Why Does the Moon Always Show the Same Face?

Tidal locking is a consequence of gravitational tidal forces. Early in its history, the Moon rotated faster than it orbited Earth. Earth gravity raised tidal bulges on the Moon (deforming its rocky body slightly), and the gravitational pull on these bulges created a torque that gradually slowed the Moon rotation until it synchronised with its orbit. This process took perhaps 100 million years. The near side and far side of the Moon are strikingly different: the near side has large dark basalt plains (maria), while the far side is almost entirely rugged highlands. The far side was first photographed by the Soviet Luna 3 probe in 1959. Tidal locking is common in the solar system — most large moons of Jupiter and Saturn are also tidally locked to their planets.