Re: [guide-user] Great Red Spot longitude setting

Bill Gray Oct 7 9:39 AM

Hi Gary,

On 10/06/2013 02:04 PM, Gary Seronik wrote:
> ...I like this suggestion. And I would like to add one more. Could
> we have a way of entering the current GRS longitude manually?

I see two big issues with this: the GRS is moving pretty fast these
days, about fourteen degrees a year; and if you set Guide back to
1990, it would be stuck with the 2013 value of the GRS longitude.

However, I think there's a simple and even elegant solution to this.
When Guide shows GRS data in tables or More Info, the words "Great Red
Spot" are linked to some text describing the feature. I can, without
much effort, add this text:

The Great Red Spot moves gradually over time. You can *click here* to
have Guide download current longitude data, thereby causing the program
to display its position and compute its transit times more accurately.

"*click here*" would very simply cause Guide to download this file:

to your Guide folder, overwriting the copy you currently have there.
I would suggest doing this; it'll keep you going until I get the "click
here" version posted. The currently posted 'grs_lon' file has been
updated using data (
and the extrapolated position revised. From the 'rGrs' graph, it
looks as if the GRS has accelerated a bit, and is now moving at a
pretty steady 14 degrees/year. (I also revised an error I made in the
explanation at the end of the file, in which I suggested that perhaps
we could find data going back to Galileo. I mis-remembered my history;
Cassini probably observed the GRS, but then there's a gap until 1831.
So I don't expect us to be able to extend the table before then.)

> (I just did this yesterday, as it happens, and I believe Guide
> was giving a longitude of around 182 whereas the actual longitude
> is 201 or something close to that.)

I didn't think it was _that_ far off. But I see that with the
default file with a last "known position" of 168 degrees in August 2011,
back when the GRS wasn't moving at its current breakneck speed, you
get something around 182 degrees... definitely something that needs

-- Bill