Celestia Forums

The rotation period Celestia provides for Jupiter is System III - the rotation period of Jupiter's magnetic field, which is tied to the solid core of the planet.
But the most prominent observable feature on Jupiter's "surface" is the Great Red Spot - which rotates a little more slowly than System III. At present (December 2002), it lies at 85 degrees west longitude in System II.
If you had a texture map with the Great Red Spot precisely at its centre, then the following would be the correct rotational definition for Jupiter, in order to show the Great Red Spot in the correct position:

However, most texture maps need a jigger factor, to allow for the fact that the GRS is not placed centrally. In general, the RotationOffset can be calculated from:

RotationOffset = -21 + (1 - 2*[distance of GRS from left edge of image]/[image width])* 180

For the default jupiter.jpg that comes with the current version of Celestia, this translates to:


UPDATE: The above text has been revised, in view of recent information that the Great Red Spot has drifted 5 degrees in longitude in the last few months (courtesy Fridger). Predicted GRS transit times using longitude 80W are available on the Sky and Telescope website - these are in advance by ~8mins, given the recent change in longitude.

Grant

granthutchison wrote: RotationOffset -16.247 #for texture map with central Great

where can i download this texture?

billybob884 wrote:where can i download this texture?

You can't, as far as I'm aware. I was just showing my working, so that folk with a mathematical bent could see where I'd come up with the "-16" term in the equation I gave.
In practice, you should either use the settings I give for the default jupiter.jpg, or figure out from the equation the correct settings for the Jupiter texture you are using.

Grant

k, thanks!

If my red spot is to the far left of my texture
What would be my Rotation offset ?

Darkmiss wrote:If my red spot is to the far left of my texture
What would be my Rotation offset ?

How far is "far left"?
What is the distance in pixels from the left edge of the image to the centre of the Great Red Spot, and what is the overall width of the image?

Grant

oooh blimey, thats a bit technical
ill report back with all the info........

Okay I have a 4096 x 2048 dds texture
and if i have done it right,
35 pixels from the left edge to left edge of red spot
78 pixels from the left edge to the center of the red spot.

Looks like

To check that our sums are OK, please set this up and then view Jupiter from Earth at 08:00UT 15 Dec 2002. This should give you a GRS position comparable to the one in the CCD image that Fridger posted over on another thread:
http://www.shatters.net/forum/viewtopic.php?t=1492

(The image is timed at 08:38, but light travel time is 38 minutes.)

Grant