Hey people
I heard from a freind that the moon was very slowly slipping away from earths gravity, about 4 cm a year or something, is this true or a load of nonsense ? if so does anyone have any info on the subject.
Thanks

the moon is indeed moving away... there was athread on here somewhere, but can't find it.

But nothing to worry about

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Yeah, that's what Apollo 11 found - it slips away at 4 cm per year, but the Moon is still going to be around for a very long time to come.

really,awsome stuff cheers for confirming guys. its quite upsetting really lol

This is true, but the moon isn't leaving. It will slowly move away from us until the Earth gets tidally locked with it, and then it will start coming back to us even more slowly. Some people indicate that it will break apart and form spectacular rings when it gets very close, but the Sun should swallow both the Earth and Moon up before it can get that close.

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ah i see, interesting , cheers

For what reason will this latter phenomenon occur? (emphasis mine) A tidally locked system is in equilibrium, barring external forces and effects of general relativity. It begs for a bit of explanation.

It all happens because of the Roche limit.

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I think it has to do with the gravitational interactions with the sun...

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I think it is general relativity - gravitational waves and all that. Either that or tidal effects of the Sun on the Earth-Moon system. Antoniseb?

A recent model of the future activity of the Sun that I saw indicates that, due to mass loss from the Sun, the Earth's orbit will have moved outward sufficiently to avoid being engulfed, even though the radius of the red-giant Sun will be greater than the Earth's present-day orbit.

I always wondered why the earth's rotation was slowing down. Thanks.

No, it's not gravitational waves. That would take far, far longer than the age of the Universe!

It has to do with tides from the Sun. I am too tired to explain it right now, but this page of mine explains why the Moon is moving away and we're slowing down.

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Hi BA (and everyone else),

I am trying to understand the scenario related by antoniseb above that after the Moon becomes tidally locked, it would somehow drift back toward the earth such that it would pass within Earth's tidal radius (Roche limit). Your page describes the evolution of the Earth/Moon system until it becomes tidally locked - which I understood. It's the other bit I was asking for substantiation.

Can anyone elaborate?

(and I was being a bit "flip" with my suggestion that GR might play some role in this particular case, since for the Earth/Moon system, orbit decay via gravitational waves would take nearly forever - as you rightly pointed out.)

It's like the_shaggy_one says, the tidal interaction of the sun on the locked earth/moon system slowly brings the moon back to the earth. But that will take a long long time

If the Moon moves farther away than about 1.26 times its present distance, solar tides will be stronger than lunar tides. Earth will slow (and the Moon will move outwards) until Earth is tidally locked with the Moon, but then the Earth will keep slowing because of solar tides. This will push the synchronous orbital radius out beyond the Moon's current position, and the torque from the lagging tidal bulge it generates on the Earth's surface will now serve to bring the Moon back inwards again.
Over on this thread I gave John Dollan a reference to a paper that discusses the whole sequence of evolution for hypothetical moons of giant extrasolar planets.

Grant Hutchison

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Why, thank you very much!!

It will take the moon about 50,000,000,000 years to make the journey from tidal lock, back to earth. In about 5,000,000,000 years, (a) the sun will do its red-giant thing and may (or may not) swallow the earth, and (b) M31 will simultaneously collide with the Milky Way, further complicating matters. I don't think the earth-moon system will remain intact long enough for our offspring moon to return home.

Thanks for the specifics Tim!

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Queue the music for Space 1999

Thanx,

I'll be sure not to make plans for that weekend!!!

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Michael