The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. 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. It then receded to a wider orbit because of tidal interaction with the Earth. The most widely accepted origin explanation posits that the Moon formed 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. However, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side ( the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon's orbit around Earth has a sidereal period of 27.3 days. Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence very slowly lengthens Earth's day and is the main driver of Earth's tides. Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.
Its surface gravity is about one-sixth of Earth's ( 0.1654 g). It lacks any significant atmosphere, hydrosphere, or magnetic field. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term. It is larger than any of the known dwarf planets and is the largest (and most massive) satellite relative to its parent planet. The Moon is the fifth largest satellite in the Solar System.
Its diameter is about one-quarter the diameter of the Earth (comparable to the width of Australia). The Moon is Earth's only natural satellite.
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