The Moon is drifting away from Earth, about 3.8 cm a year
In July 1969, before leaving the Moon, the Apollo 11 crew set up a small panel of mirrors on the surface. It is still in use. Apollo 14 and 15 left more, two Soviet Lunokhod rovers carried smaller ones, and India's Chandrayaan-3 lander added another in 2023.
These are retroreflectors: they send light straight back toward wherever it came from. An observatory fires a laser pulse at one and times how long the light takes to return, about 2.5 seconds for the round trip. Half that time, multiplied by the speed of light, gives the distance. The first returns were recorded on 1 August 1969, at Lick Observatory in California.
It is hard work. Of roughly 300 million billion photons aimed at the mirrors, only one to five come back to the telescope on a good night. But after decades of measuring, the distance is known to within millimetres, and it is changing.
The tides are the cause
The Moon's gravity raises tidal bulges in Earth's oceans. Earth spins much faster than the Moon goes around it, so the spin drags those bulges slightly ahead of the Moon. The bulge ahead pulls the Moon forward along its path, lifting it into a slightly wider orbit. In return, the Moon pulls back on the bulge, which acts as a gentle brake on Earth's spin.
So energy and spin pass from Earth to the Moon. Most of the energy is lost on the way, as heat from water moving over the sea floor in shallow seas, but the effect adds up. Earth's day is growing longer by around two thousandths of a second every century.
The rocks keep a record of it. Layered sediments laid down by tides about 620 million years ago point to a day of about 22 hours and a year of about 400 days. Fossil shells from around 70 million years ago suggest a year of 372 days.
Today's pace is unusually fast. Averaged over those 620 million years, the Moon pulled away at roughly half its present speed.
None of this is anything you could notice. Over a long lifetime, the Moon moves away by about three metres. But it means that our tides, our eclipses and the length of our day all belong to one moment in a very slow change.
What is slowly pushing the Moon away from Earth?
Tides pass some of Earth's spin to the Moon, so the Moon moves about 3.8 cm farther away each year while Earth's day slowly lengthens.
