The moon

The Moon is the only natural satellite of the Earth,[d][7] and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to thesize of its primary, having a quarter the diameter of Earth but only 1⁄81 its mass.[e] The Moon is the second densest satellite after Io, a satellite of Jupiter. It 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. The Moon is the brightest objectin the sky after the Sun, although its surface is actually very dark, with a reflectance similar to that of coal. 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 minute lengthening of the day. The Moon'scurrent orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solareclipses. This matching of apparent visual size is a coincidence. Earlier in Earth's history, the Moon was closer to Earth, and would have had an apparent visual size greater than that of the sun.
TheMoon 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 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 thefirst 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 origins, the formation of its internal structure,...