!!! you have an interest in astronomy that goes way beyond the knowledge of most. I just had to go on Wikipedia to check that out about the retrograde rotations of those 2 planets. I didn't know that (or probably did know and forgot) and I was taught by a professor of astrophysics for 3 years. Right now, I can't think why that would be. Of cause all planets, viewed from the Earth go through phases of apparent retrograde orbit.
Anyways, I think this thread has been hijacked. It's supposed to be about gas holders so enough from me.