Earth's Moon The Moon is the only natural satellite of Earth. It has no formal name other than "The Moon" although it is sometimes referred to as Luna (Latin for moon) to distinguish it from the generic "moon." The average distance from the Moon to the Earth is 384,403 kilometers (238,857 miles). The Moon's diameter is 3,476 kilometers (2,160 miles).

The rotation of the Moon is synchronous. As a result, one side of the Moon (the "near side") is permanently turned towards Earth. 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.

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 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. 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. 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.

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 Earth and the Moon form in fact a "binary planet": each one is more closely tied to the Sun than to the other. The points where the Moon's orbit...

Solar eclipses occur when a node coincides with the new Moon; lunar eclipses when a node coincides with the full Moon.
Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal force, leaving an ocean basin behind as a scar. This concept requires too great an initial spin of the Earth. Others speculated the Moon formed elsewhere and was captured into its orbit. Some propose Coformation or Condensation theory, the concept that the Earth and the Moon formed at about the same time from the accretion disk. This theory fails to explain the depletion of iron in the Moon. Yet different groups propose that the Moon formed from a debris field around Earth resulting from an asteroid or planetesimal collision.

The currently accepted theory is the Giant Impact theory, in which the Moon originated from the ejecta from the collision between a semi-molten Earth and something the size of Mars (speculatively called Theia) (Palme, 2004). 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 (Korotev, 2004). 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.

The Moon is covered with tens of thousands of craters having a diameter of at least 1 kilometer. 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 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 meters (10 to…