Why the Moon is drifting away
The Moon’s retreat begins with a tug-of-war between Earth’s rotation and the Moon’s gravity. The Moon pulls on Earth’s oceans, creating tidal bulges on opposite sides of the planet. Because Earth spins faster than the Moon travels around it, the planet’s rotation carries those bulges slightly ahead of the Moon’s position.
That small offset matters. The nearer bulge’s gravity pulls the Moon forward along its orbit. A forward pull adds orbital energy, nudging the Moon into a wider orbit; in return, Earth loses some of its rotational energy and gradually spins more slowly. The process is called tidal transfer: energy moves from Earth’s spin into the Moon’s orbit.
Why ocean tides make the difference
The bulges do not keep pace perfectly with Earth’s turning. Water sloshing across shallow seas and against coastlines meets resistance, or friction, which helps create the offset. That resistance also dissipates energy as heat. The details vary with the shape of ocean basins and the arrangement of continents, so the rate of retreat has not been constant throughout Earth’s history.
The Moon’s distance increases by roughly a couple of inches per year. That is far too little to notice from one year to the next, but the same exchange has been operating over immense spans of time. It also means that Earth’s days gradually lengthen as the planet’s spin slows.
A distant consequence for eclipses
As the Moon moves farther away, it appears slightly smaller in Earth’s sky. Today, its apparent size can match the Sun’s closely enough for the Moon to cover the Sun completely during a total solar eclipse. In the far future, the Moon will appear too small for that: even when centered over the Sun, it will leave a bright ring around the edge.
The eclipse change is not imminent. It is the visible, eventual consequence of a quiet exchange happening now: Earth’s turning helps push the Moon outward, one tide at a time.
