RE: 23h 56m vs 24h

From: Barker, Jefferson (JBARKER) (JBARKER@arinc.com)
Date: Wed Apr 03 2002 - 18:05:28 EST

  • Next message: Matson, Robert: "A note on the First Point of Aries"

    The whole objective of a satellite in a geostationary orbit is so that the
    satellite will be fixed in one spot in the sky so that a user of the system
    gets full time coverage with one satellite and doesn't have to have a
    tracking antenna.
    
    A geostationary orbit is not synchronous with the stars.  To a ground
    observer the stars appear to move slowly across the sky so a satellite that
    would stay fixed relative to the stars would also appear to move across the
    sky, thus requiring a tracking antenna.
    
    The relationship between the sidereal day (23:56:04) versus the solar day
    (24:00:00) is explained by the fact that in the one day that it takes to
    make one earth revolution, the earth has moved in it's orbit of about 0.9863
    degrees in it's one year (365 days) orbital (360 deg) revolution around the
    sun.  The point of reference for a sidereal day is an imaginary point way
    out in deep space which is considered to be fixed because it is so far away.
    It is also called the First Point of Aires - along a line from the center of
    the earth through the equatorial plane at the point when the Sun passes from
    the southern hemisphere to the northern hemisphere on the first day of
    spring (vernal equinox).  The point of reference for a solar day is the
    center of the sun.
    
    The math is available at
    http://scienceworld.wolfram.com/astronomy/SiderealDay.html
    There's also a diagram and explanation available at
    http://www.storm.ca/~tjones/sidereal.html
    
    Jeff Barker
    
      
    
    -----Original Message-----
    From: Robert G Fenske Jr [mailto:fenske@rgfpc.electro.swri.edu]
    Sent: Wednesday, April 03, 2002 3:25 PM
    To: SeeSat-L@satobs.org
    Subject: Re: 23h 56m vs 24h
    
    
    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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