Re: ISS's Varying Orbit

From: Tony Beresford (
Date: Sun Dec 03 2000 - 23:53:42 PST

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    At 13:27 4/12/00 , Garvey J. Dyer wrote:
    >Could someone please tell me why ISS's orbit changes from time to
    >time??  From my location it is normally visible in the early morning,
    >but during Shuttle missions it becomes visible in the evening.  Did
    >anyone notice that, or did I just discover something big???  :-)
    >Garvey J. Dyer
    >Bethesda, MD (USA)
    >39.0 North, 77.1 West
    garvey and other lurkers and "peekers"
    Not something big garvey, you have just noticed (partially)
    that the ISS orbit rotates relative to the Sun. In fact local
    time passses of the ISS (and MIR) repeat in an approximately
    60 day cycle. The orbit plane of the ISS moves westwards about 5
    degrees a day in a precession motion caused by the oblate shape
    of the Earth. When you add the 1 degree/day apparent motion of the Sun,
    due to the Earth's orbital motion this gives 6 degrees/day relative to
    the Sun. These effects are of course in addition to the fact that
    the Earth is rotating 15 degrees/hour underneath the satellite.
    it also depends where you are located. For both this mission
    and the previous one, there were no visible passes at convenient
    times. For this mission ALL passes over Australia occur during
    daylight hours. 
    Tony Beresford
    Adelaide, So. Australia
    34.9638S, 138.6333E
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