Some questions about the moon

[marcusI]

This seems like a knowledgeable crowd.
I heard somewhere (not verifyable) that 100,000 years ago the Earth's rotation was 23 hours. The slowdown being caused by the gravitational effect of the moon. Not knowing enough about astronomy or physics to argue, it doesn't sound too far-fetched considering that the moon causes the tides and that Earth's gravity has already stopped the moon's rotation. We hear a lot about the eventual destruction of the Earth a few billion years from now when the sun enlarges as part of it's life cycle. However, if this is true the moon sounds like a much more immediate threat. It might only take a couple million years to cause near complete destruction. An Earth that only rotates once a month will be unlikely to support much life.
Now the questions:
>Is the above true?
>What, if any, is the rate of slowing?
>If the Earth is slowed to 1 rotation a month, What will the climate be like?
>Will the Earth-moon system still be stable, or will the two collide?

[Quailman]

I've heard that they will not so much collide as the moon will come to rest on the Earth. Over the millenia, the Earth's rotation will slow, as you say, until it matches the rotational moment of the moon - the same side of the Earth will always be facing the same side of the Moon. As the Moon's orbit degrades, it will come closer and closer, but since the rotational speeds match, there won't be a catastrophic collision. The moon's inertia will keep it from crashing straight down. So eventually, it will just come in contact with the Earth at one point, and you will be able to step from one to the other.

It kind of makes it seem such a waste that we spent all those billions of dollars and lives lost sending a few men there back 40 years ago.

[Samadhi]

Answers:
1. I don't know the exact figures, but the Earth's rotation has been slowing and the moon has been receding.
2. Not sure.
3. It would suck.
4. The moon will get farther and farther away. I think that once one side is always facing the moon the distance will stabilize.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[LordKinbote]

[Courk]

[Amb]

If the moon gets too close to the earth it will absolutely destroy the dairy industry. Its a supply and demand thing.

[Thok]

[Lepton*]

Quailman is right.

The moon is getting closer. We can figure this out by looking at where the energy goes: the moon pulls on the earth and causes tides; these dissipitate a little bit of energy (and vice-versa with the earth pulling on the moon) and so the earth-moon system loses a bit of energy to friction. Since it loses energy, the moon will seem to be falling down into the earth's gravitational well.

The sad news is that the rate is pretty slow; nothing truly interesting will happen before the sun goes nova in ~5000 million years.

1. The numbers are incorrect, but the idea is valid: the earth's rotation is slowing down.
2. The rate is very slow. Too slow to be observed in any meaningful way, I think. In any case, the sun will "explode" first. (it won't actually explode)
3. It will be more like Mercury, although the atmosphere will help to soften the blow a bit. Nights will be VERY cold and days will be VERY hot. Dawn and Dust will be marked by huge windstorms, again assuming we have an atmosphere. It will probably be cold enough for the oceans to freeze during the night; if that happens, weather patterns will change in strange ways.
4. In the physics sense, the earth-moon system is not stable. However, an earth-moon meeting in this sense will probably be a somewhat-less-peaceful version of Quailman's story. The slowing down will increase as it gets close to the earth, and the atmosphere will act like water acts on a landing float plane. I would expect something like the saucer landing in Star Trek: Generations, except with a (much larger) moon instead of the forward half of the Enterprise.

I think.

[Samadhi]

I think you know better.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Dented Ford]

Round of applause for Lepton (and Quailman) for explaining something astronomic that's absolutely fascinating in a manner that's neither dumb nor overly technical, and easily brain-digested.

[Samadhi]

oh right...it's 2008 now. So it's the year of the Irony Bird
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Dented Ford]

That was not meant to be ironic

I actually like astronomy in a completely amateur way, and I like some of the abstract astronomic concepts of the way the universe works if I can understand them (some of them in a quasi-sensible Douglas Adams way). What I'm not great with is abstract equations and theoretics involving advanced physics and maths. I genuinely appreciated the explanations given.

[Courk]

*is with DF*

Though now Sam has me wondering if Qman and Lept were lying and I just don't know any better.

[worm]

i wasn't sure either way, so i cheated and used google.

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html

[marcusI]

Thanks for chiming in everybody. Unfortunately, I'm more confused than ever.
However, thanks to the link provided by worm I sent my question to NASA.
If they respond I'll post the answer here.

[Lepton*]

Oh, frig. Sorry, people who believed me.

I forgot the backreaction of the tidal bulge on the moon. Here.

In any case, the rate is still too slow to be meaningful.

[Dented Ford]

Oh frig.
I guess I'll just go back to spotting and trying to remember names of stars and constellations and the cultural/mythological stories about which ones are which for which obscure reason.
Although - that article in Wikipedia mentions that in about 2 billion years the moon and Earth will achieve spin-orbit resonance and the orbital period will be about 47 days. How much further away would that make it likely to be and how much reduction in visibility and brightness would there be? Would a full moon at that time appear significantly dimmer, maybe only not much brighter than (say) Venus? Or would there be a noticeable difference yet still far outshining anything but the sun?

[Samadhi]

[Lepton*]

...and the problem is even more difficult if you ask when it will *appear* brightest, as its "full" phase puts it very near the sun. We most often notice Venus when it is close to maximum elongation (furthest from the sun), which is when its phase is around half.

[Chuck]

There is no moon. We're seeing the reflection of the earth from some very high clouds, distorted by irregularities in the cloud patterns.

[Amb]

Which explains why Nasa had to return to the studios to film Armstrong.

[Dented Ford]

Everyone knows that Neil Armstrong's moon footage wasn't shot in a studio. It was really filmed in Iceland, round the back of a volcano.