Re: Ground Speed vs. Latitude vs. Sky Speed

Date: Sun Jul 23 2006 - 17:25:00 EDT

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    "Can someone verify that the "ground speed" of  the ISS is latitude 
    Does that make a difference how fast it moves  across one's local sky? 
    Finally, how much does the ground speed vary with  the ISS?"
    The ground speed, relative to a non-rotating Earth, is very  nearly equal to 
    the orbital speed for satellites in LEO. If you're not looking  at a satellite 
    in LEO, you can calculate its angular speed --how many degrees in  its orbit 
    does it move in one hour. Multiply that by 60, and you've got the  ground 
    speed in knots, again relative to a non-rotating Earth. The latitude  dependence 
    of the ground speed results only from the Earth's rotation. This is a  maximum 
    factor of about 1000 mph for observers at the equator. Out of about  17,000 
    mph total, this is only about 6%. 
    The biggest factor for the rate  of motion across the sky is range. If one 
    satellite is 300 miles high traveling  at 18,000 mph and a second is 150 miles 
    high traveling at almost the same speed  (slightly higher), when both 
    satellites pass nearly overhead from your observing  location, the second satellite's 
    rate of motion across your sky will be twice as  fast.
    42.0N 87.7W, or 41.4N  72.1W.  
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