Moon

The Moon (Latin: Luna) is Earth's only natural satellite.^[e][f][8] Although not the largest natural satellite in the Solar System, among the satellites of major planets it is the largest relative to the size of the object it orbits (its primary).^[g][h] It is the second-densest satellite among those whose densities are known (after Jupiter's satellite Io).
The Moon is thought to have formed nearly 4.5 billion years ago, not long after Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body.
The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the second-brightest regularly visible celestial object in Earth's sky (after the Sun), as measured by illuminance on Earth's surface. Although it can appear a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art, and mythology.
The Moon's gravitational influence produces the ocean tides and the slight lengthening of the day. The Moon's current orbital distance is about thirty times the diameter of Earth, causing it to have an apparent size in the sky almost the same as that of the Sun. This allows the Moon to cover the Sun nearly precisely in total solar eclipse. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.82 ± 0.07 centimetres (1.504 ± 0.028 in) per year, but this rate is not constant.
The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origin, the formation of its internal structure, and its subsequent history. After the Apollo 17 mission in 1972, the Moon has been visited by only unmanned spacecraft.

Name and etymology

The English proper name for Earth's natural satellite is "the Moon".^[9][10] The noun moon derives from moone (around 1380), which developed from mone (1135), which derives from Old English mōna (dating from before 725), which, like all Germanic language cognates, ultimately stems from Proto-Germanic *mǣnōn.^[11]
The principal modern English adjective pertaining to the Moon is lunar, derived from the Latin Luna. Another less common adjective is selenic, derived from the Ancient Greek Selene (Σελήνη), from which the prefix "seleno-" (as in selenography) is derived.^[12][13]

The Moon

Lunar near side

Lunar far side

Lunar north pole

Lunar south pole

 

 

  

Formation

Several mechanisms have been proposed for the Moon's formation 4.527 ± 0.010 billion years ago,^[i] some 30–50 million years after the origin of the Solar System.^[14] Recent research presented by Rick Carlson indicates a slightly lower age of between 4.40 and 4.45 billion years.^{[15] [16]} These mechanisms included the fission of the Moon from Earth's crust through centrifugal force^[17] (which would require too great an initial spin of Earth),^[18] the gravitational capture of a pre-formed Moon^[19] (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon),^[18] and the co-formation of Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metals in the Moon).^[18] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.

The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact, where a Mars-sized body (named Theia) collided with the newly formed proto-Earth, blasting material into orbit around it that accreted to form the Moon.^[21] This hypothesis perhaps best explains the evidence, although not perfectly. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don’t come to our conference unless you have something to say about the Moon’s birth." At the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular.

Formation of Moon