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, roughly 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, installed over 50 years ago, are still functional. NASA's Lunar Laser Ranging program has been firing pulses at these mirrors, and despite the low catch rate, the statistics reveal a distance measurement accurate to a few millimeters. This experiment has been running for millions of shots, and the results show a clear trend: the Moon is moving away from Earth at a steady rate.
The cause of this recession is tidal friction. The Moon's gravity creates a bulge in Earth's oceans, and as Earth rotates faster than the Moon orbits, that bulge is carried ahead. This mass tugs the Moon forward, giving it a gravitational kick. As a result, the Moon climbs into a higher orbit while Earth's rotation slows down, leading to longer days.
The comparison between the Moon's recession rate and human fingernail growth is a fascinating one. While it may seem whimsical, it's also exact. The Apollo retroreflectors, in effect, act as rulers measuring the retreat one photon at a time.
The experiment's success is a testament to the chain of events that made it possible. The Saturn V rocket, the lunar module, and the astronauts' efforts all played a crucial role. The mirrors had to withstand extreme conditions, including lunar nights and days, and the abrasive lunar dust. Despite these challenges, the retroreflectors have been quietly working for decades, without crew, funding crises, or political reauthorization.
The laser ranging data has revealed more than just the recession rate. It has tested Einstein's equivalence principle and measured the size of the Moon's fluid outer core. The Apollo samples, even those returned decades ago, continue to yield new scientific insights.
As the Moon continues to drift away, it will have significant implications for future civilizations. The last total solar eclipse on Earth will occur hundreds of millions of years from now, and after that, only annular eclipses will be possible. The Moon's recession is a reminder of the dynamic nature of our solar system and the ongoing scientific discoveries that shape our understanding of the universe.
