"Can someone verify that the "ground speed" of the ISS is latitude dependent? 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. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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