07-06-2003 04:15									#1
	Is the moon spinning off into space? Would it be better nearer or further away? Increase Your PM Length

I watched a show a while back about the early Earth and the beginning of life. In it there was a quick mention of how the moon used to be closer to the earth, creating greater tidal effects. So, the moon is moving away from the Earth. (Just remembered that it's not spinning)

Is that a good thing? Sure, eventually it might go wandering off, fall into the sun or set itself up as a planet on the same plane as the Earth. That would be a bummer. Before it wanders off its gravitational effect would become less. That means less tidal effect, smaller waves, perhaps less earthquakes?

Would that be good? Is the optimum distance from the earth still to come or come and gone?

	07-06-2003 04:23									#2
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We'll probably do the job first, but before the sun goes red giant and vaporizes us, the moon will kill us all off, since it's tidal locking us faster than it's going away.

	07-06-2003 04:25									#3
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What's "tidal locking" Mike?

	07-06-2003 07:15									#4
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.

Last edited by Lancer on 07-06-2003 at 08:15

	07-06-2003 07:48									#5
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Well considering that most major environmental changes are bad for the more advanced species, I think it would be better for us humans if the moon just stays where it is.

	07-06-2003 08:09									#6
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quote: You pulled 'tidal locking' right outa your ass you yankee gringo you!

quote: (Just remembered that it's not spinning)

Lancer, you have referred to the phenomenon of tidal locking. The reason the moon no longer spins is because the Earth has tidally locked the moon so that one side always faces the Earth.

The moon also slows down the Earth's rotation, but because the Earth is quite a bit more massive, the tidal locking takes much more longer.

I want to see MtGs calculations that show the moon tidally locking the Earth faster than the moon will drift away from the Earth.

Remember, you have to take into account, that as the moon moves farther away, there will be less tidal friction, and the earth will slow down less quickly.

	07-06-2003 08:18									#7
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Sounds like the days will keep getting longer... There ought to be 2 habitable bands around the edges of a locked earth though?

	07-06-2003 17:32									#8
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quote: Originally posted by obiwan18 I want to see MtGs calculations that show the moon tidally locking the Earth faster than the moon will drift away from the Earth. Remember, you have to take into account, that as the moon moves farther away, there will be less tidal friction, and the earth will slow down less quickly.

The Moon gains the energy to move to a higher orbit by taking it from the Earth's rotational energy. Simply conservation of angular momentum and energy. I'm not sure how much effect friction of oceans has on the dynamics though.

The Earth-Moon orbit coupling will be complete when an Earth day is as long as a Lunar month. I seem to recall that would be 56 days long (this may be way off though). If the Moon can have a stable orbit with that rotation period, which I'd imagine it can, then we won't lose the Moon.

If we did lose the Moon for whatever reason there would be more wobbling of the Earth's rotational axis, leading to extreme climate variations.

	07-06-2003 20:38									#9
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quote: Originally posted by Lancer Sounds like the days will keep getting longer... There ought to be 2 habitable bands around the edges of a locked earth though?

There's two places the energy goes from the braking of the earth's rotation - one is change in the moon's orbital distance, the other is heat here on the earth. Unless you like hot steam baths, earth won't be a nice place. This is something on the order of hundreds of millions to a billion years, though, so I'm sure if people haven't killed each other off, we'll have done something else to each other.

Obiwan - any pair of rotating objects in which one orbits the other will tidally lock each other eventually (the operative word here), as a consequence of conservation laws. While they're rotating at different angular rates from the revolution, there is a tidal locking effect. AFAIK, the only ways out of that situation is if there's an unstable orbit due to interactions with other objects, or if one of the objects changes do to collisions, stellar evolution, or what have you.

Tidal locking is fairly common - Mercury is locked to the sun, at IIRC a couple of the Galilean moons are already locked to Jupiter, and there should be a few more, but I don't know how much research work has gone into tracking the rotational periods of minor satellites in our solar system. I suspect not much, since there's not a lot of observational data.

The more massive object locks the less massive object first, but the relationship is fairly complex. It has to address sums and differences in vectors, so you need to account for not only mass but diameter, as well as distance. So if you've got two black holes, with diameters of a few centimeters, you have to be patient.

