Our nearest neighbor in space, the Moon is the only natural satellite of the Earth. Apart from the Sun, it is the brightest object in the sky. It is 384,400 kilometers from Earth and has a diameter of 3,476 kilometers. Great plains stretch over the moon's surface, dotted with huge mountains and scarred by numerous craters.

    The Moon, of course, has been known since prehistoric times. The moon was called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies.

    As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes and we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (as measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

It has the following characteristics:

Mass 0.073 � 1027 g;

Radius (equatorial) 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.

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.

The brighter surface regions (highlands) represent the original lunar crustal material shaped by saturation bombardment of meteoritic material. The brighter, densely cratered, mountainous highland regions (sometimes called terrae), are found mainly on the southern part of the Moon's near side, and over the entire far side.

Much of the Moon's surface is covered with craters. These are the result of impacts by meteors. The largest are about 200 km in diameter, the smallest are only about a metre across. Most of these craters were formed between 3000 and 4000 million years ago. Many of the craters were made by meteoric impacts. Some of the craters are large enough to encircle several cities as big as New York, with mountains higher than the Rockies rising up from their floors.

When a meteor strikes Earth, it makes a crater, just as it does on the Moon. But on Earth, wind, rain, and the motions of the crust erase or fill in these craters. Since the Moon has no atmosphere, there is no rain or wind to erode the craters. And because the Moon's interior is no longer hot and active like Earth's, there are no active volcanoes on the Moon. So craters formed by meteorites on the Moon last a long time.

The average distance of the moon from the earth is about 239,000 miles (384,000 km). The moon's diameter is about 2,160 mi (3,476 km) or about one- fourth that of earth. The moon's volume is about one-fiftieth that of the earth. The mass of the earth is 81 times greater than the mass of the moon. Thus the average density of the moon is only three-fifths, and the pull of gravity at the lunar surface only one-sixth that of the earth.

The Moon has no atmosphere. Any early atmosphere that the Moon might have had, has escaped from the Moon's feeble gravitational pull. This is only one sixth that at the surface of the Earth. Because of the lack of any atmosphere, the temperature of the Moon's surface varies between -180 C and 110 C. The Moon offers little protection from the solar wind, cosmic rays or micrometeorites and so it is not surprising that there is no form of life on the Moon.

The moon has no free water and essentially no atmosphere, and no weather exists to change its surface; yet it is not totally inert. Evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been confirmed by Lunar Prospector. There is apparently ice at the north pole as well.

The Moon's crust averages 68 km thick and varies from essentially 0 km under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth's mantle, however, the Moon's mantle is only partially molten. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the lunar near side.

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. The overall composition of the Moon is believed to be similar to that of the Earth other than a depletion of volatile elements and of iron.

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 60 km (38 miles) on the near side to 100 km (63 miles) on the far side. The regolith varies from 3 to 5 metres (10 to 16 feet) in the maria to 10 to 20 metres (33 to 66 feet) 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, although the rim of Shackleton crater is illuminated for 80% of the lunar day. 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.

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 km&sup3.

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 ([3] (http://www.lpi.usra.edu/research/clemen/clemen.html)) craters at the lunar south pole ([4] (http://www.lpi.usra.edu/research/clemen/2polar.gif)) 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.

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.

Earth and Moon orbit about their barycentre, or common centre of mass, which lies about 4700 km from Earth's centre (about 3/4 of the way to the surface). Since the barycentre 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.

It may seem curious that the inclination of the lunar orbit and the tilt of the Moon's axis of rotation are listed as varying considerably. One must be reminded here that the orbital inclination is measured with respect to the primary's equatorial plane (in this case the Earth's), and that the axis of rotation's tilt is measured with respect to the normal to the satellite's orbital plane (the Moon's). For most planetary satellites, but not for the Moon, these conventions model physical reality and the values are therefore stable.

