Hi Julian,

"...I cannot set the GRS Longitude under the #9 tab on the toolbar
or access through the Jupiter-Display-Options feature."

True. The Great Red Spot longitude is now set in a different
manner (a vast improvement, in my humble opinion) :

http://www.projectpluto.com/new.htm#grs_setting

Basically, Guide has a table of GRS longitudes for assorted
dates, stored in the file 'grs_lon.txt'. (Appended below are
comments from the bottom of 'grs_lon.txt' for interested citizens.)
For any desired date, Guide interpolates within this table.

The _Sky & Telescope_ page of GRS transits at

http://www.skyandtelescope.com/observing/objects/planets/3304091.html?page=1&c=y

states that:

"...The predictions assume the Red Spot is at Jovian System II
longitude 121°, which matches images of Jupiter acquired by
Christopher Go in March 2008."

Which matches Guide's value pretty closely (the GRS longitude is
given in 'more info' when you click on Jupiter). I'll update the
'grs_lon.txt' file to include Go's value anyway.

-- Bill

Comments at the end of 'grs_lon.txt':

This file gives the GRS longitude as a function of time. Guide
computes the System II GRS longitude for any date by interpolating
within this table. Data are given from most recent to oldest. For
each date, a year, month, day, and System II longitude are given,
followed by the source of the data. For dates after the most recent
data point, the last GRS longitude is used; for dates preceding the
table, the oldest GRS longitude is used.

The GRS longitude currently appears to be increasing at about
eight degrees per year (with plenty of fluctuations). I've added a
totally fictitious longitude for 2017 based on that drift, just to
get around the fact that otherwise, the GRS would remain at the last
listed value, which is almost certainly even less realistic than
the assumed drift.

1986 to present longitudes came from _Sky & Telescope_, which in turn
got the data from ALPO (Association of Lunar and Planetary Observers).
I checked a random sampling of my back issues of that magazine to
compile the table. Because S&T is published about two months before the
date given on the cover, I've assumed that the longitude given in (for
example) the August issue of a given year is the one that was actually
measured in June, and have arbitrarily chosen the first of the month.

Jari Suomela found some data in Bertrand Peek's book _The Planet
Jupiter_ for years 1831 to 1952. See 'grs_peek.cpp' for details.

Other sources would be welcome. Seems to me as if somebody, somewhere,
must have assembled a table going back to Galileo's observations...

17 June 2006 version: created by BJG
6 June 2007: updated with new data and 2017 "observation" added
9 Mar 2008: added 1968 and earlier data from Peek via Jari Suomela;
see 'grs_peek.cpp' for details