Re: YEC, the Moon, and comets

From: Bill Payne (
Date: Sat Oct 06 2001 - 00:52:11 EDT

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    On Fri, 5 Oct 2001 10:43:32 -0700 (PDT) Brent Foster
    <> writes:
    > 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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