Is the Moon moving away from Earth?

Apollo 11, 14 and 15 left panels of mirrors on the lunar surface. More than fifty years later, observatories still fire laser pulses at them and time how long the light takes to come back. The round trip is about 2.5 seconds, and the distance it implies is known to within a few centimetres.

Those measurements show the Moon receding by roughly 3.8 centimetres a year. The cause is tides: Earth's oceans bulge toward the Moon, the planet's rotation drags that bulge slightly ahead, and the offset hands orbital energy to the Moon while gently braking Earth's spin.

That braking is the part people miss. Days on Earth are getting longer by a couple of milliseconds per century. The experiment is one of the longest-running in science, and it is still returning data from equipment nobody has touched since the seventies.