Why Does the Same Side of the Moon Always Face Earth?


Though the moon does rotate around its axis, the speed with which it completes these revolutions match the amount of time it takes to orbit around the Earth, leading the same side to be faced toward Earth at all times. This process takes a about month, meaning the moon's days are as long as an Earth month.

Fans of progressive and psychedelic rock may know that there's a "dark side of the moon" thanks to Pink Floyd's 1973 album, but there's a reason why the same side of the moon always faces Earth. Though it may seem as if the moon is always fixed in place as it orbits around the Earth, it's actually turning in perfect harmony with its orbit.

Just as the moon's gravity affects tidal flow of bodies of water on Earth, the Earth's gravity has a tug on the moon, causing what's known as tidal friction. It's this tidal friction that causes the moon to rotate in synch with its orbit, and this is true of most of the moons in the solar system. This perfect alignment is called tidal synchronization. As of June 2014, only one moon, Saturn's Hyperion, is known to rotate without tidal synchronization.