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Measured by laser since 1969

The Moon Is Leaving, Four Centimetres a Year

Every year the Moon's orbit gets a little wider. The energy for that is coming out of the length of our day.

Earth's tidalbulge, runningslightly ahead Tows the Moonforward, andoutward
The Moon is not drifting away on its own. Earth is pushing it.

  1. Step 1 of 5

    The Moon raises a bulge, and Earth spins out from under it.

    The Moon pulls the water up Earth spinsfaster than theMoon orbits So the bulge is dragged out in front And it neverquite catchesupocean

    The Moon's pull lifts the oceans into a bulge on the near side. But Earth turns once every 24 hours and the Moon takes a month to go round, so Earth keeps dragging that bulge forward, out ahead of where the Moon actually is.

  2. Step 2 of 5

    A bulge out in front is a hand pulling from ahead.

    The bulge,running ahead Pulled veryslightly forwardsSpeeding a satellite up raises its orbit.

    That heaped-up water has mass, so it pulls on the Moon too. And because it sits ahead of the Moon, its pull has a forward component. The Moon is being tugged along its own orbit, slightly, forever.

    This is counter-intuitive: adding speed to an orbit makes it wider, and a wider orbit is a slower one. The Moon ends up further away and moving more slowly.

  3. Step 3 of 5

    The energy has to come from somewhere, and it comes from our day.

    The Earth-Moon systemEarth's spin slows: ~1.8 ms per centuryThe Moon's orbit widens: ~3.8 cm per year
    One is paying for the other.

    Dragging that bulge against the Moon's pull is friction, and it acts as a brake on Earth's spin. The Moon gains orbital energy; Earth loses rotational energy. The books balance.

    Fossil corals lay down daily growth bands. Counting them in 400-million-year-old coral gives a year of about 400 days — so the day really was about 22 hours long.

  4. Step 4 of 5

    We know the number because there are mirrors up there.

    1 Fire alaser pulseat the MoonKilometreswide onarrival2 It hits amirrorpanelLeft byApollo andSoviet craft3 A handfulof photonscome backAbout 2.5secondslater4 Halve it,times lightspeedDistance tothemillimetre
    Fifty-odd years of that measurement is where 3.8 cm a year comes from.

    Apollo 11, 14 and 15 left panels of corner reflectors on the surface, and two Soviet rovers carried more. A corner reflector sends light straight back where it came from, whatever angle it arrives at. Observatories have been firing lasers at them ever since.

  5. Step 5 of 5

    Four centimetres a year is nothing, and it adds up to everything.

    In a year3.8 centimetresIn a human lifetimeAbout 3 metresIn a million years38 kilometresSince the dinosaursThousands ofkilometres

    It is roughly how fast a fingernail grows. But it has been happening since the Moon formed. That is why the Moon once loomed enormous in the sky. It is also why eclipses will stop working in the far future: the Moon will be too small to cover the Sun.

The short version

Earth's spin drags the tidal bulge ahead of the Moon, and that bulge tows the Moon outward by about 3.8 cm a year — paid for by our day getting slowly longer.

Try it yourself

Swing a bag on a string in a circle, then pull the string in. It speeds up. Let it out and it slows down. Orbits work the same way, which is why a Moon that gains energy ends up further out and slower.

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