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Re: World's angular momentum budget



At 09:38 PM 9/9/01 +0300, Muhsin Ogretme wrote:
I remembered another question (a famous one) "why does the earth
rotate?". The common answer is because there is no agent to stop it.

That's a pretty shabby answer. It has no predictive value. That answer
would "explain" something that rotated a billion times more slowly than the
earth. It would also "explain" something that rotated a billion times faster.

The real question that needs answering is why the earth rotates "about as
much as it does".

Can we start with the idea that all heavenly objects were already rotating
when they started their lifes?

Well, once again, it doesn't do much good to say they were rotating without
saying something about _how much_ they were rotating. Everything rotates
some, if you look closely enough.

Is it a good idea to accept the rotation of the heavenly objects as an
internal property of them - as we take the concept of inertia as an
internal property of mass?

The answer is yes, for reasons having almost nothing to do with heavenly
objects. Angular momentum is a conserved quantity. It obeys a local
conservation law -- and obeys it at least as rigorously as mass obeys its
local conservation law. In practice, on a laboratory scale, it is
sometimes easier to keep mass from leaking across system boundaries and
relatively harder to keep angular momentum from leaking, but in principle
the conservation law is 100% strict:
change(stuff on inside) = -flow(stuff across boundary).

For a system the size of a planet, there is (on most days, recently)
-- very little mass crossing the boundary of the system, and
-- very little angular momentum crossing the boundary of the system.

In general, figuring out where the angular momentum came from is part and
parcel of figuring out where the mass came from. A fellow named Bill
Hartmann has had a few things to say about this:
http://www.psi.edu/hartmann/science.html
http://www.psi.edu/projects/moon/moon.html
http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html
http://www.pbs.org/wgbh/nova/tothemoon/origins2.html
http://www.xtec.es/recursos/astronom/moon/canupe.htm