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Orbit Knowledge

The Moon Is Drifting Away From Earth by About the Width of a Fingernail Every Year

Lasers bounced off mirrors that Apollo astronauts left on the Moon show the gap growing by about 1.5 inches, or 3.8 centimeters, a year. The push comes from Earth ocean tides.

By Orbit Post2 min read
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The Moon Is Drifting Away From Earth by About the Width of a Fingernail Every Year

Key points

  • The Moon is leaving, slowly.
  • Apollo 11, 14 and 15 astronauts left arrays of small mirrors, called retroreflectors, on the lunar surface.
  • The cause is the tide.

The Moon is leaving, slowly. Measurements taken over more than 50 years show it drifting away from Earth at about 1.5 inches, or 3.8 centimeters, a year. That is close to the speed at which a fingernail grows.

A ruler made of light

Apollo 11, 14 and 15 astronauts left arrays of small mirrors, called retroreflectors, on the lunar surface. The arrays need no power. Observatories in New Mexico, France, Italy and Germany fire laser pulses at them and time how long the light takes to come back. Because the speed of light is known, the timing gives the distance to within a fraction of an inch. NASA Jet Propulsion Laboratory scientists say those measurements show the distance growing by 1.5 inches a year. The Moon sits about 239,000 miles, or 385,000 kilometers, away on average, so the yearly change is tiny against the whole gap. Year after year of readings makes the drift certain.

Tides do the pushing

The cause is the tide. The Moon gravity pulls Earth oceans into bulges. Earth spins faster than the Moon orbits, so rotation drags those bulges slightly ahead of the Moon position. The bulges have gravity of their own, and that pull tugs the Moon forward in its orbit. A body pushed forward in orbit gains energy and moves into a wider path. NASA explains the same exchange from the other side. Energy added to the Moon orbit comes mainly from Earth oceans and from Earth rotation. As the Moon gains, Earth loses a little spin. Our day grows longer by a very small amount, far too small to notice in a lifetime, but real in the record.

It was not always this slow or this far

The Moon formed much closer to Earth, and early days on Earth were far shorter than 24 hours. Rock layers laid down by ancient tides, and the growth rings of fossil corals and shells, record days of well under 24 hours in the distant past. The drift rate has also changed over time as oceans, continents and sea levels changed shape. The 3.8 centimeter figure is the present day rate, measured directly, not a rate assumed to hold for all of Earth history.

What changes, and what does not

Nothing dramatic follows for anyone alive now. Total solar eclipses offer one long term change. The Moon now fits the Sun disk in our sky almost exactly, which is why total eclipses look the way they do. As the Moon moves farther away, it will appear smaller, and in the very distant future total eclipses will stop and only ring shaped eclipses will remain. That day sits hundreds of millions of years off. For now, the fact is simpler and stranger. A panel of mirrors left on the Moon in 1969 is still working, and it says our nearest neighbor is a fingernail width farther away than it was last year.

Sources