Moon
This article is about Earth's natural satellite. For moons in general, seeNatural satellite. For other uses, seeMoon (disambiguation).
The Moon, tinted reddish, during a lunar eclipse.
The Moon (Latin: Luna) is Earth's onlynatural satellite.[e][f][8] Although not the largest natural satellite in the Solar System, it is, among the satellites of major planets, the largest relative to the size of the object it orbits (itsprimary).[g][h] It is the second-densestsatellite among those whose densities are known (after Jupiter's satellite Io).
The Moon is in synchronous rotationwith 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 visiblecelestial object in Earth's sky (after theSun), as measured by illuminance on the surface of Earth. 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 onlanguage, calendars, art, andmythology. 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 cm per year, but this rate is not constant.[9]
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.
As of November 2014, the Moon is the only celestial body other than Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmannedspacecraft 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 oflunar 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. Of these, orbital missions have dominated: Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters, which have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has also seen two rovermissions: the final Soviet Lunokhodmission in 1973, and China's ongoingChang'e 3 mission, which deployed itsYutu rover on 14 December 2013.
Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under theOuter Space Treaty, free to all nations to explore for peaceful purposes.
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