Moon

The Moon is a gravity rounded astronomical body orbiting Earth and is the planet's only natural satellite. It is also the only natural satellite in the inner Solar System to be relatively round (Mars' two moons, Phobos and Deimos, are irregularly shaped) and the only spherical one of a terrestrial planet (Mercury and Venus has no moons). It is the fifth-largest satellite in the Solar System, and by far the largest among planetary satellites relative to the size of the planet that it orbits. The Moon is, after Jupiter's satellite Io, the second-densest satellite in the Solar System 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 hypothetical Mars-sized body called Theia. New research of Moon rocks, although not rejecting the Theia hypothesis, suggests that the Moon may be older than previously thought.