The Moon feels like one of the most permanent things in our sky. It has been orbiting Earth for billions of years, controlling the tides and lighting up our nights. But the Moon is actually moving away from us.
Every year, the Moon drifts roughly 3.8 centimeters farther from Earth. It sounds insignificant, but over astronomical timescales, the effect becomes much more interesting.
The reason is connected to Earth's rotation and ocean tides.
As the Earth rotates, its oceans bulge slightly because of the Moon's gravitational pull. That tidal bulge doesn't point exactly toward the Moon. Because Earth rotates faster than the Moon orbits, the bulge gets carried slightly ahead of the Moon.
The Moon's gravity pulls on this bulge, and that interaction transfers a tiny amount of Earth's rotational energy to the Moon. As a result, the Moon gains orbital energy and gradually moves into a higher orbit.
At the same time, Earth is losing rotational energy. This means Earth's rotation is very slowly slowing down, making our days longer.
Billions of years ago, a day on Earth was significantly shorter than it is today. In the distant future, Earth's rotation will continue to slow, although the process is complicated by other factors affecting our planet.
Interestingly, the Moon won't simply keep moving away forever. The system is evolving toward a state where Earth's rotation and the Moon's orbital motion would become synchronized. However, the Sun's future evolution will dramatically change the Earth-Moon system long before that theoretical equilibrium could be reached.
So the next time you look at the Moon, remember: it isn't sitting still.
It is slowly moving away from us — just a few centimeters at a time.
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