Comment 3 for bug 490019

GRS longitude is reported e.g. in astronomical magazines, maybe there is also an online source. Jupiter's rotation would have to be simulated with a special function the interpolates an additional rotation in a table. Of course, only data for <150 years (?) are available, and future is unpredictable. The table should be a text file (JD vs. longitude) to extend for users. It may not be too difficult, once the data is located... Maybe http://jupos.privat.t-online.de/ is a source, at least for recent years.
G.