We know exactly how fast the Moon is leaving

Between 1969 and 1971, the Apollo missions left several panels of retroreflecting mirrors on the Moon. The idea was simple and brilliant: fire a laser from Earth, time the bounce, and calculate the distance with millimetre precision.

Half a century later, observatories still use them. And the measurement keeps returning one steady result: the Moon is drifting away from us by about 3.8 centimetres a year — roughly the speed at which your fingernails grow.

The tides are to blame. The Moon pulls on the oceans; that displaced mass of water pulls back on the Moon, speeding it up and nudging it into an ever higher orbit. As a bonus, the same tug-of-war is slowing Earth's rotation, so our days are getting longer, little by little. Back in the age of the dinosaurs, a day lasted less than twenty-four hours.