Description

The Moon is Earth's only permanent natural satellite. It is one ofthe largest natural satellites in the Solar System, and the largestamong planetary satellites relative to the size of the planet thatit orbits (its primary). It is the second-densest satellite amongthose whose densities are known (after Jupiter's satellite Io). TheMoon is thought to have formed approximately 4.5 billion years ago,not long after Earth. There are several hypotheses for its origin;the most widely accepted explanation is that the Moon formed fromthe debris left over after a giant impact between Earth and aMars-sized body called Theia. The Moon is in synchronous rotationwith Earth, always showing the same face with its near side markedby dark volcanic maria that fill between the bright ancient crustalhighlands and the prominent impact craters. It is thesecond-brightest regularly visible celestial object in Earth's skyafter the Sun, as measured by illuminance on Earth's surface. Itssurface is actually dark (although it can appear a very brightwhite) with a reflectance just slightly higher than that of wornasphalt. Its prominence in the sky and its regular cycle of phaseshave made the Moon an important cultural influence since ancienttimes on language, calendars, art, and mythology. The Moon'sgravitational influence produces the ocean tides, body tides, andthe slight lengthening of the day. The Moon's current orbitaldistance is about thirty times the diameter of Earth, with itsapparent size in the sky almost the same as that of the Sun,resulting in the Moon covering the Sun nearly precisely in totalsolar eclipse. This matching of apparent visual size will notcontinue in the far future. The Moon's linear distance from Earthis 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 SovietUnion's Luna programme was the first to reach the Moon withunmanned spacecraft in 1959; the United States' NASA Apollo programachieved the only manned missions to date, beginning with the firstmanned lunar orbiting mission by Apollo 8 in 1968, and six mannedlunar landings between 1969 and 1972, with the first being Apollo11. These missions returned over 380 kg (840 lb) of lunar rocks,which have been used to develop a geological understanding of theMoon's origin, the formation of its internal structure, and itssubsequent history. After the Apollo 17 mission in 1972, the Moonhas been visited only by unmanned spacecraft.