Earth rotation rate

From: Matson, Robert (
Date: Wed Apr 03 2002 - 16:12:45 EST

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    Hi Jonathan,
    > But what is the point in making a geosat synchronous with the stars?
    > It's not going to stay over an area of Earth (more or less).  What
    > is the purpose for a geosat to track the stars?
    Perhaps I can explain.  You need to forget about the sun for
    the moment since it doesn't have anything to do with the earth's
    rotation rate.  Just pretend it isn't there.  The only thing
    that matters is how long it takes the earth to complete one
    revolution on its axis in an inertial reference frame.  The
    answer is 23h 56m and some odd seconds.  Since this is the
    earth rotation rate, this is the value that must be matched
    by the satellite.
    In effect, 24 hours is the average time that it takes the
    sun to "complete one apparent orbit" around the earth.  Thus
    the sun is "out of synch" with the GEO satellite to the tune
    of about 4 minutes per day.
    Now if you measure time with a sidereal clock, you'll see that
    geosynchronous satellites (with zero inclination), always
    return to the same RA/Dec at the same sidereal time each
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