The Apollo astronauts left behind retroreflectors on the lunar surface, and scientists still bounce lasers off them today. This experiment has proven the Moon is drifting away from Earth at a rate of 3.8 centimeters per year, about the speed at which a human fingernail grows. This number is not a guess; it's one of the most precisely verified facts in planetary science.
The retroreflectors are still working after more than 50 years, and NASA's Lunar Laser Ranging program has been firing pulses at them since the reflectors were placed. The catch rate is brutal, but over millions of shots, the statistics resolve into a distance measurement accurate to a few millimeters.
The principle is elementary: Light travels at a fixed speed in vacuum, and timing the pulse out and back, dividing by two, and multiplying by the speed of light gives the distance. Modern stations use hydrogen maser clocks and single-photon-counting detectors with timing resolution in the picoseconds.
Repeat the measurement for half a century, and the secular trend pops out cleanly. The Moon, on average, is farther away every year than it was the year before. Astronomy Magazine puts the figure at 1.49 inches, or 3.78 centimeters, per year.
The cause is tidal friction. The Moon’s gravity raises a bulge in Earth’s oceans, and the mass of the bulge tugs the Moon forward in its orbit, giving it a tiny gravitational kick. Angular momentum has to be conserved, so the Moon climbs into a higher orbit while Earth’s rotation slows down.
Days are getting longer as a result. Billions of years ago, an Earth day was substantially shorter than today. Coral and tidal rhythmite fossils confirm the pattern in the geological record. The Moon was closer then, and the spin-down of Earth has been ticking along ever since.
The recession rate has not been constant. Continental configuration matters. Today’s Atlantic Ocean happens to resonate near the tidal frequency, amplifying friction. In the deep past, with different ocean basins, the rate was slower.
Over geological time, the effect is dramatic. A billion years from now, the Moon will be about 38,000 kilometers farther away. The angular size of the lunar disk in the sky will be perceptibly smaller. The coincidence that lets the Moon almost exactly cover the Sun during a total eclipse is the product of where the Moon sits right now.
An earlier analysis of lunar recession and eclipses shows that every total solar eclipse human beings see is something a future civilization will not. The last total solar eclipse on Earth will occur hundreds of millions of years from now. After that, the Moon will be too small in the sky to completely cover the solar disk. Annular eclipses will be the only kind left.
The Moon does not actually drift away forever. As Earth’s rotation slows, the angular momentum exchange has a natural endpoint. Eventually, Earth would rotate so slowly that one Earth day equaled one lunar orbit. Both bodies would then be tidally locked to each other.
The endpoint is academic. The Sun becomes a red giant in roughly five billion years and probably engulfs Earth, or at least bakes it sterile, long before the Earth-Moon system can finish settling. Earth and the Moon will not survive the Sun’s evolution intact. They go down together.
The 3.8 cm/year figure is the headline, but lunar laser ranging has done a lot of other work along the way. The data has tested Einstein’s equivalence principle and constrained possible variation in the gravitational constant G over time. It has measured the size of the Moon’s fluid outer core by detecting tiny wobbles in the lunar rotation.
The Apollo samples themselves keep yielding new science. Recent analysis of rocks brought back in 1972 turned up exotic sulfur isotopes that point to material from the deep lunar mantle.
What it took to get there is easy to forget. The Saturn V had to work. The lunar module had to land. The astronauts had to walk the array out and level it within a few degrees of the local vertical. The mirror surfaces had to survive 14-day lunar nights at -170°C and lunar days at over 100°C. Today, ground stations in New Mexico, Hawaii, France, and Italy keep firing pulses.
By the time you finish reading this sentence, several pulses could have made the trip. And the Moon, by the time you go to bed tonight, will be roughly a hundredth of a millimeter farther away than when you woke up.
