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That's really cool Dennis! I always love seeing comparisons to reality.
You are right, that the position of Iapetus is pretty close, and certainly close enough for most purposes, such as identification. But my testing revealed that it is no longer as accurate as I would like it to be. I strive for the positions to always be accurate to less than an arc second, and hopefully less than 0.5" most of the time.
My testing revealed that three moons of Saturn are no longer making the grade. They are: Iapetus, Hyperion, and Titan. Titan really surprises me, as its a pretty major moon. All the other solar system moons are good.
So, no, you can't trust the timing of the upcoming transit. But I will have an update, hopefully in the next few days, that will restore its accuracy.
Clear skies,
Greg
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Thanks Greg, it really is quite fascinating and illuminating to get an insight into the research, analysis and development work that goes on under the hood with these dynamic solar system bodies, so that we get to enjoy using SkyTools on our computers.
Cheers
Dennis
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2022-04-27, 02:00 AM
(This post was last modified: 2022-04-27, 02:01 AM by bigmasterdrago.)
Looks to be a full daylight event for me! And below my horizon. Boo Hiss.
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(2022-04-27, 02:00 AM)bigmasterdrago Wrote: Looks to be a full daylight event for me! And below my horizon. Boo Hiss.
Have a look at the 17th or 18th July 2022 - I think there is also a second event that might be better for you folks in the North.
Cheers
Dennis
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2022-04-27, 03:20 PM
(This post was last modified: 2022-04-27, 04:16 PM by bigmasterdrago.)
Dennis is in a very good location for this event. I think Brisbane? From SE Texas, Saturn is only above the horizon for the first hour or so . It finishes around 03:00 UTC on 30th April, so it will be dark here, but Saturn is way below the horizon then. I've no chance of seeing the transit from SE Texas.
Condition, barring weather are much better for the Australians! Only in the last minutes is the event very low. Here is a video of the event from Brisbane using unnamed specialty software rise to set. North up, east left. It originally had the position for Iapetus off by 2,000Km but the positions are corrected in the video to exactly match SPICE: https://vimeo.com/703762684
(2022-04-27, 03:11 AM)Dennis Wrote: (2022-04-27, 02:00 AM)bigmasterdrago Wrote: Looks to be a full daylight event for me! And below my horizon. Boo Hiss.
Have a look at the 17th or 18th July 2022 - I think there is also a second event that might be better for you folks in the North. 
Cheers
Dennis Thanks. Yes, the last half will be better placed here in SE Texas as over 30° up.
But the next one on December 23rd is low here but good for you. Starts near 45°up, Gets low at 10UT.
Then we need to wait til November 26, 2035 for the next one. It will be high for SE Texas for the majority of the event near Saturn's north pole.
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2022-04-27, 06:30 PM
(This post was last modified: 2022-04-27, 06:33 PM by theskyhound.)
So I fixed Iapetus.
Or not. I have no idea why its doing that. And its not even going in front of the planet?
Sorry, Iapetus, you are no longer my favorite moon of Saturn!
Clear skies,
Greg
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Wow Greg! That there's some serious perturbations doing their mischief! Or a polynomial fit gone awry  . Need more terms, I guess  .
Just kidding, of course. Iapetus is certainly misbehaving.
I haven't looked at the 'Explanatory Supplement to The Astronomical Almanac" to see what they were using in the early 1990s. It's probably out of date by now anyway. I can see what they have if you're interested, Greg.
Phil S.
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That's likely where I got the original method. The problem with Iapetus (which would make a great name for a paper, or maybe a movie) is that it has a weird orbit that probably has some pretty complicated perturbations. I'm looking into other solutions, but this rabbit hole just got a lot deeper, and I have other things I'm supposed to be working on right now. I may have to put it back the way it was and fix it another day.
By the way, eyeballing the Horizons data, it looks like the transit will occur between 2022-Apr-29 22:00 UT and 2022-Apr-29 23:00 UT. This is an unusual and difficult calculation to do accurately, so don't just assume the other source of information is actually correct about the date and time of the transit. JPL has to be able to navigate spacecraft near the moons, so they are the gold standard.
Clear skies,
Greg
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And given what happened several years ago (assuming the news reports are correct) with a Mars fly-by where two parts of the team used metric & English units & hit the planet - nobody's perfect. Close approaches are tricky.
You could write an Iapetus papyrus.
Phil S.
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Greg, you're talking mid transit time. Correct? With all the positions corrected using SPICE, and viewing from the center of the Earth:
1st contact: 29th April at 18:24 UTC
2nd contact: 29th April at 18:34 UTC
3rd contact: 30th April at 02:53 UTC
4th contact: 30th April at 03:03 UTC
As for Iapetus having a weird orbit, not sure I understand. Weird are Ariel, Charon, Proteus, Rhea, and possibly Triton.
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