On Fri, 5 Oct 2001 10:43:32 -0700 (PDT) Brent Foster
> I may be demonstrating my ignorance of physics here, but why would
> the earth's spin rate have any effect on the moon's orbit? I
> understand that tidal friction causes a slowing of the earth's
> rotation. But I don't see how the moon's orbit would change as a
> result, as long as the mass of both objects is constant.
I'm no physicist, but this isn't rocket science. As you know, the moon
and earth are tied gravationally. The moon tugs on the closer side of
the earth and pulls the oceans toward the moon. It also pulls the entire
earth in a slight oribt around the moon, creating a tidal bulge on the
backside of the earth due to centrifigual force. The earth rotates in
the same direction as the moon revloves, but obviously much faster (once
every 24 hours for the earth, once every 27 days for the moon). As the
earth rotates, the tidal bulge (water) is dragged by friction with the
solid earth, so that the tidal bulge is pulled ahead of the moon. This
causes the earth's rotation to slow down (by 0.0016 second per century),
but at the same time the moon's speed of revolution around the earth
increases. It increases because the tidal bulge is ahead of the moon and
actually pulls it forward. Adding speed to the moon results in the moon
moving to a "higher" (further away from the earth) orbit, hence the moon
drifts further away from the earth each year, and the earth slows down a
little each year. IOW, the earth's rotational energy is being slowly
transferred to the moon's angular momentum (maybe I said that right, but
I doubt it!).
Something I don't understand from my college _Introduction to Astronomy_
by McLaughlin: "Over short intervals of time the irregular changes in
the earth's rotation completely overshadow the steady effects of tidal
friction. Thus, from September 1955 through January 1958, the length of
the day was increasing at the rate of .00043 seconds per year, about 50
times the rate due to tidal friction." My question is, what causes "the
irregular changes in the earth's rotation?" Could this be due to the
accumulation of cosmic dust and meteors?
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