Look up at the Moon tonight and you’re looking at something that is quietly running away from us. Not fast — about 3.8 centimetres a year, the speed your fingernails grow — but relentlessly, and forever. The reason is one of the most elegant pieces of physics in the sky, and it’s slowly stretching our days.
The tides are pushing it
The Moon’s gravity pulls on Earth’s oceans, raising a bulge of water on the side facing the Moon. Here’s the twist: Earth spins faster than the Moon orbits, so our planet’s rotation drags that tidal bulge slightly ahead of the Moon.
That misplaced bulge has its own gravity, and it tugs the Moon forward along its orbit. Give an orbiting object a forward tug and it climbs into a higher, wider orbit. So every single day, the tides nudge the Moon a fraction farther out.
The Moon isn’t escaping. It’s being gently thrown outward by the very tides it creates.
How we know it to the millimetre
This isn’t a guess. When astronauts visited the Moon, they left behind retroreflectors — special mirrors that bounce light straight back where it came from.
Scientists fire lasers at those mirrors from Earth and time how long the light takes to return. Because light’s speed is known exactly, the round trip reveals the Earth–Moon distance to within millimetres. Decades of these measurements show the same steady retreat: 3.8 cm per year.
The hidden cost: longer days
Physics keeps its books balanced. As the tides fling the Moon outward, they also act as a brake on Earth’s spin — friction between the ocean bulge and the seafloor bleeds away our planet’s rotational energy.
The result: Earth’s day is getting longer, by about 1.7 milliseconds per century. Run that backwards and the numbers get wild. In the age of the dinosaurs a day was roughly 23 hours. Go back hundreds of millions of years more, to the early oceans, and a day lasted only about 22 hours — and a year had over 400 days.
We even have a fossil record of it: certain ancient corals laid down daily and yearly growth bands, and counting them confirms the year once had far more, shorter days.
So will the Moon ever leave?
No — and it’s oddly comforting. Two things stop it. First, the drift is glacially slow. Second, our star has other plans: in roughly 5 billion years the Sun will swell into a red giant, almost certainly engulfing the Earth–Moon system entirely. The Moon will still be faithfully orbiting when the lights go out.
So the next clear night, take a look. That familiar face is a few centimetres farther than it was last year — and it’s the reason your days are, very slightly, longer than a dinosaur’s.