The Earth and the Moon form in fact a "binary planet": each one is more closely tied to the Sun than to the other. The plane of the lunar orbit maintains an inclination of 5.145 396° with respect to the ecliptic (the orbital plane of the Earth), and the lunar axis of rotation maintains an inclination of 1.5424° with respect to the normal to that same plane. The lunar orbital plane precesses quickly (i.e. its intersection with the ecliptic rotates clockwise), in 6793.5 days (18.5996 years), because of the gravitational influence of the Earth's equatorial bulge. During that period, the lunar orbital plane thus sees its inclination with respect to the Earth's equator (itself inclined 23.45° to the ecliptic) vary between 23.45° + 5.15° = 28.60° and 23.45° - 5.15° = 18.30°. Simultaneously, the axis of lunar rotation sees its tilt with respect to the Moon's orbital plane vary between 5.15° + 1.54° = 6.69° and 5.15° - 1.54° = 3.60°. Note that the Earth's tilt reacts to this process and itself varies by 0.002 56° on either side of its mean value; this is called nutation.

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 Moon's origin is uncertain. There were three main theories:

New and detailed information from the Moon rocks led to the theory that a Mars-sized object collided with the Earth soon after it was formed, a geyser of molten material spewed into space, and entered orbit around Earth. Some of this material fell back to Earth, but much of it coalesced to form the Moon.

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 Moon makes a complete orbit about once a month. Each hour the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most 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.

The lunar orbit is elliptical. The moon and the Earth orbit around their common center of gravity. The moon's orbital period around the earth, and also its rotation period, is 27.322 days. Since the period of the orbit is the same as the Moon's rotational period (or lunar day), we always see the same side of the Moon facing us. 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.

This is not a coincidence but something that happened naturally in the evolution of the earth/moon relationship. It is a characteristic shared by the moons of the other planets; for example Titan rotates on its axis once every 15 days 23 hours, the same time that it takes to complete one orbit around Saturn.

The pull of the moon's gravity causes the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. This causes two small bulges in the sea water, one in the direction of the Moon and one directly opposite. Because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day.

The difference in the moon's gravitational pull on the solid earth and on the waters of the oceans is the main factor in producing tides; the lowest (neap) tides occur when the gravitational forces of the moon and the sun act at right angles and the highest (spring) tides occur when they pull in the same or, paradoxically, opposite directions. Thus, spring tides occur with new moons and full moons (in this context 'spring' has no connection with the season).

But, the Earth is not completely fluid either. The Earth's rotation carries the Earth's bulges slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and the Moon is not exactly along the line between their centers, producing a torque on the Earth and an accelerating force on the Moon.

This causes a net transfer of rotational energy from the Earth to the Moon, slowing down the Earth's rotation by about 1.5 milliseconds/century and raising the Moon into a higher orbit by about 3.8 centimeters per year.

The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth's rotation is now being slowed by the Moon's influence so in the distant past the Moon's rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon's rotation rate was slowed to match its orbital period (such that the bulge always faced the Earth) there was no longer an off- center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earth's rotation will be slowed to match the Moon's period, too, as is the case with Pluto and Charon.

Actually, the Moon appears to wobble a bit (due to its slightly noncircular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. There is no "dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles).

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. This synchronous rotation is a result of torque having slowed down the Moon's rotation in its early history, a process known as tidal locking.

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.

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

The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. Tidal flow is synchronized to the Moon's orbit around Earth. This synchronous rotation is only true on average because the Moon's orbit has definite eccentricity. When the Moon is at its perigee, its rotation is slower than its orbital motion, and this allows us to see up to an extra eight degrees of longitude of its East (right) side. Conversely, when the Moon reaches its apogee, its rotation is faster than its orbital motion and reveals another eight degrees of longitude of its West (left) side. This is called longitudinal libration. The tidal bulges on Earth caused by the Moon's gravity lag behind the apparent position of the Moon, due to the impedance of the ocean system - effectively the inertia of the water and the friction as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. Because the lunar orbit is also inclined to the Earth's equator, the Moon seems to oscillate up and down (as a person's head does when indicating "yes") as it moves in celestial latitude (declination). This is called latitudinal libration and reveals the Moon's polar zones over about seven degrees of latitude. Finally, because the Moon is only at about 60 Earth radii distance, an observer at the equator who observes the Moon throughout the night moves by an Earth diameter sideways. This is diurnal libration and reveals about one degree's worth of lunar longitude.

