Lunar Laser Ranging: Measuring the Moon’s Distance

Lunar laser ranging measures the distance between Earth and the Moon with extreme precision.
Astronauts from Apollo 11, 14, and 15 placed special retroreflectors on the lunar surface.
These reflectors bounce laser beams sent from Earth straight back to their source.
The round-trip travel time of the laser reveals the Moon’s distance.
The average Earth–Moon distance is about 384,400 km (238,855 miles).
Measurements are accurate to within just a few centimeters.
Lunar ranging proved that the Moon is slowly drifting away from Earth at 3.8 cm per year.
It provides critical tests of Einstein’s general relativity.
The experiment has been running continuously since 1969, making it the longest-running lunar science experiment.
Lunar laser ranging helps refine our understanding of Earth-Moon dynamics and orbital evolution.