How Do Scientists Know the Distance between the Earth and the Moon?

As the Moon orbits the Earth, the distance between these two bodies is always changing. At its closest, it's about 225,000 miles (360,000 km) and at its farthest, it's about 250,000 miles (405,000 km). Scientists can keep track of the precise distance between the Earth and the Moon thanks to the Lunar Laser Ranging experiment, set up by the Apollo 11 astronauts in 1969.

The Lunar Laser Ranging Retroreflector Array is basically a mirror-studded panel. It was left on the Moon by astronauts Neil Armstrong and Buzz Aldrin of Apollo 11. It measures the distance between the Earth and the Moon in a fairly simple, but very precise, way. Scientists send a laser pulse from Earth to the retroreflector array on the Moon. The mirrors on the array reflect the laser pulse back, providing a very accurate measurement of the distance. The Lunar Laser Ranging experiment is the only Apollo experiment that continues to provide data from the Moon.

More about the Earth and the Moon:

• The Earth's circumference (the distance around the Earth at the Equator) is approximately 25,000 miles (40,000 km).
• Neil Armstrong and Edwin “Buzz” Aldrin stepped onto the Moon on 20 July 1969.
• The point at which the Moon is closest to the Earth is called the "perigee" and the point at which the Moon is farthest from the Earth is called the "apogee."