Astronomy
Using Lunar Laser Ranging to Measure the Moon's Recession
Quick fact
Lunar laser ranging has measured the Moon's distance with millimeter precision and shows that it is receding at about 3.8 centimeters per year — roughly the rate your fingernails grow!
Why this is interesting
Have you ever wondered how we know the Moon is slowly drifting away from Earth? The answer lies in bouncing laser beams off mirrors left on the lunar surface!
Read the full explanation
Understanding Using Lunar Laser Ranging to Measure the Moon's Recession
Lunar laser ranging works like a cosmic radar gun, but instead of sending a radio wave and waiting for its reflection, we send a laser pulse from Earth toward a special mirror placed on the Moon by astronauts. Because light travels at a known speed, we can calculate the distance by timing how long the pulse takes to go there and come back. This distance, measured over months and years, changes as the Moon moves in its orbit and also very slowly increases. That increase is the Moon's recession. Think of it as measuring how far a ball rolls each second by using a high-speed camera timing its position — the more precise the timing, the finer the movement we can detect.
A deeper explanation
The Moon's recession is a result of tidal interactions between Earth and the Moon. The Earth's rotation drags the tidal bulge ahead of the Earth-Moon line, and the gravitational pull of this bulge exerts a torque that transfers angular momentum from Earth's spin to the Moon's orbit. This causes the Moon to spiral outward. Lunar laser ranging makes this tiny drift observable by using retroreflectors — arrays of corner-cube prisms — which reflect light back exactly in the direction it came from. A laser pulse is fired, and the telescope detects the returned photons. By measuring the round-trip time, we can determine the distance to within a few millimeters. Over decades, these data reveal an average recession rate of about 3.8 centimeters per year. This measurement is crucial for testing gravitational theories (e.g., the equivalence principle) and for refining models of the Earth-Moon system's evolution, including the history of the Moon's orbit and the length of Earth's day.