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2 Bodies Appulse
#1
On the Special Events page, Two Bodies Appulses, I am trying to get appulses between Saturn and Iapetus.
I can get Saturn/Titan and Mars/Phobos. But not Saturn/Iapetus.
Parms: Above/Below Horizon, Day or Night, and "Show All Events".
Even though Saturn/Iapetus can be selected, is there a restriction on which satellites can be computed?
Thanks, Steve
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#2
Hi Steve,

That's a pretty unusual use of the appulse finder. I am not sure that anyone has tried finding appulses between the planet and a moon before. I doubt that I have. I'm happy to hear that it works for so many!

As for why Iapetus might be failing, have you tried making the search period longer? Iapetus has a 79 day orbital period. The appulse finder is very sensitive to the search time. If too short it is prone to fail. There is code internally that adjusts the parameters to keep this from happening, but it is set on a case by case basis and it may not be set up for planetary satellites. Try a search period of 160 days or more.

So now I am really curious: why are you interested in knowing when the moons are closest to the planet?
Clear skies,
Greg

SkyTools Developer
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#3
Greg,
First, I am looking at this because I am helping Larry Mitchell develop the TSP Advanced list. The subject is Bernard objects. Bernard discovered Iapetus, so Larry wants to put this on the list of objects.

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.

As for making the search period longer, I just want it for the TSP week.

FYI, also going on the list, maybe, is Amalthea and Phobos/Diemos at Eastern/Western elongations.

Hope that explains why I am looking at this.
Thanks Greg,
Steve
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#4
I ran it on my copy of ST3 V3.2j Pro and found appulses approximately every 5 weeks. I set it to 1 degree, picked Saturn (top) and Iapetus (bottom), started on 1/01/2018 and used a period of 9 months. I got these dates and spacing...

Jan 23, 1.4'
Mar 3, 1.4'
Apr 14, 1.4'
May 22, 1.5'
July 1, 1.7'
Aug 8, 1.7'
Sept 18, 1.6'
Joe Stieber
http://sjastro.org/
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#5
(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.
Clear skies,
Greg

SkyTools Developer
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#6
(2018-01-29, 05:38 PM)Joe Stieber Wrote: I ran it on my copy of ST3 V3.2j Pro and found appulses approximately every 5 weeks. I set it to 1 degree, picked Saturn (top) and Iapetus (bottom), started on 1/01/2018 and used a period of 9 months. I got these dates and spacing...

Jan 23, 1.4'
Mar 3, 1.4'
Apr 14, 1.4'
May 22, 1.5'
July 1, 1.7'
Aug 8, 1.7'
Sept 18, 1.6'

Thanks Joe. However, I am looking for values for only one week, the first full week of May, during the Texas Star Party.
Steve
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#7
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.
Joe Stieber
http://sjastro.org/
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#8
(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.
Greg,
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
Steve

(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,

Steve
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