The Moon Is Drifting Away by 3.8 Centimeters a Year
Apollo astronauts left mirrors on the Moon. Laser shots off them show it slips farther every year.
On July 21, 1969, Buzz Aldrin set a 46-cm panel of corner-cube mirrors on the Sea of Tranquility. Apollo 14 and Apollo 15 left similar retroreflectors. Every clear night since, observatories have fired laser pulses at them, timed the round-trip, and watched the Moon inch away.
The figure, averaged over five decades of ranging: 3.8 centimeters per year.
The engine is Earth's tides. As the oceans bulge toward the Moon and drag along the spinning planet beneath them, that bulge tugs the Moon forward. Angular momentum is conserved: the Moon climbs into a slightly higher orbit, and Earth's day lengthens by roughly 2 milliseconds per century.
Wind the clock the other way and the numbers bite. Around 600 million years from now, the Moon will be too small in Earth's sky to fully cover the Sun. No more total solar eclipses. The annular ring we already get in some eclipses will be all that's left.
The ranging experiment is one of the longest-running physical measurements in science. The retroreflectors have no power, no moving parts, and no expiration date. Fire a laser, wait about 2.5 seconds, catch a few photons coming back. They are, in the most literal sense, bookkeeping rocks.
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