fixed GRS

Did some Jupiter Great Red Spot software simulation work.

Last night, at the member's night at the David Dunlap Observatory, after the talks, Chris asked me about the GRS position as it related to Stellarium and Sky Safai. We discussed that the light speed option needs to be on in Stellarium. It is not, by default.

It sounded like Chris had been relying on Stellarium for the GRS transit times but then discovered these did not match other tools. I did some research. The image below, from Stellarium 0.11.0 under Windows, is with the Jupiter "rotation" value set to 250.

Sky and Telescope's interactive GRS tool showed that the spot would transit at 9:48 PM EDT on 8 April 2013. I used this as my benchmark. It also noted that the longitude value used for these calculations was 190. Check. For an experiment, I set the rotation value to 190 in Stellarium.

Stellarium. I already knew that number used by this software was hocus-pocus. It does not seem to correspond to the longitude value in any way. But I started to wonder if there was, perhaps, a relationship. But I don't think I have any historical data, at this point, that I can use. Maybe, with some better notes, I might see a pattern. In the meantime, I found—through trial and error—a good number in the SSYSTEM.INI text file is 105. That's 85 different that the longitude... Hmmm. Almost 90... Anyway.

[ed: Hold the phone... Chris disagrees. He says 140 works for him...]

SkyTools. I remembered a notice from the developer Greg back in December recommending we update the value to 188. Jumped into the software Preferences and found the value 192. Good enough.

TheSky 6. Fired up the application and zoomed into Jupiter. Found a tan-coloured oblate disc. Ah. No image. Couldn't remember if it supported realistic images and GRS positioning. Moving on...

StarryNight 6 Pro. Looked in the extensive settings of the software and didn't see anything for changing the GRS location. Sheesh, you can change everything else this commercial application! Why is the GRS setting not easily accessed through the graphical user interface?! After a Google search, I located the JUPITERGRS.TXT file, added a comment referring to the old value of 134.0, and entered the new value 190. It didn't work! Huh?

Edited the value to 190.0. Surely the decimal wasn't an issue. Still didn't work. Phoned Mr X, er, Phil. And as we chatted, I realised the problem. I removed the comment! It seems that the text file must not have any other number in it. Man! All the more reason to include this setting in the GUI, so "regular" users can't screw it up, like I did. Thanked Phil his help.

Sky Safari. Steve and Phil confirmed it looked correct. This suggests it automatically updates. Good to know. Image provided by Steve.

Sent Chris a quick summary.

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Eric confirmed that he used 190 in his StarryNight.