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

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. Wai…