Hi Oliver,

As Denis has mentioned, you can set this value by right-clicking on
Jupiter, then Display, then Options. However, great minds are working
alike. I've started on making a little file with historical data on the
GRS longitude at certain dates. The idea would be that when you set Guide
to a date in the past, it can interpolate within the data. For the present
and future, it could use the most recent observation or a reasonable
extrapolation. That last makes some sense, because at least in recent
years, the GRS longitude has steadily increased.

I'd probably just provide the file as part of the usual Guide updates.
It need not be very large.

-- Bill