Jupiter GRS timing in Software

[Solanum]

I've just been observing the GRS through my scope and this page:
http://www.skyandtelescope.com/observing/objects/planets/3304091.html?page=1&c=y
gives the centred transit as 10.10pm local time, which is about when it looked centred to me.

Now when I looked at Jupiter in Xephem earlier in the evening it had the GRS centred at about 8.40pm. Checking this in Stellarium showed the same time.

Both pieces of software are reporting UTC and local time correctly, so does anyone have an explanation for why they are 1.5 hours off reality? The only ones I can think of is that they are using images that aren't centred correctly, or that the GRS has shifted significantly since the images they are using were taken. Either way, neither is any use for predicting transits of the GRS.

Or have I done something stupid....?

[iceman]

The GRS is always drifting in longitude, due to the East-West wind movement in the belts and West-East wind movement in the zones.

The software "shows" the position of the GRS based on its current longitude when it crosses the "middle" (Central Meridian) of the planet. As explained above, it's always changing and so the software needs to be regularly (once every few months) updated with the "current" longitude of the GRS so that it shows it transiting the CM at the correct time.

The current value of the longitude of the GRS is 119° in System II. So see if your software has the ability to either automatically or manually update this value - then you'll ge the GRS showing at the correct position at the correct time.

That S&T page is accurate and can be used to predict transit times of the GRS, otherwise you could download a free program called Jupiter 2 (http://www.astrosurf.com/rondi/jupiter/) which is what I use. It lets you update the position of the GRS, and gives you visual (and ephemeris) representation of the GRS as well as the position of the galilean moons so you can plan in advance (or check in retrospect) transits and shadow transits. I use it almost daily when the skies are clear.

Hope that helps.

[Kal]

Anyone have an idea of how accurate the NASA planet view simulator is?

http://space.jpl.nasa.gov/

[Solanum]

Thanks Iceman, that's basically what I was meaning with my second option... I'll stick with the S+T website then! Jupiter 2 looks excellent, but appears to be Windows only. Though Borland did spend some effort making there IDEs cross platform, so I guess it might be portable if the author was so inclined.

[Solanum]

Kal, as far as Jupiter is concerned it appears to be about 30 minutes out in the earth view based on the above timings.