Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> The moon is also receeding from the earth due to tidal forces.
> 
>   But isn't this recession decelerating? It will stop when the Earth is
> tidally locked with the Moon.

I think that's how it usually works. I'm not sure in this case, given that 
the sun asserts a greater gravitational influence on the moon than the earth 
does.

> Thus the Moon is not going to get very much
> farther than it already is.

I think we have a ways to go before the earth is locked with the moon. :-)

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > Darren New <dne### [at] sanrrcom> wrote:
> >> The moon is also receeding from the earth due to tidal forces.
> > 
> >   But isn't this recession decelerating? It will stop when the Earth is
> > tidally locked with the Moon.

> I think that's how it usually works. I'm not sure in this case, given that 
> the sun asserts a greater gravitational influence on the moon than the earth 
> does.

  I don't think that's how it works.

  The Sun asserts a certain gravitational influence on the Earth-Moon system.
That doesn't affect how the Earth and the Moon influence each other.

  If we measure the absolute strength of gravity caused by the Sun, it
might be stronger than the strength of gravity between the Earth and the
Moon, but Sun's gravity is the same for both the Earth and the Moon.
Basically Sun's gravity "sees" the Earth-Moon system as one object.

> > Thus the Moon is not going to get very much
> > farther than it already is.

> I think we have a ways to go before the earth is locked with the moon. :-)

  Just a blink, when we are dealing with astronomical times...

-- 
                                                          - Warp

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Warp wrote:
>   The Sun asserts a certain gravitational influence on the Earth-Moon system.
> That doesn't affect how the Earth and the Moon influence each other.

If the moon drifts far enough from the earth, the gravity from the sun will 
be stronger than the gravity of the earth, so something strange is likely to 
happen.

>   If we measure the absolute strength of gravity caused by the Sun, it
> might be stronger than the strength of gravity between the Earth and the
> Moon, but Sun's gravity is the same for both the Earth and the Moon.
> Basically Sun's gravity "sees" the Earth-Moon system as one object.

Hmmm. Maybe.

>> I think we have a ways to go before the earth is locked with the moon. :-)
>   Just a blink, when we are dealing with astronomical times...

True, but not really a blink when you're dealing with the timespans of 
intelligent life on Earth.  I've already forgotten what we started out 
talking about, tho, so I'm gonna go get more coffee. :-)

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   The Sun asserts a certain gravitational influence on the Earth-Moon system.
> > That doesn't affect how the Earth and the Moon influence each other.

> If the moon drifts far enough from the earth, the gravity from the sun will 
> be stronger than the gravity of the earth, so something strange is likely to 
> happen.

  I'm not a physicist, but I still think you have got some errors in your
thinking.

  You seem to think that the Sun exerting a gravitational force on the Moon
larger than the Earth does is somehow significant, but I don't think that's
how it works. In fact, I think it doesn't matter too much whether the Sun's
gravitational pull is stronger or weaker than the Earth's. (The difference
might affect the speed at which things happen, not whether they happen at
all.)

  There might be some tidal forces in play, caused by the Sun's gravitational
pull being stronger for the object closer to it (when it is closer to it)
than the other object, which is farther from it, but that doesn't have
anything to do with the Sun's gravitational pull being larger than the
Earth's.

  I don't know how strong the tidal forces caused by the Sun are at the
altitude of the Earth-Moon system, but I bet it's not very strong, and
probably won't rip the system apart (or else it would probably have done
so already).

  But as said, I'm not a phycisist. A pro could be laughing at me right now
for making such foolish statements. :)

-- 
                                                          - Warp

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Warp wrote:
>   I'm not a physicist, but I still think you have got some errors in your
> thinking.

Quite possibly. But look at it this way: Say the altitude of the moon over 
the earth was 1000x what it is now, and the moon was orbiting appropriately 
slower? What if the outside of the moon's orbit was close to where Venus is? 
Certainly there would be different dynamics. Hence, in the limit, there's 
obviously some point where the influence of the sun overwhelms the influence 
of the earth. I don't know where that point is, mind, or whether it would 
occur before or after the planets were tidally locked 100%.

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   I'm not a physicist, but I still think you have got some errors in your
> > thinking.

> Quite possibly. But look at it this way: Say the altitude of the moon over 
> the earth was 1000x what it is now, and the moon was orbiting appropriately 
> slower? What if the outside of the moon's orbit was close to where Venus is? 
> Certainly there would be different dynamics. Hence, in the limit, there's 
> obviously some point where the influence of the sun overwhelms the influence 
> of the earth. I don't know where that point is, mind, or whether it would 
> occur before or after the planets were tidally locked 100%.

  This sounds a lot like the three-body problem... :P

  Anyways, as I said, the recession of the Moon is, AFAIK, decelerating,
and it's quite slow to begin with, so I think the Earth is not going to
lose the Moon ever (unless some external forces come into play to disturb
the whole thing).

-- 
                                                          - Warp

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   I'm not a physicist, but I still think you have got some errors in your
> > thinking.

> Quite possibly. But look at it this way: Say the altitude of the moon over 
> the earth was 1000x what it is now, and the moon was orbiting appropriately 
> slower? What if the outside of the moon's orbit was close to where Venus is? 
> Certainly there would be different dynamics. Hence, in the limit, there's 
> obviously some point where the influence of the sun overwhelms the influence 
> of the earth. I don't know where that point is, mind, or whether it would 
> occur before or after the planets were tidally locked 100%.

  Ah, I found the exact term you are talking about:
http://en.wikipedia.org/wiki/Hill_sphere

-- 
                                                          - Warp

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Interesting tidbit: Did you know that Moon's orbit around the Sun is
completely convex? (In other words, the curvature of the orbit never
changes sign.)

-- 
                                                          - Warp

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Warp wrote:
>   Ah, I found the exact term you are talking about:
> http://en.wikipedia.org/wiki/Hill_sphere

Kewl!  So if the moon gets close to taking 7 months instead of 1, we could 
be in trouble. :-)

I had read in a reliable source that the moon is actually more attracted to 
the Sun than the Earth when it's at eclipse, but I never did the math 
myself, and I'm too lazy to look at the math here to see if that's 
sufficient to disrupt the orbit anyway. :-)

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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Warp wrote:
>   Interesting tidbit: Did you know that Moon's orbit around the Sun is
> completely convex? (In other words, the curvature of the orbit never
> changes sign.)

Yes, that's what I'd read about. In essence, it's orbiting the sun as it 
wobbles back and forth across the earth's orbit, rather than actually 
orbiting the earth. That's why I thought we might be much closer to losing it.

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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