It has no formal name other than "The Moon" although it is occasionally called Luna (Latin for moon) to distinguish it from the generic "moon". The words moon and month come from the same Old English root word.

The Moon is 384,403 kilometers (238,857 miles) distant from the Earth. Its diameter is 3,476 kilometers (2,160 miles).

In 1969, Neil Armstrong and Buzz Aldrin became the first humans to land on the Moon.

Table of contents
1 The two sides
2 Orbit
3 Origin
4 Physical characteristics
5 Eclipses
6 Observation of the Moon
7 The exploration of the Moon
8 Human understanding of the moon
9 See also
10 External links

The two sides

The Moon has synchronous rotation. As a result, one side of the Moon (the "near side") is permanently turned towards Earth. The other side, the "far side", mostly cannot be seen from Earth, except for small portions near the limb which can be seen occasionally due to libration. Most of the far side was completely unknown until the era of space probes.

The far side is sometimes called the "dark side". In this case "dark" means "unknown and hidden" and not "lacking light"; in fact the far side receives as much sunlight as the near side, but at opposite times. Spacecraft are cut off from direct radio communication with the Earth when on the far side of the Moon.

The distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features.

Four orthographic views of the Moon
Near side
Far side


The Moon makes a complete orbit about once a month. Each hour the Moon moves in the sky by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most other satellites of other planets in that its orbit is close to the plane of the ecliptic and not in the Earth's equatorial plane.

The time it takes to make a complete orbit with respect to the stars is a sidereal month; the time it takes to reach the same phase is called a synodic month. These differ because in the meantime the Earth and Moon have both orbited some distance around the Sun.

Since the Moon's rotational period is exactly the same as its orbital period, we always see the same face of the Moon pointed towards Earth. This synchronous rotation is a result of friction having slowed down the Moon's rotation in its early history, a process known as tidal locking. As a result of tidal locking, Earth's rotation is also gradually being slowed down by the Moon, and the Moon is slowly receding from Earth as Earth's rotational momentum is transferred to the Moon's orbital momentum. The gravitional attraction that the Moon exerts on Earth is the cause of tides in the sea. Tidal flow is synchronised to the Moon's orbit around Earth.

Earth and Moon orbit about their barycenter, or common center of mass, which lies about 4700 km from Earth's center. Since the barycenter is located below the Earth's surface, Earth's motion is more commonly described as a "wobble". When viewed from Earth's North pole, Earth and Moon rotate counter clockwise about their axes; the Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise.

The Moon's orbital plane about Earth is inclined by 5 degrees with respect to Earth's orbital plane about the Sun (the ecliptic plane). The Moon's orbital plane along with its spin axis rotates clockwise with a period of 18.6 years, always maintaining the 5 degree inclination. The points where the Moon's orbit crosses the ecliptic are called the "lunar nodes": the North (or ascending) node is where the Moon crosses to the North of the ecliptic; the South (or descending ) node where it crosses to the South. Solar eclipses occur when a node coincides with the new Moon; lunar eclipses when a node coincides with the full Moon.


The inclination of the Moon's orbit makes it rather unlikely that the Moon formed along with Earth or was captured later; its origin is the subject of strong scientific debate. The most generally accepted theory states that the Moon originated from the collision between the young Earth and an impactor the size of Mars (sometimes called Theia) and was formed from material ejected from Earth as a result of the collision. This is called the Giant Impact theory. New simulations published in August 2001 support this theory.[1] This theory is also corroborated by the fact that the Moon has all the same minerals as Earth, albeit in different proportions.

The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history.

Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards Earth.

Physical characteristics


More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that one component of lunar rocks, KREEP (K-potassium, Rare Earth Elements, and P-phosphorus), represents the last chemical remnant of that magma ocean. KREEP is actually a composite of what scientists term "incompatible elements": those which cannot fit into a crystal structure and thus were left behind, floating to the surface of the magma. For researchers, KREEP is a convenient tracer, useful for reporting the story of the volcanic history of the lunar crust and chronicling the frequency of impacts by comets and other celestial bodies.

The lunar crust is composed of a variety of primary elements, including uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminum and hydrogen. When bombarded by cosmic rays, each element bounces back into space its own radiation, in the form of gamma rays. Some elements, such as uranium, thorium and potassium, are radioactive and emit gamma rays on their own. However, regardless of what causes them, gamma rays for each element are all different from one another — each produces a unique spectral "signature", detectable by a spectrometer.

A complete global mapping of the Moon for the abundance of these elements has never been performed. However, some spacecraft have done so for portions of the Moon; Galileo did so when it flew by the Moon in 1992. [1]

Surface geography

The Moon is covered with tens of thousands of craters having a diameter of at least 1 kilometre. Most are hundreds of millions or billions of years old; the lack of atmosphere or weather or recent geological processes ensures that most of them remain permanently preserved.

