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Old Mar 13th 2009, 02:16 AM   #1
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Kepler (conservation of angular momentum) and the moon,

Hi,

The moon is receding at about 4cm/year.
Apparently because it is speeding up.

Does the moon's orbit then violate Kepler's law of planetary motion?
How can the integral remain equal for sweeps of the moons orbit if the moon is moving away.

Is this an example of Kepler's law being broken perhaps because the orbit is not stable?

Are there any authoritative internet sites with more information?

Thanks
Regards
Craig.
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Old Mar 15th 2009, 10:06 PM   #2
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Challenge!

Hi,

Is my question too vague or off topic.
Surely some of you fine physicists have some idea of how Kepler's law relates to the moon.

I am really surprised there are no suggestions, so I have to assume I have done something wrong.

Any thoughts?

Thanks
Craig.
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Old Mar 15th 2009, 11:51 PM   #3
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Somewhere I read that moon is gaining energy from the tidal effect taking place on earth.
What you are talking about integral, I suppose you are talking about areal velocity of the moon. That the areal velocity remains constant, is derived using law of conservation of angular momentum. And angular momentum remains conseved only in absence of external torqe which is not the case with the moon as some exrenal torque is provided by tides. So I think in the case of the moon areal velocity will not be constant.
I am not sure what I have said above, hence I request other members to discuss the matter.
Thanks
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Old Mar 16th 2009, 12:36 AM   #4
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External Torque,

Hi Parvez,

That makes sense. The integral I am talking about is the fact that for an elliptical (I assume stable) orbit the orbiting planet (or moon) sweeps the same area of the elipse for any given time interval. In other words, the orbiting body will speed up and slow down in it's orbital path in such a way as to always sweep the same area of the elipse for a given time interval. The area is the integral. This is Kepler's second law which is conservation of angular momentum.

I think you are definitely on the right track. The second law (as I have it stated) does not mention external torque, though it's pretty obvious that if you have other forces at play the law is not going to hold.

Let's see if anyone else has any ideas.

Cheers
Craig.
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