The Apollo astronauts left behind retroreflectors on the lunar surface, and these mirrors have been crucial in measuring the Moon's recession from Earth. This phenomenon, where the Moon is slowly drifting away from our planet, is a result of tidal friction caused by the Moon's gravitational pull on Earth's oceans. The round-trip time of laser pulses bounced off these retroreflectors has been precisely measured, confirming the Moon's recession rate at an astonishingly accurate 3.8 centimeters per year. This rate is so precise that it's akin to the speed at which a human fingernail grows. The retroreflectors, installed during the Apollo missions, have been remarkably durable, with some still functioning after over 50 years. The experiment's success lies in its simplicity: a cube corner with three mirrors meeting at right angles, ensuring no electronic components to fail. NASA's Lunar Laser Ranging program has been continuously firing pulses at these mirrors, despite the low catch rate, and the data has revealed much more than just the recession rate. It has tested Einstein's equivalence principle, constrained variations in the gravitational constant, and even measured the size of the Moon's fluid outer core. The Apollo samples themselves continue to yield new scientific insights, with recent analysis uncovering exotic sulfur isotopes from the deep lunar mantle. The retroreflectors, left behind by the astronauts, have become a powerful tool for measuring the Moon's retreat, one photon at a time. This measurement is a testament to the ingenuity of the Apollo missions and the ongoing efforts to explore our celestial neighbor.
