Moon (Earth)

The Moon is Earth's only proper natural satellite. It is one-quarter the diameter of Earth (comparable to the width of Australia), making it the largest natural satellite in the Solar System relative to the size of its planet. It is the fifth largest satellite in the Solar System and is larger than any dwarf planet. The Moon orbits Earth at an average lunar distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter. Its gravitational influence produces Earth's tides and slightly lengthens Earth's day. The Moon is classified as a planetary-mass object and a differentiated rocky body. It lacks any significant atmosphere, hydrosphere, or magnetic field, and its surface gravity is about one-sixth of Earth's (0.1654 g). Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

The Moon's orbit around Earth has a sidereal period of 27.3 days, and a synodic period of 29.5 days. The synodic period drives its lunar phases, which form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis (a lunar day) is the same as the synodic period, resulting in its same side (the near side) always facing Earth. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective (its libration).

The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it reflects direct sunlight, is contrasted by the relatively dark sky, and has a large apparent size when viewed from Earth, the Moon is the brightest celestial object in Earth's sky after the Sun. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

The first manmade object to reach the Moon was the Soviet Union's Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' NASA Apollo program, which conducted the first manned lunar orbiting mission with Apollo 8 in 1968. Beginning with Apollo 11, six human landings took place between 1969 to 1972. These missions returned lunar rocks which have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history; the most widely accepted origin explanation posits that the Moon formed about 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothetical Mars-sized body called Theia.