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🌘 Why the Moon is drifting away from Earth

The Moon retreats about 3.8 centimetres a year, and we know that because Apollo left mirrors on it and observatories have been firing lasers at them since 1970. But run that rate backwards and the Moon touches Earth 1.6 billion years ago —

5
lessons
~18 min
to learn
Adults
level
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What you’ll learn

  1. The measurementShow how the Moon's recession is actually measured, and how hard that measurement is.Five reflector arrays left on the Moon return laser pulses to a handful of observatories, giving the distance to about a millimetre.
  2. The ocean does the pushingTrace the mechanism from a late tidal bulge to a widening lunar orbit.Earth's rotation drags the tidal bulge ahead of the Moon, and the bulge's gravity tugs the Moon forward into a wider, slower orbit.
  3. The day is getting longerFollow the other half of the exchange — the energy budget, and the slowing of Earth's spin.Tides dissipate 3.7 terawatts, only about three per cent of it reaching the Moon, and the day lengthens by roughly two milliseconds a century.
  4. Running the tape backwardsExplain why today's rate cannot be extrapolated into the past, and what the rocks record instead.A constant-rate rewind puts the Moon at Earth 1.6 billion years ago, far too recent; today's continents simply make unusually effective tides.
  5. What actually happens nextSeparate what the drift really changes from what it does not.Tidal locking holds and the Moon keeps its face to us, but total solar eclipses are on a very long clock.

Questions this course answers

About how fast is the Moon receding from Earth?

Lunar laser ranging gives 38.30 ± 0.09 millimetres per year — close to the rate fingernails grow.

How is the Earth–Moon distance actually measured?

Observatories fire a laser at retroreflector arrays left by Apollo and Lunokhod, and time the round trip.

What makes a corner-cube prism so useful on the Moon?

Three mirrored faces at right angles send light straight back to its source, so the array never needs aiming.

Why does Earth's tidal bulge sit slightly ahead of the Moon?

Earth turns once a day while the Moon takes a month, so the spinning planet carries the bulge ahead of it.

What does the offset bulge do to the Moon?

The displaced water is mass sitting ahead of the Moon, and its gravity pulls the Moon forward.

A forward tug adds energy to the Moon's orbit. What happens to the Moon's speed?

Added orbital energy means a larger orbit, and a satellite in a larger orbit travels more slowly.

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