The Apollo astronauts left mirrors on the Moon, and every night since, observatories have been bouncing lasers off them to measure the exact distance, which is why we know the Moon is drifting away at about the rate your fingernails grow

One of the quietest Apollo experiments is still doing science without a battery, a radio, or a moving part.

It is not a telescope on the Moon. It is not a buried instrument or a forgotten antenna. It is a panel of small corner-cube reflectors, set on the lunar surface by Apollo astronauts so that light sent from Earth can be sent back to Earth.

The basic idea is simple enough to explain with a stopwatch. Fire a short laser pulse at the Moon. Wait for a tiny fraction of that light to return. Measure the round-trip travel time. Since light moves at a known speed, that time becomes a distance.

The result is one of the longest-running physical measurements in space science: lunar laser ranging, a technique that has turned the Earth-Moon distance into a clock-like measurement repeated across more than half a century.

The mirrors are not ordinary mirrors

The Apollo 11 astronauts left the first lunar laser ranging retroreflector on the Moon in July 1969. Apollo 14 and Apollo 15 later placed larger arrays. These are often called mirrors in everyday language, but they are more precise than flat bathroom mirrors.

Each array is made of corner-cube reflectors. A corner cube sends incoming light back toward the direction it came from, even if the reflector is not perfectly aimed. That is what makes the experiment possible. A laser beam fired from Earth spreads enormously by the time it reaches the Moon, and only a very small number of photons come back. The reflectors make those returning photons much easier to identify than light scattered from the rough lunar surface.

The first successful returns from the Apollo 11 reflector were reported in 1969. A 1973 paper in Science described the lunar laser ranging experiment and its early results. Later reviews, including a 1994 Science paper by J. O. Dickey and colleagues, treated lunar laser ranging as a continuing Apollo legacy rather than a completed mission souvenir.

How a pulse becomes a distance

The Moon is about 384,400 kilometers from Earth on average, but the real distance is always changing. The Moon follows an elliptical orbit. Earth rotates. Observatories move with the ground beneath them. Tides, atmospheric delay, lunar orientation, and the exact reflector position all matter.

That is why lunar laser ranging is not just “shine a laser and divide by two.” The raw measurement is the round-trip time of light from an observatory to a reflector and back. Turning that into a useful Earth-Moon distance requires detailed models of Earth rotation, lunar motion, station position, atmospheric conditions and relativistic effects.

The modern version of the experiment can be astonishingly precise. The Apache Point Observatory Lunar Laser-ranging Operation, known as APOLLO, began science-quality observations in 2006. In a 2023 dataset paper, James Battat and colleagues reported that APOLLO measured the Earth-Moon separation by recording the travel time of photons returning from five lunar retroreflector arrays, including the Apollo arrays and two Soviet Lunokhod arrays.

That paper, available as an arXiv preprint, reported a median nightly accuracy of 1.7 millimeters for APOLLO measurements. That does not mean the Moon itself sits still to within a millimeter. It means the ranging system can measure the line-of-sight distance to the reflector with extraordinary precision, after all the geometry and timing are handled.

The Moon is moving away

The famous number is about 3.8 centimeters per year. That is the rate at which the Moon is receding from Earth, on average, because of tidal interactions between the two bodies.

The comparison to fingernail growth is not exact biology. Fingernails vary from person to person. But as a scale image, it works: the Moon’s average outward drift is roughly the same order as the annual growth of human fingernails.

The cause is not the Moon “escaping” in the dramatic sense. The cause is angular momentum moving through the Earth-Moon system. Earth’s gravity raises tides in the oceans and in the solid Earth. Because Earth rotates faster than the Moon orbits, the tidal bulge is carried slightly ahead of the Earth-Moon line. That displaced mass pulls on the Moon, giving it a little orbital energy. The Moon moves outward. Earth’s rotation slows slightly in return.

This is not a fast process on human timescales. In a year, the change is smaller than the width of two fingers. Across millions and billions of years, it matters.

Not literally every observatory every night

The title’s “every night since” captures the continuity of the experiment, but the operational reality is more practical. Lunar laser ranging depends on weather, telescope access, lunar phase, equipment, and the geometry between Earth, Moon and reflector. Different observatories have contributed at different times.

What is true is that the measurement has continued across decades. Observatories have repeatedly sent laser pulses to lunar reflectors and used the returning photons to refine the Moon’s orbit, the motion of Earth, and tests of gravity.

The APOLLO project is one high-precision chapter in that longer record. Earlier ranging work came from places including McDonald Observatory in Texas and the Observatoire de la Cote d’Azur in France. The full archive is valuable because it stretches over time. A short experiment can measure a distance. A decades-long experiment can reveal a trend.

A small signal from a huge distance

The experiment sounds clean, but the returning signal is tiny. A laser pulse that begins as an intense beam on Earth spreads over hundreds of thousands of kilometers. It hits a small reflector array on the Moon. Only a few photons return to the telescope, and they must be separated from background light and detector noise.

That is part of what makes the achievement so elegant. The Moon is not wired to Earth. The reflectors do not call home. They simply sit there, in dust and sunlight and cold, waiting for light to arrive from one particular direction.

Even the reflectors have changed over time. A 2024 analysis by Sanchit Sabhlok and colleagues, also available as an arXiv preprint, argued that lunar dust has likely reduced the optical performance of the Apollo retroreflectors. The experiment still works, but the Moon has not preserved the hardware as a museum display under glass.

Why this old experiment still matters

The recession of the Moon is the fact most people remember, but lunar laser ranging does more than measure a widening gap.

It has helped refine the Moon’s orbit. It has been used to test aspects of general relativity. It has informed studies of the Moon’s rotation and interior. It has also provided one of the cleanest reminders that Apollo’s scientific legacy was not limited to returned rocks and photographs.

The reflectors are passive, but the experiment is not passive. It requires telescopes, lasers, clocks, models, weather windows and patient analysis. The Apollo astronauts left hardware on the Moon. Generations of observers on Earth turned that hardware into a long measurement of a moving system.

That is the real scale of the story. A panel placed by astronauts in 1969 still helps show that the Moon is inching away from Earth, not by the drama of a visible departure, but by a few centimeters a year, measured one faint returning pulse at a time.