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This page refers to Earth's Moon. For other moons in the solar system, please see natural satellite. See also: the Luna program of unmanned space missions.


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Orbital characteristics
Mean radius384,400 km
Revolution period27d 7h 43.7m
Is a satellite ofEarth
Physical characteristics
Equatorial diameter3,474.8 km
Surface area38 million[?] km2
Mass7.349 × 1022 kg
Mean density3.34 g/cm3
Surface gravity1.62 m/s2
Rotation period27d 7h 43.7m
Axial tilt1.5424°
Surface temp.
K250 KK
Atmospheric characteristics
Atmospheric pressure3 × 10-13kPa
Carbon dioxide
Crust composition

The Moon is the largest satellite of the Earth, and is occasionally called Luna (Latin for moon) to distinguish it from the more general "moon". The words moon, and month, come from the same Old English root word.

The color image of the Moon to the right was taken by the Galileo spacecraft at 9:35 a.m. PST December 9, 1990, at a range of about 350,000 miles. (click here for full-sized image)

The side of Luna that faces away from the Earth is properly called "the farside". It is sometimes referred to as the "dark side" of the Moon. In this case "dark" means unknown and hidden; it also refers to the "communications black out" which occurs as a spacecraft travels on the far side; this black out is a result of the moon's mass blocking radio signals. This term, "dark side", is often erroneously interperted as referring to a lack of solar radiation. Sol can be seen from the far side. Most of the far side cannot be seen, from Terra, because the planet and its moon have a synchronic relationship; a small portion of the far side can be seen, from Earth, due to libration[?].

The near side, of Luna, is covered with ~30,000 craters having a diameter of at least 1 kilometer. The largest crater on Luna, and indeed the largest known crater within the solar system, forms the South Pole-Aitken basin. This crater is located on the farside, near the south pole, and is some 2,240km in diameter, and 13km in depth.

Table of contents

The Moon and the Celestial Sphere The Moon makes a complete orbit of the celestial sphere about every four weeks. Each hour the moon moves in the sky a distance close to its perceived angular size, or by about 0.5º. The Moon always remains within a path, called the Zodiac, which extends about 8º on either side of the ecliptic. Luna crosses the ecliptic about once every 2 weeks.

Brief History of Lunar Understanding During the ancient period, it was not uncommon for cultures to believe that Luna died each night, thus descending into the underworld; other cultures believed that the moon chased Sol (and vice-versa). By the medieval period, some believed that Luna was a "perfectly smooth" sphere; and some believed that there were oceans there (see: maria). As late as the 1920s (or so), it was believed that Luna might have a breatheable atmosphere (or so science-fiction, of the period, seems to indicate). In 1969, the first humans claimed to have landed on the moon.

Physical characteristics

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 the Earth. This synchronicity 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, the Earth's rotation is also gradually being slowed down by the Moon, and the Moon is slowly receding from the Earth as the Earth's rotational momentum is transferred to the Moon's orbital momentum. The gravitional attraction that the Moon exerts on the Earth is the cause of tides in the sea. Tidal flow is synchronised to the Moon's orbit around the Earth.

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

The Moon's orbital plane about the Earth is inclined by 5 degrees with respect to the 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 the Earth or was captured later; its origin is the subject of strong scientific debate. The most 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 (http://physicsweb.org/article/news/5/8/13) published in August 2001 support this theory . This theory is also corroborated by the fact that the Moon has all the same minerals as the 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 the Earth.


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.

Moon craters (magnify)
Over time, comets and meteorites continually bombard the Moon. Water-rich meteorites and comets, largely water ice, may leave significant traces of water on the lunar surface. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. 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. 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. Such components could make space operations as well as human colonization on the Moon possible. Although the equatorial Moon rock collected by Apollo astronauts contained no traces of water, the recent 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 - one billion cubic meters, or an amount the size of Lake Erie. 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.

Compared to that of the Earth, the Moon has a very small 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 the Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).

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. 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.

The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgasing - 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.

The Moon in myth 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, along with Isil[?] in J. R. R. Tolkien's invented Middle-earth mythology.

The term lunacy[?] is derived from Luna because of the folk belief in the Moon as a cause of periodic insanity. Folklore also stated that lycanthropes 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.

Observation of the Moon By what can only be a truly extraordinary coincidence, the apparent size of the Moon as seen from the Earth is almost exactly the same as the apparent size 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[?].

Moon surface (magnify)
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] (http://www.lhup.edu/~dsimanek/3d/moonillu.htm)

Various lighter and darker colored areas 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. 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. The lighter-colored highlands are called terrae.

See also: Lunar phase.

The exploration of the Moon

Apollo 17 astronaut Harrison Schmitt standing next to boulder at Taurus-Littrow during third EVA. (magnify)
The far side of the Moon was first seen on September 15, 1959 when the unmanned Soviet probe Luna 2 was launched into an orbit over it.

Humans first landed on the Moon on July 20, 1969 as the culmination of a Cold War-inspired space race between Russia and America. The first astronaut on the Moon was Neil Armstrong, captain of Apollo 11. The last man to stand on the Moon was scientist Harrison Schmitt, who as part of Apollo 17 walked on the Moon in December 1972.

The European Space Agency and China both have plans to launch probes to explore the Moon in the near future. European spacecraft Smart 1 is scheduled for liftoff in mid-July 2003. The probe will be testing a new form of propulsion, an ion thruster. It will survey the lunar environment and create an X-ray map of the Moon. [2] (http://news.bbc.co.uk/2/hi/science/nature/2818551.stm) 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. [3] (http://space.com/missionlaunches/china_moon_030304)

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