Mercury would not lock the sun (ignoring the sun's evolution, which would happen beforehand anyway) since it's internally driven convection contributes far more to the sun's internal motion that does Mercury's rather tiny mass and net difference of tidal gravitational vectors.

With the earth-moon pair, the earth's core convection doesn't count much, if at all, because it's not primarily an internally driven process. The relative masses, distance and diameters are close enough between earth and moon that both objects are likely to survive undisturbed long enough for mutual tidal locking to occur.

	07-06-2003 20:44									#10
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quote: Originally posted by Nacht Well considering that most major environmental changes are bad for the more advanced species, I think it would be better for us humans if the moon just stays where it is.

I agree, to the extent that if it isn't broken why fix it - but I think we are better off having an understanding of how the system works just in case it needs a kick in the right direction.

	07-06-2003 21:27									#11
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Yes
The moon would eventually tidal lock the Earth is true.
But I seem to recall that the the timeframe was very long, even in astronomical terms....

	07-06-2003 21:36									#12
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The shortest estimate I've seen is ~350 million years, assuming a fairly simple model of the earth's interior. (The inner core is rotating faster than the the mantle & surface already, due to internal convection and the amount of tidal braking that has occurred so far.

	07-06-2003 21:44									#13
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This would just mean that the earth will have 30*24 hour long days, not that it will completely stop.

	07-06-2003 21:50									#14
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quote: Originally posted by MichaeltheGreat We'll probably do the job first, but before the sun goes red giant and vaporizes us, the moon will kill us all off, since it's tidal locking us faster than it's going away.

For some reason I find that sad. We're gonna have to kill our own moon in self defense.

	07-06-2003 22:03									#15
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Kid, if we want to survive, we've got to think big.

I say, chop the moon off, and build asteroid sized spaceships.

	07-06-2003 23:27									#16
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quote: Originally posted by Jaguar Warrior For some reason I find that sad. We're gonna have to kill our own moon in self defense.

That would be even nastier. One of the biggest reasons that the glancing collision hypothesis was finally adopted for the origins of the moon is that it's the best explanation for the rather large difference between the earth's surface rotation and the rotation of the earth's core. The difference between magnetic pole and rotational axis, plus the degree of wobble in the earth's rotation, would make for a tectonically and climatologically really, really nasty place.

	08-06-2003 02:44									#17
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We could cause the moon to spin again using a couple of Death Star engines. If we got it to spin really fast, the Earth would speed up too? Does the deal work the other way round?

	08-06-2003 03:23									#18
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You'd be real hard pressed to keep it in a stable orbit. (speed is a function of orbital distance.) You'd just mess up the surface, too.

	08-06-2003 03:30									#19
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You just leave that to me.

	08-06-2003 03:31									#20
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So we're doomed?

	08-06-2003 03:33									#21
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Yup

	08-06-2003 03:34									#22
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Screwed silly.

	08-06-2003 03:35									#23
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Put that way, it doesn't sound so bad.

	08-06-2003 03:38									#24
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Like all of astronomy, it depends on the objects involved.

	08-06-2003 05:05									#26
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If this were even just ten million years away, I'm sure that human technology could in at most a million years have figured out a way to solve the problem...

	08-06-2003 06:06									#27
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Unless conservation laws are wrong, (for which there's absolutely no evidence, and we use them all the time), there simply is no solution.

We wouldn't be here at all, in all likelihood, if the moon hadn't gotten formed when some other now departed Mars-like planet played got to play billiard ball with the earth, courtesy of Jupiter.

	08-06-2003 07:49									#28
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Balls. Random stellar billiards brought this about, people will bring about a fix.

	08-06-2003 12:24									#29
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quote: So if you've got two black holes, with diameters of a few centimeters, you have to be patient.

Thanks for the improved explanation, MtG.

One effect of the Moon slowly drifting away from the earth is that there will eventually be no eclipses of the sun.

So if you are an eclipse hunter, it is bad for the moon to move farther away from the sun.

	08-06-2003 15:08									#30
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quote: Originally posted by MichaeltheGreat There's two places the energy goes from the braking of the earth's rotation - one is change in the moon's orbital distance, the other is heat here on the earth. Unless you like hot steam baths, earth won't be a nice place.

I don't see your point there. The Earth has been slowing down its rotation rate since the first moonrise. Its not heated up at any noticeable rate, and I don't see why it would start now.