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-   -   Will the Moon escape Earth's orbit in billions of years? (http://www.physicsforums.com/showthread.php?t=364921)

 Richard87 Dec21-09 04:50 PM

Will the Moon escape Earth's orbit in billions of years?

I've heard that the moon is very slowly moving away from Earth. Will it eventually escape Earth's orbit?

 mgb_phys Dec21-09 05:09 PM

Re: Will the Moon escape Earth's orbit in billions of years?

No, the moon will only move away from Earth until it reaches an orbital resonance when the length of the Earth's day and the Moon's month will be equal - that's in about 50bn years.
In theory the moon could then move closer to Earth, but since they are both going to be consumed by the sun before that - it's a bit academic.

 SHAMSAEL Dec24-09 10:48 AM

Re: Will the Moon escape Earth's orbit in billions of years?

Quote:
 Quote by mgb_phys (Post 2501501) No, the moon will only move away from Earth until it reaches an orbital resonance when the length of the Earth's day and the Moon's month will be equal - that's in about 50bn years. In theory the moon could then move closer to Earth, but since they are both going to be consumed by the sun before that - it's a bit academic.
It would also be worth mentioning that the 50 billion years it would take for this to happen is longer than the current age of the universe.

 fluidistic Dec24-09 03:40 PM

Re: Will the Moon escape Earth's orbit in billions of years?

Quote:
 Quote by mgb_phys In theory the moon could then move closer to Earth, but since they are both going to be consumed by the sun before that - it's a bit academic.
Could you please shed some light on this assertion?
You involved something like "resonance", knowing only resonance in circuits, I think about it as the time where the Moon is the farther from the Earth. But I don't understand why it would then come closer to it, considering tides effects, i.e. some "loss" of gravitational energy in detriment to friction. Feel free to correct me if I said something wrong. My goal is to learn.

 mgb_phys Dec24-09 03:58 PM

Re: Will the Moon escape Earth's orbit in billions of years?

The Earth isn't a perfect sphere, it has bulges around the equator (and the moveable water in the oceans) this means that the moon doesn't simply go around a point. it feels a stronger pull from one side of the earth than the other, ie from the nearer bulge.
The effect of this is to slow the Earth's spin slightly and transfer this angular momentum into the moon, which makes the moon move further away.
Eventually enough of the Earth's spin will have been transferred and the moon will have moved far enough away that it's month (it's orbit around the earth) and the Earth's rotation are the same.
At this point the bulges on each side of Earth are at the same distance and no longer have any effect.

Now any angular momentum that is lost from the system, to interactions with passing comets, or the other planets will decrease the moons distance.

ps. Orbital mechanics , especially tidal effects, are complicated and not very interesting/useful plusit's long time since I had to learn any of this junk, so someone else could probably explain it better !

 fluidistic Dec24-09 05:05 PM

Re: Will the Moon escape Earth's orbit in billions of years?

Quote:
 Quote by mgb_phys (Post 2504891) The Earth isn't a perfect sphere, it has bulges around the equator (and the moveable water in the oceans) this means that the moon doesn't simply go around a point. it feels a stronger pull from one side of the earth than the other, ie from the nearer bulge. The effect of this is to slow the Earth's spin slightly and transfer this angular momentum into the moon, which makes the moon move further away. Eventually enough of the Earth's spin will have been transferred and the moon will have moved far enough away that it's month (it's orbit around the earth) and the Earth's rotation are the same. At this point the bulges on each side of Earth are at the same distance and no longer have any effect. Now any angular momentum that is lost from the system, to interactions with passing comets, or the other planets will decrease the moons distance. ps. Orbital mechanics , especially tidal effects, are complicated and not very interesting/useful plusit's long time since I had to learn any of this junk, so someone else could probably explain it better !
Thank you very much. I should sharpen a bit more my English. I've understood something, I just hope it's what you mean.
By the way, it might look like something uninteresting to you, but I assure you that not knowing what really happens is frustrating and makes your curiosity growing! So I don't find this uninteresting. :biggrin:

 mgb_phys Dec24-09 05:08 PM

Re: Will the Moon escape Earth's orbit in billions of years?

Quote:
 Quote by fluidistic (Post 2504938) By the way, it might look like something uninteresting to you, but I assure you that not knowing what really happens is frustrating and makes your curiosity growing!
The concept is interesting, it's just all the equations you have to learn, and the complications of the reference frames and which angles are measured from where, and so on that is tedious.

The wiki article on lunar orbit is a good starting point.

 Janus Dec24-09 06:28 PM

Re: Will the Moon escape Earth's orbit in billions of years?

Quote:
 Quote by fluidistic (Post 2504881) Could you please shed some light on this assertion? You involved something like "resonance", knowing only resonance in circuits, I think about it as the time where the Moon is the farther from the Earth. But I don't understand why it would then come closer to it, considering tides effects, i.e. some "loss" of gravitational energy in detriment to friction. Feel free to correct me if I said something wrong. My goal is to learn.
It is tidal interaction that causes the Moon to recede. The Moon raises tidal bulges on the Earth. Friction between the bulges and the rotating Earth drag the tidal bulges out of alignment with the Moon. The out of line bulges pull forward on the Moon, giving it more energy which lifts it into a higher orbit. The Earth slows its rotation in response.

This all happens because the Earth rotates faster than the Moon orbits. So when the Earth slows enough to match its rotation rate to the Moon's orbit, the situation stabilizes.

However, the Sun still raises tidal bulges on the Earth, which continue to slow the Earth's rotation. Now the Earth is rotating slower than the Moon orbits. Again, friction between the lunar tidal bulges and the Earth come into play. This time however, the bulges lag behind the Moon, pulling it backward, and into a lower orbit.

This interplay between Solar and Lunar tides will continue forcing the Moon into a lower and lower orbit.

 fluidistic Dec24-09 07:21 PM

Re: Will the Moon escape Earth's orbit in billions of years?

Quote:
 Quote by Janus (Post 2505009) It is tidal interaction that causes the Moon to recede. The Moon raises tidal bulges on the Earth. Friction between the bulges and the rotating Earth drag the tidal bulges out of alignment with the Moon. The out of line bulges pull forward on the Moon, giving it more energy which lifts it into a higher orbit. The Earth slows its rotation in response. This all happens because the Earth rotates faster than the Moon orbits. So when the Earth slows enough to match its rotation rate to the Moon's orbit, the situation stabilizes. However, the Sun still raises tidal bulges on the Earth, which continue to slow the Earth's rotation. Now the Earth is rotating slower than the Moon orbits. Again, friction between the lunar tidal bulges and the Earth come into play. This time however, the bulges lag behind the Moon, pulling it backward, and into a lower orbit. This interplay between Solar and Lunar tides will continue forcing the Moon into a lower and lower orbit.
Thanks. Very nice explanation, I get it.

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