The moon is a satellite of planet Earth. The moon revolves around the Earth in an elliptical orbit, which means that the moon-Earth distance is not constant: it changes depending on the moon's location in the orbit.

After careful measurements, it has been found that at its farthest point in its orbit (apogee), the moon is about 405,000 kilometers away from Earth. In comparison, at the nearest point in its orbit (perigee), the moon is about 360,000 kilometers from the earth. Thus, on average, moon-Earth separation is about 382,500 kilometers.

Before the first lunar mission took place, the distance was calculated by a study of planetary motion or by using a telescope. 

Astronauts then left glass reflectors on the surface of the moon, and very precise distance measurements could be carried out by determining the time it took light to reflect back from these reflectors.