The largest crater on the Moon, and indeed the largest known crater within the solar system, forms the South Pole-Aitken basin. This crater is located on the far side, near the south pole, and is some 2,240 km in diameter, and 13 km in depth.

The dark and relatively featureless lunar plains are called maria, latin for seas, since they were believed by ancient astronomers to be water-filled seas. They are actually vast ancient basaltic lava flows that filled the basins of large impact craters. The lighter-colored highlands are called terrae. Maria are found almost exclusively on the Lunar nearside, with the Lunar farside having only a few scattered patches. Scientists think that such asymmetry of the lunar crust most likely accounts for the Moon's off-set center of mass. Crustal asymmetry may also explain differences in lunar terrain, such as the dominance of smooth rock (maria) on the near side of the Moon.

Blanketed atop the Moon's crust is a dusty outer rock layer called regolith. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 38 miles (60 km) on the near side to 63 miles (100 km) on the far side. The regolith varies from 10 to 16 feet (3 to 5 meters) in the maria to 33 to 66 feet (10 to 20 meters) in the highlands.

In 2004, a team led by Dr. Ben Bussey of Johns Hopkins University using images taken by the Clementine mission determined that four mountainous regions on the rim of the 73 km wide Peary crater at the Moon's north pole appeared to remain illuminated for the entire Lunar day. These unnamed "mountains of eternal light" are possible due to the Moon's extremely small axial tilt, which also gives rise to permanent shadow at the bottoms of many polar craters. No similar regions of eternal light exist at the less-mountainous south pole. Clementine's images were taken during the northern Lunar hemisphere's summer season, and it remains unknown whether these four mountains are shaded at any point during their local winter season.

Presence of water

Over time, comets and meteorites continually bombard the Moon. Many of these objects are water-rich. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. However, it has been hypothesized that significant traces of water remain on the moon, either on the surface, or embedded within the crust. The results of the Clementine mission suggested that small, frozen pockets of water ice (remnants of water-rich comet impacts) may be embedded unmelted in the permanently shadowed regions of the lunar crust. Although the pockets are thought to be small, the overall amount of water was suggested to be quite significant — 1 km³, or an amount the size of Lake Erie.

Some water molecules, however, may have literally hopped along the surface and gotten trapped inside craters at the lunar poles. Due to the very slight "tilt" of the Moon's axis, only 1.5°, some of these deep craters never receive any light from the Sun — they are permanently shadowed. Clementine has mapped ([1]) craters at the lunar south pole ([1]) which are shadowed in this way. It is in such craters that scientists expect to find frozen water if it is there at all. If found, water ice could be mined and then split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator. The presence of usable quantities of water on the Moon would be 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. Neither the Lunar Prospector nor more recent surveys, such as those of the Smithsonian Institution, have found direct evidence of lunar water, ice, or water vapour. Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.

Magnetic field

Compared to that of Earth, the Moon has a very weak magnetic field. While some of the Moon's magnetism is thought to be intrinsic (such as a strip of the lunar crust called the Rima Sirsalis), collision with other celestial bodies might have imparted some of the Moon's magnetic properties. Indeed, a long-standing question in planetary science is whether an airless solar system body, such as the Moon, can obtain magnetism from impact processes such as comets and asteroids. Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core — evidence that will help scientists better understand the Moon's origins. For instance, if the core contains more magnetic elements (such as iron) than Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).


The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing — the release of gases, for instance radon, which originate deep within the Moon's interior. Another important source of gases is the solar wind, which is briefly captured by the Moon's gravity.


By what can only be a truly extraordinary coincidence, the angular diameter of the Moon as seen from Earth is almost exactly the same as the angular diameter of the Sun, so that total solar eclipses are possible where the Moon almost completely covers the Sun and the solar corona becomes visible to the naked eye.

Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that several million years ago the Moon always completely covered the Sun on solar eclipses so that no annular eclipses occurred. Likewise, in several million years the Moon will no longer cover the Sun completely and no non-annular eclipses will occur.

Eclipses happen only if Sun, Earth and Moon are lined up. Solar eclipses can only occur at new moon; lunar eclipses can only occur at full moon.

See also Solar eclipse and Lunar Eclipse.

Observation of the Moon

The Moon (and also the Sun) appear larger when close to the horizon. This is a purely psychological effect (atmospheric refraction and its larger distance actually causes the image of the Moon near the horizon to be slightly smaller); it is assumed that size judgments for overhead objects were not important during evolution of the cognitive apparatus and are therefore inaccurate. [1] The angular diameter of the Moon from Earth is about one half of one degree.

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. Craters and mountain chains are also prominent lunar features.

During the brightest full moons, the Moon can have an apparent magnitude of about -12.6. For comparison, the Sun has an apparent magnitude of -26.8.

The moon is most clear at night, but can sometimes be seen during the day.

See also: Lunar phase.

