Why Is Only One Side of the Moon Called the Dark Side of the Moon?

The Moon rotates on its axis and so has day and night much like the Earth does. However, the period of the Moon's rotation and its orbit around the Earth take the same amount of time, so the near side of the Moon is always facing the Earth. The far side, or "dark" side, is never visible from the Earth's surface.

The Moon takes about 29 days to make a complete orbit around the Earth, which is also the length of time it takes to make one full rotation around its axis. Scientists believe that the gravitational pull of the Earth has gradually slowed the Moon's rotation down to its current speed, a phenomenon known as tidal locking. Due to a slight oscillation of the Earth and Moon relative to each other, about 18 percent of the far side of the Moon is occasionally visible from Earth. The far side of the Moon has been seen and photographed by astronauts in lunar orbit. In 1959, the unmanned Soviet probe Luna 3 took the first photographs of the far side. The far side was first seen directly in 1968 by the astronauts of Apollo 8. Since that time, it has been extensively mapped and photographed.