Earth and the Moon form the Solar System's best-studied gravitational two-body system. The Moon orbits at an average distance of 384,400 km, completing one circuit every 27.3 days while keeping the same face toward Earth through tidal locking — a process still gradually slowing our planet's rotation. The Moon's gravity raises ocean tides, stabilises Earth's axial tilt against chaotic swings, and may have been essential for the development of complex life. The leading giant-impact hypothesis holds that the Moon formed when a Mars-sized body struck the young Earth about 4.5 billion years ago, ejecting debris that coalesced in orbit.