Help with understanding transits

[billyharris72]

Hi all. Just in from a brief session with Jupiter, terminated due to poor seeing. I did catch the GRS though, and it has left me rather confused. At around midnight I saw the spot, slightly to the left of the centre of the planetary disk, so past transit (with a reversed image) but still pretty central. Looking up a list of transit times online suggests the spot transited about 90 minutes previously.

What I'm not getting is that if jupiter turns on its axis about once every ten hours, the spot should cross from one side of Jupiter to the other in 5 hours, and it should take 2.5 hours from transit to the edge of the disk. So why would the spot not appear more than halfway to the edge of the Jovian disk. (I've just had a thought that it might be to do with the fact that Jupiter is a sphere, but don't know). Can anyone enlighten me?

Billy.

[billyharris72]

Update - pretty clear that my above suggestion does nor explain it. In fact, as midnight UK last night is equal to 01:00 UTC, about 2.5 hours after the listed transit, the GRS should have been leaving the face of Jupiter.

So, given the seeing last night, was my GRS spotting a case of averted imagination? Be good to know when others were viewing it.

Billy.

[Dave In Vermont]

This will be the second time this morning when someone could use a very nice little free software-program for Jupiter's GRS-spotting (and Moons):

http://astrosurf.com/rondi/jupiter/

I'm sure someone will be along who can answer your direct question better than I - I'm to burned-out still from a long night here in Podunk.

Enjoy!

Dave

[laudropb]

Midnight last night would infact be 23:00 UT