I was reading articles on the Moon and I came across this article which states that the Moon moves away from Earth at the rate of 2.5 CM/Year whereas in the other article it states that it moves away at the rate of 1.6 Inches (4 CM)/Year.

Is the moon moving away? Did they prove this behavior? If yes, than what's the real rate of moving?

  • @drat thats the reason I asked, how they can possible calculate such a small distance...
    – Mr. Alien
    Jun 20, 2014 at 7:30
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    Thanks to very short laser pulses they can have incredible accuracy. This experiment claims to have an accuracy of 15 mm in detecting the lunar orbit.
    – drat
    Jun 20, 2014 at 7:33

1 Answer 1


They use laser ranging to determine the distance of the moon (http://physics.ucsd.edu/~tmurphy/apollo/apollo.html). They bounce Earth-based lasers off of reflectors left on the moon by the Apollo astronauts.

"Measurements show that the Moon is receding from Earth at a rate of about 3.8 centimeters per year." (http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html)

Here is the original paper publishing the results: http://physics.ucsd.edu/~tmurphy/apollo/doc/Dickey.pdf.

This level of accuracy is possible because they are bouncing a laserbeam off of mirrors on the moon. They know how fast light travels. They know the geometry of the beam's path. So, they can work out the Earth-moon distance.

  • 4
    It is, perhaps, useful to add that the mechanism of this behavior is known. The moon's differential gravitation pulls the oceans of the world out of their natural shape and if there wasn't a lot of land in the way, this would result in two neat bulges along the line from the Earth to the moon; Earth's rotation creates a frictional force that tries to pull the bulges ahead of the that nominal line and the moon pulls them back. But if the moon pulls them back, they pull the moon forward: energy and angular momentum transferred (ever so slowly) from the Earth's rotation to the moon's orbit. Jun 20, 2014 at 15:27
  • @dmckee That's also related to how the moon got locked with one side facing the Earth, right?
    – user5582
    Jun 20, 2014 at 18:27
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    @Articuno It is the mechanism of tidal locking, though in the case of the moon we may (I'm unsure of the time frame and how long the moon may have retained a liquid core) be talking about plastic dissipation in a deforming solid rather than dissipation by fluid drag which is the main effect slowing the Earth. Jun 20, 2014 at 20:16

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