Great Red Spot position when offset is given

[Michael Covington]

I am somewhat puzzled by line 718 of Planet.cpp and am wondering whether the call to qAbs is correct.

This is the routine for "drifting" the position of the Great Red Spot when an offset has been provided by the user.

The drift, apparently in degrees, seems to be 0.2483 * qAbs(JD - 2456172)

If I have understood it correctly, this seems to assert that the drift reversed direction in 2012. Other sources show a steady drift.

Also, I'm wondering about the coefficient 0.2483. The actual drift of the GRS is only about 0.04 degree per day relative to System II. The rest of the coefficient might come from the difference between System II (used for GRS measurements) and System III (used for the rotation period in ssystem.ini), but even then I'm not sure I understand its value.

[Alexander Wolf (alexwolf)]

This is a legacy code to compensate for the drift of the GRS. I think we can use the approach of CdC planetarium and introduce explicit configurable options for the GRS as a special case.

P.S. Stellarium uses an inner coordinate system for all planets.

[Michael Covington (a-mc)]

Yes; now that you have the -1 option (compute position of GRS from scratch), I would advocate that when the user specifies anything other than -1, the offset should simply stay constant, as with any other planet, to keep from puzzling the user. This drift certainly puzzled me when I didn't know it was built in.

[Alexander Wolf (alexwolf)]

I deleted a outdated code for drift of GRS.

[Michael Covington (a-mc)]

Thanks!

[Michael Covington (a-mc)]

By the way, I fully support your decision to use System III. It is the most physically real of the three coordinate systems. The other two are basically moving longitude systems that try to track different parts of the atmosphere. System II, as I understand it, was originally intended to track the Great Red Spot, but...

[Alexander Wolf (alexwolf)]

Since version 0.14.80.0 you can fill settings for GRS in GUI.

[Michael Covington (a-mc)]

Thanks!

On 2016-04-09 03:41, Alexander Wolf wrote:
> Since version 0.14.80.0 you can fill settings for GRS in GUI.
>

--
Michael A. Covington, Ph.D.
Consultant, Covington Innovations, Athens, Georgia, USA
http://www.covingtoninnovations.com/michael