The Moon Is Leaving Us — 3.8 Centimeters Every Year
Answers: “is the moon moving away from earth?”
Apollo astronauts left more than flags and footprints on the Moon. They left mirrors — panels of precisely angled reflectors, facing Earth.
For over 50 years, observatories have fired lasers at those mirrors and timed the reflection to millimeter precision. The measurements say something unambiguous: the Moon is moving away from us, about 3.8 centimeters per year. Roughly the speed your fingernails grow.
The thief is the tide. The Moon’s gravity raises bulges in Earth’s oceans — but Earth rotates faster than the Moon orbits, dragging those bulges slightly ahead of the Moon. That offset mass tugs the Moon forward, boosting its orbit. In exchange, the ocean bulges drag against Earth’s rotation like a brake.
It’s an exchange with consequences on both sides:
- Earth’s days are lengthening — by about 2 milliseconds per century. When the dinosaurs reigned, a day was about 23 hours; go back further and Earth spun in under 19.
- Growth rings in fossil corals record it: 420 million years ago, a year had over 400 days
- Right after its formation ~4.5 billion years ago, the Moon may have hovered just tens of thousands of kilometers away — filling the sky, driving tides hundreds of meters high
There’s a poetic casualty coming: total solar eclipses are temporary. They exist only because the Moon currently appears exactly the same size as the Sun in our sky — a staggering coincidence. As the Moon recedes, it will eventually look too small to fully cover the Sun. In roughly 600 million years, Earth will host its final total eclipse.
We happen to live in the narrow age when the sky performs its best trick. Every eclipse you see is one fewer left in the universe’s stock.