Thank you, Bill.

Just a new way for getting this information. I had read your notes on
Project Pluto and Sky & Telescope's page for this information. In the past,
I usually got the GRS longitude from Sky & Telescope or Jupos and plugged it
into Guide using the old method.

Thanks for the background on the determination of the GRS longitude.

Julian

----- Original Message -----
From: "Bill J Gray" <pluto@...>
To: <guide-user@yahoogroups.com>
Sent: Sunday, April 27, 2008 9:56 AM
Subject: Re: [guide-user] GRS Settings


> 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
>
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