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I was wondering if there is some way to update the longitude of Jupiter's Great Red Spot in SkyTools 4?

In SkyTools 4.0d Imaging, the current GRS longitude is 289.4° compared to 312.3° per WinJUPOS (the ST4 value can be determined from the Object Information box for Jupiter; take the current central meridian value and subtract the GRS angle). The 22.9° difference between ST4 and WJ results in ST4 showing a current GRS central meridian transit about 38 minutes early. I can't find any way to initiate an update or enter the value manually.

Another glitch I noticed. I like the addition of the "Display Direction Indicator" to charts, with the equatorial version in ST4 showing N and E on arrowed lines connected at a right angle. They are correctly oriented on-screen (in various directions depending on the view and time), but not when printed. With both the print preview screen and a paper printout, N is always parallel the vertical edge and E is always parallel to the horizontal edge, regardless of the on-screen orientation. I'm using a Dell i7 laptop with Win 10 and a Brother wireless laser printer.

My apologies if I missed either of these items in any earlier posts.
Hi Joe,

My version of ST4.0d Imaging says that the Jupiter GRS longitude data was updated 4 June 2019 when I started ST4i. I haven't checked to see if the updated info is correct. How quickly does the GRS longitude drift?

Phil S.
I can't find where ST4 gives the last time the GRS longitude was last updated. If it's the splash box on startup, it flashes by so quickly on my system that I can't really read it. During its brief appearance, I see something about comets and minor planets, but didn't notice the GRS or a date (but again, it flashes by very quickly).

Did you check what the longitude actually was on your system? As I noted before, in the Jupiter "Object Information" box, it lists the "Central Longitude" and the "GRS Angle" for the date and time setting. Subtract the GRS Angle from the Central Longitude and you get the GRS longitude.

For example, as I write this at 10:30 am EDT on 30-June-2019, ST4 gives the Central Longitude as 36.7° and the GRS Angle as -252.7°, so the GRS longitude would be 36.7 - (-252.7) = 289.4°. At the same time, WinJUPOS shows the GRS longitude as 312.3°, a 22.9° difference, which at 1.65 minutes of Jovian rotation per degree of longitude, the GRS precedes the actual position by 38 minutes in ST4. Based on visual observations, I have confidence in the WinJUPOS GRS longitude values.

The GRS longitude drifts about 2° per month, although the movement can be erratic. See the JUPOS page which plots the GRS longitude vs. time and is the source of the WinJUPOS value...

Note that the 289.4° currently used in my version of ST4 nominally matches with July of 2018 in the JUPOS plot. Out of curiosity, I went back in ST4 to 30-June-2018 and then forward to 30-June-2020 (both at 10:30 am EDT) and found the GRS longitude was 286.9° and 291.8° respectively. That shows an increase of 2.5° in the past year and an increase of 2.4° in the next year. I know that one needs to be careful projecting the GRS longitude that far back or forward, but these deltas are far smaller than expected (just a bit more than the nominal monthly drift). So, there seems to be some time-dependent adjustment factor in ST4, but not enough.

I'm still wondering exactly how ST4 sets (and/or updates) the GRS longitude and if there is any way of manually setting it, like in ST3.
Hi Joe,

My ST4.0d Imaging shows the same info for Jupiter's central longitude & GRS at 10:30am 30 Jun 2019 as you listed.

To see what's been updated when ST4i starts, click the little green (or flashing red when something's been updated) circle at the bottom left of the Target Selection window. A list of the updates will pop up. It's nearly impossible to see what's updated while ST4i starts (usually novae & supernovae), but clicking the little circle lets you open the pop up box. I never noticed the update to the GRS data until I saw your message.

Hope this helps,

Phil S.

Thank you very much for the tip about clicking the little green/red circle to get the update info! Obviously, I had not found it on my own.

Anyway, with my ST4, the list of items in the update notification box shows: "2019 Jun 9 18:16 Jupiter GRS longitude updated"

That's a relatively recent update, so the question now is... what's the source of the GRS longitude that ST4 uses?

So how this works is SkyTools 4 has a data file that tracks the GRS over time historically. As a result, you can enter any date after 1965 and the GRS can be calculated for that date. As time goes on I add new values to the data file. When that happens the file is downloaded when you start ST4. The file is called GRS.dat and resides in your "SkyTools 4" folder in Documents.

One thing that's going on is that the GRS longitude stopped changing between May and June, and my last update was the start of June. As a result, when it extrapolated to the future it could underestimate the position after it started moving again.

But in testing, that alone does not appear to explain why the current values don't seem to be correct. I updated with the latest data and now it appears to be suggesting GRS longitudes that are over 220 degrees in mid-June, and that doesn't make sense. I am looking into what is causing this. I suspect the file is being downloaded as binary rather than text and that is affecting the format and so the values being read.

I will post here when I have an explanation and a fix.
Hi Joe,

Glad I could help. I run ST4i almost every day & got the GRS update message on 4 June as noted above. As you probably know things are updated when the program starts.

I couldn't find any info in the Help system about the GRS data. In fact the GRS didn't appear in the Help index nor did the Help Search for 'GRS' or 'Great Red Spot' find any entries. There were 5 entries for Jupiter, but none of them were related to the GRS.

This looks like a question for Greg C.

Phil S.

So it turns out that the code was badly broken, which is upsetting. I wrote this code several years ago and I thought that it was tested extensively at the time. An update will be forthcoming.

By the way, I also added the current calculated GRS longitude in parentheses after the GRS angle on the Object Info.

Thanks for your quick response to my inquiry, and it wasn’t my intention to upset you!

That’s a great idea to put the current GRS longitude in parentheses after the angle. I presume it will appear with the next ST4 update (which I’m not expecting instantly).

I did find the updated GRS.dat file. I made a copy and was able to open it with Win 10 Notepad and see the latest entry as “2019.400 312.30,” which corresponds with WinJUPOS 312.3° longitude for today.

However, when I go to Jupiter’s Object Information box, the calculated GRS longitude is 335.6° today, now off 23.3° in the opposite direction, so ST4 shows the central meridian transit about 38 minutes late instead of early. But as you said, you have some broken code to repair — I’ll let the topic rest and give you a chance to work on it.
Hi Joe,

Shouldn't today (30 June 2019) be closer to 2019.5 than 2019.400 though?

Glad to hear that a fix will be coming although I don't have any imaging systems with a long enough focal length to image Jupiter properly. I could put the camera on the 13.1" Odyssey 1 dob with a 1.4x teleconverter to see what happens. I was amazed that the EOS 5D Mk4 with a 24-105 mm f/4 zoom lens showed Jupiter's disc and 3 moons shot hand held at 1/10 sec. Unfortunately the disc was severely overexposed.

Phil S.
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