While trying to understand what I’m seeing on Jupiter through telescope, I’m using software to simulate the view to understand it better. But I’m learning that just because a program shows a rotating image of Jupiter doesn’t mean it’s correct. Every simulator I try shows me different images, and some of them like Stellarium seem flat-out wrong. Here’s an example.
Jupiter will be interesting about 11pm local time tonight (ie: 2012-12-28 07:00Z). I know from this table that there’s a transit of the Great Red Spot that’s centered at 7:06Z, and I know from this calculator that Europa will be making a transit (moon transit: 04:58Z to 07:26Z. shadow transit: 06:12Z to 08:44Z). So I’d expect the image at 07:00Z to show the Great Red Spot, Europa inside Jupiter’s disc, and Europa’s shadow also inside Jupiter’s disc.
But what I actually get is different for different software. Europa is in a different place in different programs; I think that’s because Jupiter is 35 light minutes from Earth right now. So if a program doesn’t simulate the speed of light, it’ll show an image 35 minutes ahead of what I’d see on earth.
But a couple of programs have the Great Red Spot off by several hours. They all simulate rotation, but do some of them have the wrong facing? I triple checked all this and I think I am using each program correctly. In particular I was careful about local time vs. UTC. It seems like a crazy flaw. I’m probably using the software wrong somehow, but I can’t figure out how.
Update: a bit more research shows the GRS position is a known problem, at least in Stellarium. Here’s a fix, you edit the value of rot_rotation_offset in ssystem.ini for Jupiter. Unfortunately editing that file seems to have no effect in Stellarium 0.11.4 on my Mac, I wonder what I’m doing wrong. Interestingly the GRS wanders, it seems to drift about 15° per year. That may explain why Starry Night (below) is close but not quite correct.
Update 2: the file to edit on Stellarium on Mac is ~/Library/Application Support/Stellarium/data/ssystem.ini. I set rot_rotation_offset = 92 and now the Great Red Spot is in the middle of the planet during the transit at 2012-12-28 7:06Z (when Stellarium is set to simulate light speed).
The Great Red Spot is entirely missing; hunting around Stellarium shows me a transit about 4.5 hours before 07:00Z, which as far as I know is completely wrong. Stellarium does show the transit of Europa, but seems to have scale or timing off a little; Europa is shown already past Jupiter entirely. And it doesn’t simulate the shadow. This discussion also notes the discrepancy, and one poster theorizes it has to do with the speed of light. That’s what tipped me off to think about speed of light. Stellarium has an option to turn on light speed simulation, when I turn it on Europa looks more like in the right place. Why would I ever want to turn that off in an Earth-based planetarium simulator?
This image has a lot in common with the Stellarium image. Again the GRS is entirely missing, although I did verify their simulator will show it at different times. Europa is also a bit further past the disc of Jupiter than I’d expect to see, presumably the speed of light issue. No shadow. This tool is aimed at showing you JPL spacecraft, so I guess I shouldn’t be surprised if Jupiter isn’t shown correctly. But they do call it a “solar system simulator” and it seems odd they wouldn’t do a speed of light correction. Here’s a link to the exact image I generated.
My new favorite iOS app shows exactly what I’d expect to see. GRS in place, the shadow, the moon still inside Jupiter’s disc. I hope this is what I see if I go out tonight.
Sky & Telescope’s iOS app also shows about what I’d expect to see. Even the shadow. No surprise this is so like Sky Safari+: I believe it’s the same developer.
An old version of Starry Night special edition. The Red Spot is in the wrong place, although it’s close. Europa and shadow look about right. I’ve only spent two minutes with Starry Night, don’t know it yet, need to tinker more.