The exploration of the Moon

lunar module prepares to descend towards the surface of the Moon]]

astronaut Harrison Schmitt standing next to boulder at Taurus-Littrow during third EVA]]

The first man-made object to reach the Moon was the unmanned Soviet probe Luna 2, which crashed into it on September 13, 1959, at 21:02:24 Z. The far side of the Moon was first seen on October 7, 1959, when the Soviet probe Luna 3 had photographed it. Luna 9 was the first probe to soft land on the Moon and transmit pictures from the Lunar surface on February 3, 1966. The first artificial satellite of the Moon was the Soviet probe Luna 10 (launched March 31, 1966).

Humans first landed on the Moon on July 20, 1969 as the culmination of a Cold War-inspired space race between the Soviet Union and the United States of America. The first astronaut on the Moon was Neil Armstrong, commander of the American mission Apollo 11. The last man to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972. See also: A full list of lunar astronauts.

The Apollo 11 crew left a 9 by 7 inch stainless steel plaque on the moon, to commemorate the landing and provide basic information of the visit to any other beings who may eventually see it. The plaque reads:

Here men from the Planet Earth first set foot upon the moon, July 1969, A.D.
We came in peace for all mankind.'

The plaque depicts the two sides of planet Earth, and is signed by the three astronauts, as well as US President Richard Nixon.

In February 2004, US President George W. Bush called for a plan to return manned missions to the Moon by 2020. The European Space Agency and People's Republic of China both have plans to launch probes to explore the Moon in the near future, too. European spacecraft Smart 1 was launched September 27, 2003 and is expected to reach lunar orbit in early 2005. It will survey the lunar environment and create an X-ray map of the Moon. [1] China has expressed ambitious plans for exploring the Moon and is investigating the prospect of lunar mining, specifically looking for the isotope Helium-3 for use as an energy source on Earth. [1] For more information about China's first Moon mission, see Chang'e program. Japan and India are on the waiting list for the Moon, too. Japan already outlined its upcoming missions to our neighbour Lunar-A (http://www.jaxa.jp/missions/projects/sat/exploration/lunar_a/index_e.html) and Selene (http://www.jaxa.jp/missions/projects/sat/exploration/selene/index_e.html). Even a manned lunar base is planned by the Japanese Space Agency (Jaxa).

In 2001 Philippe Lheureux published his claim that photographs taken by NASA astronauts on the Moon were actually faked on Earth. In that same year, the Fox Network aired a special entitled "Conspiracy Theory: Did We Land on the Moon?". Apollo moon landing hoax accusations such as these have been addressed by many astronomers, most notably Phil Plait. In September 2002, Buzz Aldrin also memorably and very effectively answered the Moon Hoax claims of Bart Sibrel.

Human understanding of the moon

Myth and folk culture

ancient times, it was not uncommon for cultures to believe that the Moon died each night, thus descending into the underworld; other cultures believed that the Moon chased the Sun (and vice-versa).

The Moon has figured prominently in various mythologies and folk beliefs. The numerous lunar deities are often female such as the Greek goddesses Selene and Artemis, their Roman equivalents Luna and Diana or the Thracian Bendis. However males are also found, such as Nanna or Sin of the Mesopotamians, Thoth of the Egyptians and the Japanese god Susanowo, and Tecciztecatl of the Aztecs, along with Isil and the guidesman Tirion in J. R. R. Tolkien's fictional Middle-earth cosmology.

The words lunacy, lunatic, and loony are derived from Luna because of the folk belief in the Moon as a cause of periodic insanity. Folklore also stated that shapeshifters such as werewolves and weretigers, mythical creatures capable of changing form between human and beast, drew their power from the Moon and would change into their bestial form during the full Moon.

John Heywood's Proverbes (1546) commented that "The moon is made of a greene cheese", but was probably being sarcastic. [1]

Scientific understanding

By the medieval period, before the invention of the telescope, some believed that the Moon was a "perfectly smooth" sphere.

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 craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today.

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. The possibility that the Moon could contain vegetation and be inhabited by "selenites" was seriously considered by some major astronomers even into the first decades of the 19th century.

In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon. Almost at the same time however (during 18341836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.

There remained some controversy over whether features on the Moon could undergo changes. Some observers claimed that some small craters had appeared or disappeared, but in the 20th century it was determined that these claims were illusory, due to observing under different lighting conditions or due to the inadequacy of earlier drawings. It is however known that the phenomenon of outgassing occasionally occurs.

During the Nazi era in Germany, the bizarre 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 in 1959, and was extensively mapped by the Lunar Orbiter program in the 1960s.

See also

External links

Space Missions


Myth and folklore


The Solar System
Sun | Mercury | Venus | Earth | Moon | Mars | Asteroids | Jupiter | Saturn | Uranus | Neptune | Pluto
(For other objects and regions, see: List of solar system objects, Astronomical objects)