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2 Bodies Appulse
(2018-01-29, 06:13 PM)theskyhound Wrote:
(2018-01-28, 11:49 PM)SteveGoldberg Wrote: Greg,
What I am trying to find is where Iapetus is during TSP in relation to Saturn. Specifically the RA/Dec. When I pulled up the Info box for Saturn on each day of TSP, there was the PA and distance of Iapetus and a handful of moons. That helps. If I remember (I'm not at home but at our observing site doing work, not observing with the moon out) and I don't have access to my XLS I created. But I think the distance from the Info box was way less than 1 degree, so I expected the appulses to show up.

Hi Steve,

This isn't the right tool for what you are trying to do. An appulse is the moment in time when two bodies are closest to each other, which is usually the worst possible time to observe the moon of a planet. Iapetus has a nearly 80 day orbital period. There is no reason to expect there to be an appulse during the week of TSP, which is likely why it isn't showing up.

Regarding the Object Info, look up Iapetus itself in the Designation Search tool rather than Saturn. It will display the position of Iapetus instead of Saturn when you do that.  

I suggest using the Interactive Atlas. You can target either Saturn or any one of its moons individually. Of course the moons are going to move from night to night, and even during the course of a night. You can make a chart with a trail showing the position of Iapetus, labelled by date/time. To do this, target Iapetus, select Motion Trails from the tool bar. Choose Target Object Only and a time period. You may be surprised at the motion with regard to the sky. It will likely move in more or less a straight line, and by quite a bit each night. This is because most of the motion against the sky comes from the motion of the planet rather than the Iapetus.

There is also another tool that might be useful to you. Rather that search for appulses, switch to the Satellite Elongations tab. Select the moons you are interested in. An elongation is the time when a moon is farthest from a planet, and these are usually the best times to view the moon, although for Saturn, it won't really matter for the inner most moons that are close to the rings like Mimas.
Thanks for the response. I did use the Object Info box for Iapetus and saw the RA/Dec values.
I also found the answer using the Ephemeris Positional for Iapetus.
So, for now, I have the answer we've been looking for.
Thanks for your help, Greg

(2018-01-30, 11:39 PM)Joe Stieber Wrote: Steve,

The point of my table of appulses between Saturn and Iapetus in the first nine months of 2018 was...

(1) that ST3 will indeed generate the date and spacing of each appulse (and the time, which I didn't tabulate)

(2) there are no appulses during the TSP.

As Greg noted, Iapetus has an approximate 80-day period, and the appulses I listed were roughly 5 weeks apart -- or roughly half the period Greg noted. For each revolution of Iapetus around Saturn, there are two appulses, one with Iapetus on either "side" of Saturn.

I suspect that you think an appulse of Saturn and Iapetus will necessarily occur during the TSP, but that just isn't the case. If you limited your search period to the TSP dates, that's why none showed up.

As Greg also pointed out, you should just use SkyTools to find the position(s) of Iapetus during the available period(s) of observation at the TSP, and go with that.
Thanks Joe.
We were looking for the RA/Dec of Iapetus during TSP. We found the solution using the Ephemeris Positional feature.
So, I was approaching the problem using the wrong function.
Learn something every day.
Thanks again for helping,


Messages In This Thread
2 Bodies Appulse - by SteveGoldberg - 2018-01-28, 08:42 PM
RE: 2 Bodies Appulse - by theskyhound - 2018-01-28, 09:04 PM
RE: 2 Bodies Appulse - by SteveGoldberg - 2018-01-28, 11:49 PM
RE: 2 Bodies Appulse - by theskyhound - 2018-01-29, 06:13 PM
RE: 2 Bodies Appulse - by SteveGoldberg - 2018-01-31, 01:40 AM
RE: 2 Bodies Appulse - by Joe Stieber - 2018-01-29, 05:38 PM
RE: 2 Bodies Appulse - by SteveGoldberg - 2018-01-30, 10:31 PM
RE: 2 Bodies Appulse - by Joe Stieber - 2018-01-30, 11:39 PM

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