The moon is Earth’s only satellite. At about one-quarter the diameter of the Earth, it lacks any significant atmosphere, hydrosphere, or magnetic field, and has a surface gravity about one-sixth that of Earth’s. It orbits the Earth at an average distance of 384,400 km and its gravitational influence is the main driver of the Earth’s tides. During each synodic period of 29.5 days, the visible surface illuminated by the Sun varies, resulting in the lunar phases that form the basis for the months of the lunar calendar.