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Author Topic: Is the Moon slowly drifting away from Earth?  (Read 6693 times)

Offline geo driver

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Is the Moon slowly drifting away from Earth?
« on: 18/02/2010 02:34:20 »
the moon makes the tides, do tides have a effect on the rotation of the earth? if so as the moon is drifting away eventually would the earth speed up, or slow down, and would we have a load of stagnant water about the place?
« Last Edit: 18/02/2010 15:56:50 by chris »


 

Offline Geezer

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Re: Is the Moon slowly drifting away from Earth?
« Reply #1 on: 18/02/2010 03:21:26 »
Em, I think the Moon might get closer to the Earth over time Geo.

I don't think the tidal sloshing (technical term) directly affects the system because there is no permanent displacement of mass. It just kind of rocks backwards and forwards. However, it seems to me that all that sloshing back and forth does generate some friction between the water molecules. That friction produces heat and, therefore, some energy was transferred from the Moon to the Earth.

Over a long time this would seem to create a certain amount of drag on the Moon which will tend slow it down and move its orbit closer to Earth.

It's likely I have overlooked some other significant factors here, so don't be surprised if my theory gets shot down in about ten minutes.
 

Offline JimBob

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Re: Is the Moon slowly drifting away from Earth?
« Reply #2 on: 18/02/2010 03:28:28 »
No - it is on it's way "ouit" as the Canadians say.

If the earth eventually looses the tides it will be a LONG time away.

« Last Edit: 18/02/2010 15:57:12 by chris »
 

Offline Geezer

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Re: Is the Moon slowly drifting away from Earth?
« Reply #3 on: 18/02/2010 05:10:48 »
Bugger! Not even ten minutes.

But wait a minute (he said before he had even bothered to check out the links provided), surely the Earth/Moon system was "warmed up a bit" and that energy was ultimately dissipated into space. IOW, there was an increase in entropy, so the kinetic energy of the system was reduced.

Also, as far as I know, all the artificial satellites will eventually crash into Earth. I'm not aware of any that will wander off into space.
 

Offline LeeE

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Re: Is the Moon slowly drifting away from Earth?
« Reply #4 on: 18/02/2010 14:52:36 »
Artificial satellites in orbit around the Earth are much closer to Earth than the moon and will eventually de-orbit because they're not entirely outside the Earth's atmosphere.  They're also much smaller than the moon, so the effects of the atmospheric friction, as small as it is, are much greater than for the moon.
 

Offline Geezer

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Is the Moon slowly drifting away from Earth?
« Reply #5 on: 18/02/2010 20:43:47 »
Thanks Dave. The Cornell site explains it well. It seems I neglected the rotation of the Earth  ;D

I suppose there must be some thermal energy lost by the earth too (although the Cornell site makes no mention of it) but presumably it's only a small effect.
 

Offline Geezer

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Is the Moon slowly drifting away from Earth?
« Reply #6 on: 19/02/2010 18:33:41 »
Yes. The Earth slows down and the Moon speeds up.

I think it's because the tidal effect vectors the gravitational force just enough so that the Earth is "pulling" the Moon slightly as it rotates.

It's a little bit like spinning a pebble attached to a string around your head. You can only make it rotate if you move your hand with a circular motion. The center of mass of the Earth is offset from its axis of rotation by the tidal effect, so the gravitational force does not act towards the axis of the Earth. If it did act in a line through the Earth's axis I think it  would be impossible for the Earth to supply energy to the Moon.
 

Offline LeeE

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Is the Moon slowly drifting away from Earth?
« Reply #7 on: 20/02/2010 07:52:57 »
Umm... the moon isn't speeding up: a higher (bigger) orbit actually corresponds to a lower orbital speed.
 

Offline Geezer

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Is the Moon slowly drifting away from Earth?
« Reply #8 on: 20/02/2010 16:51:02 »
You're right Lee. According to the Cornell dude, the Moon's orbit increases so I suppose its orbit time is increasing. But does that not also mean it is travelling through space slightly faster?  ???
 

Offline LeeE

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Is the Moon slowly drifting away from Earth?
« Reply #9 on: 21/02/2010 12:53:35 »
Aha! - its speed relative to what? ;D
 

Offline geo driver

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Is the Moon slowly drifting away from Earth?
« Reply #10 on: 22/02/2010 22:17:43 »
so in a long long time to come will the earth slow down, or speed up?  with out the moon to keep it in check, and with a last "sling shot" perhaps would the momentum increases as there is nothing holding onto it ? or would the earth slow down just in time for the sun to become so large that it consumes the earth?
 

Offline geo driver

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Is the Moon slowly drifting away from Earth?
« Reply #11 on: 22/02/2010 22:20:41 »
ok too admit i heared in passing the darwin thing, but did not know what it ment cheers guys, although i have not a clue what dave was going on abouit "... as the canadians would say

or aboot as the scots would
 

Offline Geezer

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Is the Moon slowly drifting away from Earth?
« Reply #12 on: 23/02/2010 08:14:42 »
Kepler's Third Law is the relationship between the period, P, and the semi-major axis, a. 

P² = a³

where the square of the period (in years) equals the cube of the semi-major axis (in Astronomical Units). Likewise,

P = √a³

The angular velocity is the inverse,

ω = 1/P = 1/√a³

and the linear speed is

v = ωr = ωa = 1/√a


So, both angular and linear velocity are the inverse to a power of the distance from the central body.

Thanks Dave! Got it (I think!)
 

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Is the Moon slowly drifting away from Earth?
« Reply #12 on: 23/02/2010 08:14:42 »

 

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