The Moon is an
astronomical body that
Earth, being Earth's only
natural satellite. It is
the fifth-largest natural satellite in the
Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its
Io, the Moon is the second-
densest satellite among those whose densities are known.
The Moon is thought to have formed about 4.51 billion years ago,
not long after Earth. The 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 called
The Moon is in
synchronous rotation with Earth, always showing the same face, with its
near side marked by dark volcanic
maria that fill the spaces between the bright ancient crustal highlands and the prominent
impact craters. As seen from the Earth, it is the second-brightest regularly visible
celestial object in Earth's sky, after the Sun. Its surface is actually dark, although compared to the night sky it appears very bright, with a
reflectance just slightly higher than that of worn
asphalt. Its gravitational influence produces the
body tides, and the
slight lengthening of the day.
The Moon's average
orbital distance at the present time is 384,402 km (238,856 mi),
 or 1.28 light-seconds. This is about thirty times the diameter of Earth, with its
apparent size in the sky almost the same as that of the Sun (due to it being 400x farther and larger), resulting in the Moon covering the Sun nearly precisely in total
solar eclipse. This matching of apparent visual size will not continue in the far future, because the Moon's distance from Earth is slowly increasing.
Luna programme was the first to reach the Moon with
unmanned spacecraft in 1959; the United States'
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
lunar rocks which have been used to develop a
geological understanding of the Moon's origin,
internal structure, and
later history. Since the
Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft.
Within human culture, both the Moon's natural prominence in the earthly sky, and its regular cycle of
phases as seen from the Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in
lunar based calendar systems,