The Moon

The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; 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 (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

The Moon is Earth's only natural satellite. The average distance from the Earth to the Moon is 384,403 kilometres (238,857 miles), which is about 30 times the diameter of the Earth. The Moon has a diameter of 3,474 kilometres (2,159 miles) — slightly more than a quarter that of the Earth. This means that the volume of the Moon is only 1/50th that of Earth. Its gravitational pull is about a 1/6th of Earth's. The Moon makes a complete orbit around the Earth every 27.3 days, and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the lunar phases that repeat every 29.5 days. The gravitational attraction, and the centrifugal forces generated by the rotation of the Moon and Earth around a common axis, the barycentre, is largely responsible for the tides on Earth. The energy dissipated in generating tides is directly responsible for the reduction in potential energy in the Moon-Earth orbit around the barycentre, resulting in a 3.8 cm yearly increase in the distance between the two bodies. The Moon will continue to move slowly away from the Earth until the tidal effects between the two are no longer of significance, whereupon the Moon's orbit will stabilize.

The Moon is the only celestial body that humans have orbited (other than the Earth) or landed on. The first artificial object to escape Earth's gravity and pass near the Moon was the Soviet Union's Luna 1, the first aritificial object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966. The US Apollo program has achieved the first (and only) manned missions to date, resulting in six landings between 1969 and 1972. Human exploration of the Moon ceased with the conclusion of the Apollo program, although as of 2007, several countries have announced plans to send either people or robotic spacecraft to the Moon. On 4 December 2006, NASA outlined plans for a permanent base on the Moon as part of preparation for a voyage to Mars. Construction of the base is scheduled to take about five years, with the first preliminary missions by 2020.

The Moon has no formal English name, although it is occasionally called Luna (Latin for "moon") to distinguish it from the generic "moon" (referring to any of the various natural satellites of other planets). Its astronomical symbol is a crescent (?). The related adjective for the Moon is lunar (from the Latin root), or the adjectival prefix seleno- or suffix -selene (as in the Greek deity Selene). The word moon is Old English from older Germanic origins, with links back to Latin mensis, and back further to the PIE root of me- as in measure (time), with reminders of its importance in measuring time in words derived from it like Monday, month and menstrual.

The Lunar Surface

The Moon is in synchronous rotation, meaning that it keeps nearly the same face turned away from Earth at all times. Early in the Moon's history, its rotation slowed and became locked in this configuration as a result of frictional effects associated with tidal deformations caused by the Earth. Nevertheless, small variations resulting from the eccentricity of the lunar orbit, termed librations, allow a total of about 59 per cent of the lunar surface to be visible from Earth at one time or another.

The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side should not be confused with the dark side, which is the hemisphere that is not being illuminated by the Sun at a given moment. The far side of the Moon was first photographed by the Soviet probe Luna 3 in 1959. One distinguishing feature of the far side is its almost complete lack of maria.

Maria
The dark and relatively featureless lunar plains humans can clearly see when the Moon is full are called maria (singular mare), Latin for seas, since they were believed by ancient astronomers to be filled with water. Nearly all of these are actually vast solidified pools of ancient basaltic lava. The erupting lava flowed into the depressions formed by colliding meteors / comets, the impact basins. (Oceanus Procellarum is a major exception in that it does not correspond to a known impact basin). Maria are found almost exclusively on the near side of the Moon, with the far side having only a few scattered patches covering only about 2% of its surface, compared with about 31% on the near side. The most likely explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer. Several provinces containing shield volcanoes and volcanic domes are found within the near side maria.

Terrae
The lighter-colored regions of the Moon are called terrae, or more commonly just highlands, since they are higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are believed to be the surviving remnants of the impact basin's outer rims. In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.

From 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, at least during the hemisphere's summer season. These peaks of eternal light are possible because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of Shackleton crater is illuminated for about 80% of the lunar day. Another consequence of the Moon's small axial tilt is regions that remain in permanent shadow at the bottoms of many polar craters.

Lunar Ice

The continuous bombardment of the Moon by comets and meteoroids has most probably added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time, because of the Moon's weak gravity. However, because of the slightness of the axial tilt of the Moon's spin axis to the ecliptic plane - only 1.5 degrees - some deep craters near the poles never receive light from the Sun and are thus in permanent shadow (see Shackleton crater). Water molecules that ended up in these craters could be stable for long periods of time.

Clementine has mapped craters at the lunar south pole that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow. Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions. Estimates for the total quantity of water ice are close to one cubic kilometre.

Lunar Ice

Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar have indicated that some of the near-polar Clementine radar data might be associated with rocks ejected from young craters rather than the presence of water. To date, the question of how much water there is on the Moon, if any, has not been resolved.

The Moons Formation

Several mechanisms have been suggested for the Moon's formation. Early speculation proposed that the Moon broke off from the Earth's crust because of centrifugal forces, leaving a basin (presumed to be the Pacific Ocean) behind as a scar. This fission concept, however, requires too great an initial spin of the Earth. Others speculated that the Moon formed elsewhere and was captured into Earth's orbit. However, the conditions required for this capture mechanism to work (such as an extended atmosphere of the Earth for dissipating energy) are improbable. The coformation hypothesis posits that the Earth and the Moon formed together at the same time and place from the primordial accretion disk. In this hypothesis, the Moon formed from material surrounding the proto-Earth, similar to the formation of the planets around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of metallic iron in the Moon. A major deficiency with all of these hypotheses is that they cannot easily account for the high angular momentum of the Earth–Moon system.

Today, the giant impact hypothesis for forming the Earth–Moon system is widely accepted by the scientific community. In this hypothesis, the impact of a Mars-sized body on the proto-Earth is postulated to have put enough material into circumterrestrial orbit to form the Moon. Given that planetary bodies are believed to have formed by the hierarchical accretion of smaller bodies to larger ones, giant impact events such as this are thought to have affected most planets. Computer simulations modelling this impact are consistent with measurements of the angular momentum of the Earth–Moon system, as well as the small size of the lunar core. Unresolved questions regarding this theory have to do with determinating the relative sizes of the proto-Earth and impactor, and with determining how the material from the proto-Earth and the impactor was distributed when it formed the Moon. The formation of the Moon is believed to have occurred 4.527 ± 0.01 billion years ago, about 30–50 million years after the origin of the solar system.