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:
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RotationPeriod 9.927953 #System II
Obliquity 2.22
EquatorAscendingNode 337.77
RotationOffset -21.247 #for texture map with central Great Red Spot
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:
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RotationPeriod 9.927953 #System II
Obliquity 2.22
EquatorAscendingNode 337.77
RotationOffset 16 #for default jupiter.jpg
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
