I'm probably reinventing the wheel here, but haven't found any reference - maybe this should be a FAQ?
Anyway, altering the rot_rotation_offset value in the Jupiter section (name=Jupiter) of the ssystem.ini file in the Data folder changes the time at which the GRS crosses the meridian.
The present longitude of the GRS is 207 degrees. A rot_rotation_offset value of 265 gives the correct position (trial and error!) Lower values bring the Spot up earlier and vice versa. A higher longitude value makes the actual position earlier.
So it would appear that subtracting the actual longitude from 472 gives the correct GRS position in Stellarium. (I'm using version 0.12.4)
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This is a FAQ on Jupiter's "Red Spot". The red spot of course is an atmospheric disturbance and its position is generally governed by the same forces that govern tornadoes on earth. There is no fixed algorithm for calculating its position. As consequence we need to periodically place a correction for its offset from observation in the table of orbital parameters found in the ssystem.ini file of stellarium. The last update to the ssystem.ini file put the rotation offset at 283 degrees which is probably no longer current.
It doesn't seem to be working exactly that way now (0.14.2). I put in an offset of 35 for the present position of the GRS, and it agrees well with my pictures from the last year or so. Higher offsets make the GRS reach the meridian sooner, not later.
I note that in the latest version, ssystem.ini uses a rotation period for Jupiter that is apparently System III. Curiously, when I changed it to the figures for System II or for the recent average rotation of the GRS itself, I could not find an offset that would make the GRS position come out correct in all three of the observations I was comparing. I am slightly puzzled, although I do have Stellarium working well for the intended purpose.
Please try set rot_rotation_offset=-1
OK, I just noticed that the ssystem.ini in Program Files is different from the one in appdata (presumably carried over from earlier versions). It has rot_rotation_offset=-1 and a comment "Use GRS patch." What will that do for me? Is there a computation of the GRS position being done separately then? Will I be able to adjust it?
Just tried it. rot_rotation_offset=-1 is not bad, but I get appreciably greater accuracy (with observations spanning the past year or so) with rot_rotation_offset=35, the value I had arrived at by trial and error.
Figured it out by looking at the source code. If I supply a positive number such as 35, Stellarium continues to adjust for empirical GRS drift. That is why it is tied to the System III or in earlier versions System II rotation period. If I supply -1, it uses an algorithm to estimate the location of the GRS.
Update: Over longer time periods, rot_rotation_offset=-1 is better and I am sticking with it.