The
Moon is
Earth's only
natural satellite[nb 4][6] and the
fifth largest satellite in the
Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its
primary (though
Charon, which orbits the
dwarf planet Pluto, is proportionally larger), a quarter the diameter of Earth and
1⁄81 its mass. The Moon is the second densest satellite after
Io. It is in
synchronous rotation with Earth, always showing the same face; the
near side is marked with dark volcanic
maria among the bright ancient crustal highlands and prominent
impact craters. It is the brightest object in the sky after the
Sun, although its surface is actually very dark, with a similar reflectance to 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'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.
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.
