DID YOU KNOW: The Moon is moving away from Earth at a rate of 3.8 cm per year, and that changes everything

Maxime Marquette

2026-06-24 13:08:43

Ocean tides: the quiet force driving separation

The explanation is both simple and counterintuitive. The Moon’s gravity slightly distorts the Earth, creating two bulges of water at opposite poles—the tides. These bulges are not perfectly aligned with the Moon: because the Earth rotates on its axis faster than the Moon orbits it, the tidal bulges are slightly ahead of the Moon’s position. This slightly offset bulge of water exerts a gravitational pull on the Moon, slightly accelerating it in its orbit. When an orbiting object accelerates, it rises to a higher orbit—and moves farther away from its planet.

Conversely, this interaction transfers energy from the Earth’s rotation tothe Moon’s orbit. The Earth gives up a tiny fraction of its rotational energy to the Moon with each tidal cycle. As a result, our day is getting very slightly longer. According to Lunar Laser Ranging measurements, the length of a terrestrial day is increasing by about 2.3 milliseconds per century. Over hundreds of millions of years, this effect accumulates: geological studies of ancient sedimentary rocks suggest that 620 million years ago, a terrestrial day lasted only 22 hours.

A planetary energy transfer in action

This gravitational dance is, in fact, a perfect example of the conservation of angular momentum. The total energy of the Earth-Moon system remains constant, but it is redistributed: the Earth’s rotational energy is converted into the Moon’s orbital energy. Every tidal current rubbing against the ocean floor, every wave breaking on a shore, dissipates a tiny fraction of energy—and imperceptibly pushes the Moon into a higher orbit. Earth’s oceans are thus the unwitting drivers of the Moon’s receding orbit.

This phenomenon is called tidal friction. It occurs throughout the solar system: it is what synchronized the Moon’s rotation with its orbit (the Moon always shows us the same face), it is what locked Mercury into a 3:2 ratio with its orbit, and it is what keeps Jupiter’s moons in a precisely synchronized gravitational dance. Tidal friction is one of the great timekeepers of the cosmos.

I find it mind-boggling that every wave on every beach for billions of years has contributed, infinitesimally, to pushing the Moon farther away. The ocean breaking on the pebbles beneath your feet is a microscopic cog in a planetary mechanism that spans billions of years. The scale of it is dizzying.

Past consequences: The Moon used to be much closer

A Giant Moon in the Sky of the Past

If the Moon is moving away at a rate of 3.8 cm per year, it must have been much closer in the past. One billion years ago, the Moon was about 20,000 kilometers closer than it is today. Four billion years ago, it was probably less than 200,000 kilometers away—barely more than half the current distance. In the sky of the early Earth, the Moon must have appeared twice as large in apparent diameter as it does today, dominating a sky likely swept by titanic tides.

These tides of unprecedented strength played a crucial role inthe geological—and perhaps biological—history of the early Earth. Tides several tens of meters higher than today’s would have churned the primordial oceans intensely, mixing nutrients and alternately exposing and submerging vast coastal areas. Some researchers suggest that this regular and intense churning of coastal zones may have fostered the emergence and increasing complexity of the earliest forms of life.

Total eclipses: a temporary luxury

One of the most spectacular effects of the Earth-Moon distance concerns total solar eclipses. By an extraordinary coincidence, the Sun and the Moon appear to be exactly the same apparent size in our sky, allowing the Moon to perfectly cover the Sun’s disk during a total eclipse. This coincidence stems from the fact that the Sun is 400 times larger than the Moon, but also 400 times farther away from us. It is a purely fortuitous balance.

However, this balance is temporary. As the Moon gradually moves farther away, its apparent diameter in our sky decreases imperceptibly year after year. According to calculations published by Space.com, in about 620 million years, the Moon will be too small to completely cover the Sun, and total eclipses will be a thing of the past. Our distant descendants—if humanity survives that long—will never again see the solar corona unfold against a darkened daytime sky. We are living at the precise moment when this is still possible.

Total eclipses are often described as the most awe-inspiring astronomical experience one can have. And realizing that we inhabit precisely the brief window of time in Earth’s history when they are possible—that is one of those facts that transforms the night sky into something deeply personal.

Future Consequences: Toward a Gravitational Lock

The Distant Fate of the Earth-Moon System

If we fast-forward into the future—well beyond the Sun’s expected lifespan—the Earth-Moon system converges toward a final state of equilibrium known as mutual gravitational locking. The Moon will move as far away as about 560,000 kilometers, and the Earth will slow its rotation until one Earth day lasts about 47 days. At that point, the Earth would always show the same face to the Moon—just as the Moon already always shows us the same face. The two bodies would be mutually locked in relative stillness.

But this future will likely never come to pass. Two major obstacles stand in the way. First, in about 1 billion years, the Sun will be bright enough to vaporize Earth’s oceans—and without oceans, there will be no more tidal friction to push the Moon away. The rate of separation will slow down drastically. Then, in about 5 billion years, the Sun will turn into a red giant, likely absorbing or radically disrupting the Earth-Moon system. The mutual locking scenario is a future doomed never to happen.

What This Means Right Now

For now, the 3.8 cm per year has no perceptible impact on our lives. The tides will remain roughly the same for millions of years. Our night sky will not visibly change on a human timescale. But this knowledge offers us something precious: the awareness that the cosmos is not static. Every object evolves, every orbit changes, every gravitational relationship transforms. The Moon we’re looking at tonight isn’t exactly the same as the one our ancestors saw ten thousand years ago—and it will be different ten thousand years from now as well.

NASA’s Artemis program, which aims to return astronauts to the Moon, plans, among other things, to deploy new retroreflectors on the lunar surface. These small mirrors will enable even more precise measurements of the Earth-Moon distance, refining our understanding of the Moon’s internal structure and the gravitational mechanics of the solar system. Fifty years after Apollo, the mirrors from 1969 continue to speak. And soon, new voices will join them.

What moves me about this story of the Moon slowly fading away is that it reminds us how fortunate we are to have been born now. Not two billion years ago, when the Moon dominated a sky of titanic storms. Not in six hundred million years, when eclipses will have disappeared. Now—at this precise moment when everything is still more or less perfectly in place.

Conclusion: A distance that tells the story of Earth

Every millimeter tells a story

Those 3.8 centimeters per year are not just a trivial figure. They tell the story of the Moon’s violent birth from a cataclysmic impact, the billions of years of friction between the ocean and gravity, the gradual slowing of our planet, and the future disappearance of total eclipses. Every laser measurement sent to the Apollo mirrors is a question posed to Earth’s past and future. And every answer—a few photons returning after a 2.5-second journey—broadens our understanding of celestial mechanics.

Look up tonight

The next time you see the Moon rise, remember: tonight, it is 3.8 centimeters farther away than it was exactly one year ago. In two billion years, it will be tens of thousands of kilometers farther away, and total eclipses will exist only in scientific archives. We’re living in the perfect window—the one where the Moon is still close enough to shape our tides, stabilize our axis, and completely cover the Sun during those rare and awe-inspiring moments known as total eclipses. That deserves a moment of gratitude as we gaze at that white disk floating in the evening sky.

By Maxime Marquette, columnist