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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?

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I don't think it's a good question for Skeptics. I don't see where scepticism can be applied here as there is no real opposition to the statement. This is a well-known fact and quite frankly easily googlable. This is like asking "is sun surface temperature REALLY 5000C?", well, basically any question with "really" added to it. –  sashkello Jun 20 '14 at 7:13
@sashkello As long as many people believe the claim, it is notable enough for us to answer it here. –  user5582 Jun 20 '14 at 7:17
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 '14 at 7:33
@Articuno Again, that basically means I can put any, literally ANY statement here, add "really" to it and it will become a good fit for Skeptics? I am not convinced... Maybe it should be discussed on meta. –  sashkello Jun 20 '14 at 8:31
I believe this should be migrated to Physics.SE. The accepted (and thus far only) answer here is no different from what would be given there. The question offers no objections to the claim or reasons why it should be disbelieved; if it did, then it might well be appropriate here (and then any answer could address those objections, befitting a skeptics site). As it is, there is nothing "skeptical" about the question or answer. ... –  iamnotmaynard Jun 20 '14 at 17:48

1 Answer 1

up vote 20 down vote accepted

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.

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@Cruncher No, current telescopes aren't big enough to obtain that high a resolution. You'd need about a 25 metre Earth-based telescope to be able to resolve even the LM, and decidedly bigger to make out anything smaller. The Giant Magellan Telescope coming online in 2020 might be able to do it. Or failing that, the Thirty Meter Telescope or the European Extremely Large Telescope slated to go operational in 2022 should be big enough. Or we can always wait for the Oppressively Colossal Telescope. –  Compro01 Jun 20 '14 at 14:27
@Cruncher We do have some pretty pictures of landing sites from the Lunar Reconnaissance Orbiter though. –  Compro01 Jun 20 '14 at 14:27
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. –  dmckee Jun 20 '14 at 15:27
@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. –  dmckee Jun 20 '14 at 20:16
I think the Moon is in a stable orbit, but the lander is causing that part of the Moon to deflate. We popped the Moon, people! –  Larry OBrien Jun 21 '14 at 19:56

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