Solar eclipse.  A solar eclipse is caused by the Moon blotting out the Sun, casting a round shadow on the Earth. If only part of the Sun is blotted out, a partial eclipse occurs.

The true angular size of the moon's diameter is about 1/2�, which also happens to be the sun's apparent diameter. This coincidence makes possible total solar eclipses. A solar eclipse is when the sun is hidden by the moon; a lunar eclipse is when the moon is in Earth's shadow. In a total eclipse, the Moon completely covers the disc of 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 total 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. The orbital plane of the Moon is inclined to that of the Earth about the Sun and so eclipses are only seen when New Moon or Full Moon occur when the Moon is near to the crossing points of these planes.

By a strange coincidence, the sun has a diameter 390 times greater than the moon and it is 390 times further away; the two bodies therefore appear to be the same size in the sky and when the moon is directly between the sun and the earth it almost completely covers the sun's disk, causing a total solar eclipse. It is one of the most glorious sights in nature; when the brilliant face of the sun is obscured, the pearly corona can be seen stretching outward together with masses of red hydrogen gas in the flares; the sky darkens and the planets and bright stars can be seen.

Lunar eclipses occur when the earth lies directly between the sun and the moon. The earth, the sun and the moon return to the same relative positions every 18 years, 10.25 days. This simple rule makes lunar eclipses relatively easy to predict. Some ancient peoples were able to forecast them successfully and some mariners in the 15th century were experts. When anchored off the Americas, Columbus told the natives that the moon would 'change her colour and lose her light'; when the eclipse duly arrived, the natives treated Columbus like a god.

Moonlight is reflected sunlight and the various phases of the moon depend upon the relative positions of the sun, earth and moon; for example, a full moon must be in the opposite part of the sky to the sun and rise close to the time of sunset.

As the Moon orbits the Earth it goes through a sequence of phases as the proportion of the visible illuminated hemisphere changes. The Moon shines by reflecting the light from the Sun and shows the characteristic phases during each orbit of the Earth. Near New Moon, when the sunlit portion of the Moon is small, the phenomenon of `the old Moon in the young Moon's arms' is often seen. This is caused by sunlight being reflected towards the Moon by the Earth and being reflected back again to the Earth. We are seeing Earthshine, the equivalent of moonlight on the Earth.

The first man-made object to reach the Moon was the unmanned Soviet probe Luna 2, which crashed into it on September 14, 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. Three astronauts travelled to the moon in the Apollo 11 spacecraft, launched from Earth by a giant Saturn rocket. The main capsule orbited the moon, while astronauts Neil Armstrong and Edwin Aldrin flew down in a small landing craft. The first man on the moon was Neil Armstrong, on July 20th 1969.

From 1969 to 1972, six Apollo missions sent 12 astronauts to the Moon's surface. They gathered lunar rocks and soil and brought them back to Earth. They also left behind four special mirrors aimed at Earth. Astronomers beamed pulses of laser light to the Moon and measured the time it takes for the light to reflect back to Earth. This technique has established the exact distance from Earth to the Moon to within a fraction of an inch.

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.

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

Moon samples have been brought to Earth from these six manned missions as well as from three Luna missions (nrs. 16, 20, and 24).

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 entered lunar orbit on November 15, 2004 . It will survey the lunar environment and create an X-ray map of the Moon.

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.

Japan and India are on the waiting list for the Moon, too. Japan already outlined its upcoming missions to our neighbour Lunar-A and Selene . Even a manned lunar base is planned by the Japanese Space Agency (JAXA). India will first try an unmanned orbiting satellite, called Chandrayan.

The Moon (and also the Sun) appear larger when close to the horizon. This is a purely psychological effect (see Moon illusion). 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.

For any location on Earth, the highest altitude of the Moon on a day varies between the same limits as the Sun, and depends on season and lunar phase. For example, in winter the Moon comes highest when it is full, and the full moon comes highest in winter.