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Did you know? The Moon is drifting away from Earth, about 3.8 cm a year

Scientists measure the Moon's distance with lasers and mirrors left by Apollo astronauts. It is receding 3.8 cm a year. Here is why, and what it means.

By TAU TAK? Team Published 5 min read
A bright, sharp full Moon with its craters and dark plains clearly visible, the Moon that is slowly moving away from Earth
Image: Gregory H. Revera / Wikimedia Commons, CC BY-SA 3.0
In this article
  1. How do scientists know? They fire lasers at the Moon
  2. Harder than finding a needle in a haystack
  3. Why is the Moon drifting away?
  4. Longer days, but don't get excited yet
  5. Goodbye, total solar eclipses (eventually)
  6. The bottom line

The Moon you saw last night is actually a little farther away than it was a year ago. It's not sulking. According to NASA, the Moon is moving away from Earth at about 3.8 cm a year.

Tiny? Very. Clemson University in the US compares it to the rate at which our fingernails grow. But add it up over millions of years and the story changes.

How do scientists know? They fire lasers at the Moon

Yes, actual lasers. The technique is called Lunar Laser Ranging.

In July 1969, the Apollo 11 astronauts set up a special mirror panel in the Sea of Tranquility. It's called a retroreflector, and it bounces light straight back toward wherever it came from.

  • The Apollo 11 panel holds 100 fused silica corner cubes, each 3.8 cm across. Funnily enough, that's the same as the distance the Moon drifts each year.
  • Apollo 14 left another 100-cube panel, while Apollo 15 carried a bigger one with 300 cubes.
  • The panels need no power at all. More than 50 years on, they still work.

Observatories on Earth fire laser pulses at these panels and time how long the light takes to make the round trip. From that, the Moon's distance can be worked out to within a few millimetres.

The square Apollo 11 retroreflector panel with its grid of small mirrors, sitting on the dusty lunar surface
Image: NASA / Wikimedia Commons, Public domain

Harder than finding a needle in a haystack

It sounds simple, but it's seriously tricky:

  • By the time it reaches the Moon, the laser beam has spread to about 6.4 km (4 miles) wide.
  • Even in good conditions, only about one photon of light makes it back every few seconds.
  • The average distance between the centres of Earth and the Moon is about 385,000 km.

According to NASA JPL, the observatories doing this work are in New Mexico (US), France, Italy and Germany.

Did you know? The Soviet Union also sent mirrors to the Moon, aboard the Lunokhod 1 rover in 1970. Its French-built reflector went "missing" after 1971. In 2010, physicist Tom Murphy of UC San Diego and his team found it again using the 3.5-metre telescope at Apache Point Observatory in New Mexico, and got about 2,000 photons back on their very first try.

Why is the Moon drifting away?

The answer: ocean tides.

The Moon's gravity pulls on Earth's oceans, raising tidal "bulges". The interesting part is that these bulges don't sit directly beneath the Moon. According to NASA JPL, the highest tide is actually slightly east of the Moon.

The gravitational pull between that bulge and the Moon does two things at once:

  • Earth's spin slows down. Our days get longer.
  • The Moon gets nudged into a slightly wider orbit, about 3 mm a month.
Two laser beams streaking across a starry night sky toward the Moon from NASA Goddard's laser ranging facility
Image: NASA Goddard Space Flight Center / Wikimedia Commons.jpg), Public domain

Longer days, but don't get excited yet

If you're hoping for extra sleep, hold on. According to a 2018 study in PNAS by Stephen Meyers (University of Wisconsin-Madison) and Alberto Malinverno (Columbia University), Earth's days are getting longer by about 2 milliseconds every 100 years.

On geological timescales, though, that adds up. The same study estimates that 1.4 billion years ago, a day on Earth lasted only about 18 hours. Imagine trying to finish a work week in days that short.

Goodbye, total solar eclipses (eventually)

This is the coolest consequence. Total solar eclipses happen because the Moon just happens to look big enough in our sky to cover the whole Sun. As the Moon moves away, it looks smaller.

According to Sky & Telescope, citing calculations by astronomer Jean Meeus:

  • The Moon has to move about 23,000 km farther away (at its closest point) before it can no longer fully cover the Sun. At 3.8 cm a year, that takes more than 600 million years.
  • Starting about 620 million years from now, total eclipses become "on-again, off-again".
  • The last total solar eclipse is expected about 1.2 billion years from now.

So there's plenty of time to chase eclipses. No need to panic-buy a plane ticket tomorrow.

The bottom line

The Moon isn't going to "run away" any time soon. But every time you trim your nails, remember that the Moon has drifted roughly that far from us too. And we only know this thanks to a few mirror panels astronauts left behind more than 50 years ago.

If you could leave one thing on the Moon for future scientists, what would it be?

Sources

  1. NASA JPL, The Apollo experiment that keeps on giving (24 Jul 2019)
  2. NASA Goddard (Eclipse Web Site), Measuring the Moon's Distance
  3. Space.com, Lost Soviet reflecting device rediscovered on the Moon
  4. Sky & Telescope, When will the last total solar eclipse occur?
  5. Discover Magazine, The Moon's gradual retreat is lengthening Earth days
  6. Clemson University News, National Moon Day: Mirrors on the moon and four other facts

Tags#moon#space#apollo#science

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