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EDIT: Yup, totally misguided here: http://www.physlink.com/Education/AskExperts/ae429.cfm

Drifting away.

So, no lunar catastrophe for me (without some other handwavium)


I've read somewhere that the Moon's orbit will eventually degrade (maybe, it's the ocean tides that're causing orbital degradation?).

So, approximately when does this occur? I assume the downward trajectory will happen before the Moon gets close enough to be skimming Earth's rarefied atmosphere.

The only way(s) to stop it would be to speed up the moon's orbital period, or to drag it further out (is that correct?). Or, I guess?, decrease mass's gravitational pull in the Earth/Moon's area (mmm, magic)? Any other options?

When it comes crashing down, it won't be exactly like an asteroid strike - because usually those are coming in at non-trival speeds. However, it's 2,159mi in diameter, and 7.34767309 × 10^22 kilograms in mass - so big boom. But how big? How does that compare to an asteroid strike?

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    $\begingroup$ Wut? You got it wrong, the Moon is getting away from Earth, not the opposite ! $\endgroup$ – Vincent Dec 25 '14 at 2:16
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    $\begingroup$ You should write a self-answer. That was good link to read. $\endgroup$ – Peter M. - stands for Monica Dec 26 '14 at 15:25
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Assuming a lunar catastrophe does happen…

Depending on how the Moon comes crashing down, the timeline will look a little something like this.

First, the Moon will seem to grow as it comes crashing down. The growth will, over the course of this stage, be visible to the eye, eventually.

Then, tidal forces will overcome gravitational binding of the Moon, breaking it up into tiny pieces, forming a ring system around the Earth. The rings may or may not be visible with the naked eye from the surface of the Earth. There's a lot of lunar rock up there, but it'll be pretty close, and shaded by the Earth during most prime viewing hours (i.e. nighttime).

Finally, the lunar rock will decay and enter the atmosphere. Exactly how this will work out is anyone's guess, but unlike a meteor hitting the Earth, it will have fairly low energy, as in order to move from the present orbit of the Moon to an orbit that interacts with the atmosphere in any serious way a lot of energy has to go away.

The fundamental problem:

The Moon has a lot of kinetic energy. Slowing it down appreciably requires a lot of energy. If you don't want to wait for the Moon to decay on its own (which it will do, eventually, but the time scales involved are astronomical) anything you do to the Moon to slow it down will affect the Earth much more than the eventual crashing down.

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  • $\begingroup$ Could you elaborate on tidal forces breaking Moon? $\endgroup$ – user58697 Dec 25 '14 at 7:35
  • $\begingroup$ @user58697 The physics are once again complicated, but the gravitational pull of the Earth on the near side of the Moon is stronger than the pull on the far side, because the distance to the far side is greater. At some point, the difference between the near and far side gravitational pull becomes larger than the gravitational force holding the moon together, and it breaks up. $\endgroup$ – Williham Totland Dec 25 '14 at 12:38
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    $\begingroup$ @user58697 Google the roche limit, or it's been discussed a lot of times on this site too. $\endgroup$ – Tim B Dec 25 '14 at 16:51
  • $\begingroup$ You missed a big problem: As the moon comes down the tides get bigger. Before the moon hits the Roche limit those tides will be catastrophic. Nobody is going to be around to see the moon break up. $\endgroup$ – Loren Pechtel Oct 28 '17 at 4:50
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The rotation of the earth (24 hours) is faster than the moon's orbit: 28 days. This means the earth is actually accelerating the moon through a tidal leash sending it further and further away and with a longer orbit.

In 563 million years the moon will no longer fully eclipse the sun. It will be too far away.

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  • $\begingroup$ Hi Francis! This answer could probably be even better if you simply Edit to show us how you arrived at the figure 563 million years. $\endgroup$ – a CVn Oct 28 '17 at 19:17
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    $\begingroup$ I googled "when will the moon no longer eclipse the sun". My orbital mechanics is not up to calculating it from first principles. I could learn, but just saw the question, all the wrong answers and jumped in. $\endgroup$ – Francis Cagney Oct 29 '17 at 9:22
  • $\begingroup$ A good step then might be to simply link to the page that you found. Keep in mind that Google results differ for different people based on previous activity and whatever other data Google has collected on you, so you can't assume that someone googling for the same phrase (or even you, doing it from a different computer) will get the same results. $\endgroup$ – a CVn Oct 29 '17 at 10:07
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    $\begingroup$ Hi Michael, Thanks for your feedback. Here is the link I quoted: popularmechanics.com/space/moon-mars/a27824/…. However I'd suggest this detail is not relevant to the question. The question is wrong and based on a mis-conception of orbital mechanics. $\endgroup$ – Francis Cagney Oct 30 '17 at 11:22
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    $\begingroup$ We get questions every once in a while (sometimes every once in a short while) that, on closer inspection, do turn out to be based in a flawed understanding of whatever the OP is trying to do something with. Such questions may be bad because of lack of research (as proper research would have uncovered the flaw), but answers that explain why the premise of the question is flawed can be useful to future readers who wonder about the same thing. Just saying. :-) $\endgroup$ – a CVn Oct 30 '17 at 12:05

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