Re: 23h 56m vs 24h

From: Robert G Fenske Jr (fenske@rgfpc.electro.swri.edu)
Date: Wed Apr 03 2002 - 16:25:01 EST

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    On Wed, 3 Apr 2002, Jonathan T Wojack wrote:
    
    > 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?
    
    	As others have essentially pointed out, being synchronous with the
    stars is equivalent to being synchronous with the Earth's rotation, which is
    23h 56m.
    
    	The reason we all use 24 hours for a day is that it takes the Earth
    an extra 4 minutes to rotate back to its position relative to the Sun as
    this exaggerated diagram will (I hope) show:
    
                               /-\
               1 E------------|Sun|
                              /\-/
                             /
                            /
                           /
                          / (takes 4 more minutes to rotate to this line)
                         /
                      2 E---------------
    
    At position 1 the Sun is overhead for somebody.  23h 56m later the Earth has
    rotated around once so the person is again pointing in the same spatial
    direction (and a geo sat with a 23h 56m period will stay with this person).
    But the Earth has moved to position 2 in its solar orbit so it takes 4 more
    minutes (on average) for it to rotate so that the Sun is overhead for the
    person.
    
    
    Robert Fenske, Jr.   rfenske@swri.edu    Sw     |The Taming the C*sm*s series:
    Southwest Research Institute            /R---\  |
    Signal Exploitation & Geolocation Div  | I    | |"The Martian canals were the
    San Antonio,Texas USA  ph:210-522-3931  \----/  | Martians' last ditch effort."
    
    
    
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