Starry Night uses a file called "JupiterGRS.txt" located in the Sky Data folder to determine the mean longitude of the GRS. To change the GRS's position you can open this file and enter a new mean longitude. The GRS drifts non-algorithmically so the GRS mean longitude will need to be updated from time to time. The latest mean longitude is available from Sky & Telescope's excellent Great Red Spot web page.
For Version 6: the File "JupiterGRS.txt" can be found in the SkyData folder.
On Windows ---> the SkyData folder can be found in the Starry Night program folder.
On Mac ---> the SkyData folder can be found by right clicking the Starry Night program and choosing "Show Package Contents" ---> Resources ---> SkyData
For Version 7: the File "JupiterGRS.txt" can be found in the SkyData folder.
On Windows ---> the SkyData folder can be found in (Windows) \Users\<YourUserName>\AppData\Local\Simulation Curriculum\Starry Night Prefs
On Mac ---> the SkyData folder can be found in (OS X) /Users/<YourUserName>/Library/Application Support/Simulation Curriculum/Starry Night Prefs
When the file is opened it will say this:
// Enter the mean longitude of the Great Red Spot on the following line. Visit // the Starry Night Pro website at http://www.starrynightpro.com to get the // latest value. 134.0
(Please note the link to www.starrynightpro.com is broken. An alternative web link is provided below.)
The number in your file may be different. It is the number that needs to be changed.
Go to this webpage to find the current GRS number: http://www.skyandtelescope.com/observing/objects/planets/3304091.html?page=1&c=y
Go to this webpage if the above link is broken: http://www.skyandtelescope.com/observing/transit-times-of-jupiters-great-red-spot/
On the webpage above (approximately halfway down) it says: "The predictions assume the Red Spot is at Jovian System II longitude 173°, based on historical trends noted by JUPOS."
Given that the current prediction is 173° the number 134.0 found in our example file is in error and needs to be changed to 173.0. So, change the file to say the following:
// Enter the mean longitude of the Great Red Spot on the following line. Visit // the Starry Night Pro website at http://www.starrynightpro.com to get the // latest value. 173.0
Save the file. And, the GRS position will be corrected. In the future, if the GRS is again out of position, repeat the process.