September 26, 2007: Geostationary satellites

[Undertoad]

This was an Earth Science Pic of the Day a few days ago.

This is 8 and a half hours of an open shutter pointed at the sky. Now,
that makes clear what the trails are - stars, captured as they move
across the sky while the earth rotates. But then, the dots that don't
have any trail are geostationary satellites, in geostationary orbit.

They don't have any trail because they move along with the rotation
of the Earth. They orbit at exactly the equator. That means stationary
dishes on Earth can "see" them at all times, which is why there are so
many of them. People don't want to move their dishes!

So if you follow: the camera is stationary on Earth, except that it's not
stationary at all; it's moving with the rotation of the Earth. The
satellites appear to be stationary in the picture, but that's only
because they are moving in exact harmony with the motion of the
camera. And the things that appear to move completely across the
image are stars, entirely stationary.

Well, except that they probably aren't -- but with our short span of
attention, our 70 years, we can't sit around long enough to notice
that motion... we can't perceive it.

So why does XM Radio have three satellites while Sirius appears to
have none? Sirius' satellites use a different type of orbit, a Tundra orbit.
This allows two of their three satellites to be visible to all of North
America at any given time.