The Moon Is Leaving Us — An Inch and a Half a Year
Image: the full Moon — a photograph of the actual Moon, by Luc Viatour (CC BY-SA 3.0, via Wikimedia Commons).
The Moon is leaving Earth. Slowly — 3.8 centimeters a year, about the speed your fingernails grow — but measurably, relentlessly, every single year.
How we know
During Apollo 11, 14 and 15, astronauts left retroreflectors on the lunar surface: arrays of quartz mirrors designed to bounce a laser beam straight back to where it came from. Observatories on Earth have been firing lasers at those mirrors for over five decades, timing the round trip with millimeter precision. The verdict, published in the lunar laser ranging record (Dickey et al., 1994): the Moon is receding at 3.82 ± 0.07 cm per year. It is one of the most precisely measured motions in all of science.
Why it's happening
The cause is tides. The Moon's gravity pulls a bulge of ocean water toward itself — but Earth rotates faster than the Moon orbits, so friction drags that bulge slightly ahead of the Moon. The off-center bulge tugs the Moon forward, feeding it orbital energy and pushing it outward. Nothing is free: the energy comes from Earth's own spin, which is slowing down. Our days are getting longer by about 1.7 milliseconds per century.
It used to be huge in the sky
Rewind 4.5 billion years. A Mars-sized world named Theia collided with the young Earth, and the debris coalesced into the Moon — possibly as close as 22,500 kilometers, versus 384,400 today. Back then it filled the sky, whipped tides hundreds of meters high, and spun Earth so fast that a day lasted only a few hours.
We know this from Earth's own records. Fossil corals from 400 million years ago show growth rings implying a year had 420 days — shorter days, a faster-spinning planet, a closer Moon.
What happens next
In a few billion years the tides will weaken and full Moon eclipses will end (the Moon will sit too far away to fully cover the Sun). Given ~50 billion years, Earth and Moon would settle into mutual tidal lock — a day as long as a month. But the Sun will swell into a red giant in about 5 billion years, long before that dance finishes.
The Moon is leaving. And it has been writing the story of our planet's rotation in rock and coral the whole way.