On Feb 21, 2008, at 10:29 AM, Jim Jerzycke wrote:
The Sirius birds are in an "Inclined Orbit". It's explained more in the WikiPedia entry, about half way down the page. http://en.wikipedia.org/wiki/Sirius_radio
What I read says it's in a "geosynchronous highly elliptical" orbit. Strictly speaking, you can't have a truly geosynchronous orbit that's elliptical (it must be circular). It must also have an inclination of zero.
But: the inclined elliptical orbit causes the satellite to speed up relative to Earth's rotation during parts of its orbit, and slow down during others. This is what causes the East-West motion of the figure- eight. The inclination is what gives the North-South motion. The two are combined in such a way as to give a nice figure-eight pattern over the surface of the Earth.
The Wikipedia entry refers to this as a "Tundra Orbit":
http://en.wikipedia.org/wiki/Tundra_orbit