The Moon is the only natural satellite of the Earth and the fifth largest moon
in the Solar System. It is the largest natural satellite of a planet in the
Solar System relative to the size of its primary, having 27% the diameter and
60% the density of Earth, resulting in 1⁄81 its mass. Among satellites
with known densities, the Moon is the second densest, after Io, a satellite
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 brightest
object in the sky after the Sun, although 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 minute
lengthening of the day. The Moon's current 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 solar
eclipses. This matching of apparent visual size is a coincidence. The Moon's
linear distance from the Earth is currently increasing at a rate of 3.82±0.07
cm per year, but this rate is not constant.
The Moon is thought to have formed nearly 4.5 billion years ago, not long after
the 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 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 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 origins, the
formation of its internal structure, and its subsequent history.
After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned
spacecraft, notably by the final Soviet Lunokhod rover. Since 2004, Japan, China,
India, the United States, and the European Space Agency have each sent lunar orbiters.
These spacecraft have contributed to confirming the discovery of lunar water ice in
permanently shadowed craters at the poles and bound into the lunar regolith. Future
manned missions to the Moon have been planned, including government as well as
privately funded efforts. The Moon remains, under the Outer Space Treaty, free to
all nations to explore for peaceful purposes.