The Moon's surface. U.S. National Aeronautics and Space Administration (NASA).

The Moon is Earth's only natural satellite. Reflecting light from the Sun, the Moon is often the brightest object in the night sky.

The Moon orbits Earth at an average distance of approximately 240,000 miles (385,000 km). With revolution and rotation periods of approximately 27.32 Earth days, the Moon is in synchronous orbit about the earth. This synchronous orbit maintains a "near side" and "far side" of the Moon. The "near side" faces Earth, while the far side is not visible from Earth. Although Russian space probes—and later many American probes—took the first pictures of the far side of the Moon years earlier, it was not until the flight of Apollo 8 that United States astronauts became the first humans to directly view the far side of the Moon.

Orbital dynamics between the Sun, Moon, and Earth cause different...