Pluto Charon Mutual Events

[Calculus]

The opportunity of observing Pluto-Charon mutual events occurs only once every 124 years. In Celestia, the sequence of events for the 1987 period cannot be seen. It must be bug in calculation of Pluto/Charon orbits.

[chris]

The opportunity of observing Pluto-Charon mutual events occurs only once every 124 years. In Celestia, the sequence of events for the 1987 period cannot be seen. It must be bug in calculation of Pluto/Charon orbits.

Pluto's orbit is not calculated to the same precision as other planets. But the real problem is that Charon's orbit is approximated by an ellipse. I'll add a better calculation soon.

--Chris

[granthutchison]

But by far the biggest contributor to the absence of Pluto-Charon events in Celestia's 1987 is the erroneous position of Pluto's axis of rotation.
I spent a bit of time converting the RA&dec coordinates for Pluto's north pole to coordinates relative to the plane of Pluto's orbit, but for some reason I'm still about 12 degrees out.
However, a purely fudged "correction" of

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   EquatorAscendingNode 231

generates the mutual events required, though these bear no relationship to "real" events because the orbital position of Charon is still incorrect.

Grant

[Calculus]

At least we can have an idea how it looks.
Thanks Grant

[granthutchison]

Right, I've got the hang of the orientation of Pluto's axis. My mistake was thinking that Celestia defaults to setting the planetary equator parallel to the plane of the planet's orbit - in fact, the equator is set parallel to the ecliptic. So the Obliquity and EquatorAscendingNode have to be calculated relative to the ecliptic, not Pluto's orbit. This means that the Obliquity provided in solarsys.ssc is wrong (it's the usual figure given for axis inclination relative to orbit), and my calculated EquatorAscendingNode came out wrong, too .
So: if you insert the following in the Pluto definition

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   Obliquity         115.604 # to ecliptic plane: 122.53 to plane of orbit
   EquatorAscendingNode   228.335 # in ecliptic plane, from vernal equinox



it'll orientate Pluto's rotation axis correctly, and the Pluto-Charon mutual events will occur on schedule without need for fudging.

I should also be able to come up with a proper position for Charon, given a bit of thought. Watch this space ...

Grant

[Phule]

So how would the .ssc look?

[granthutchison]

So how would the .ssc look?

Sorry, I don't understand what you mean.

Grant

[Calculus]

Just add these lines exactly as they are in the ssc file.
If you are running windows, you can use the notepad

[granthutchison]

Ah, OK. I see.
Yes, just slip those two lines into the Pluto definition, but remember to comment out or delete the Obliquity value that's already in there.

Grant

[Guest]

I've now orientated Pluto's prime meridian according to the Explanatory Supplement to the Astronomical Almanac. Since the prime meridians of Charon and Pluto point at each other, I then just hauled Charon around in its orbit until it sits over Pluto's prime meridian, and turned it to face in the appropriate direction. The full definitions for Pluto and Charon now read as follows:

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"Pluto" "Sol"
{
   Texture "pluto.jpg"
   BumpMap "plutobump1k.jpg"
   BumpHeight 2.5
   Radius 1137

   CustomOrbit "pluto"
   EllipticalOrbit
   {
   Period            248.54
   SemiMajorAxis     39.48168677
   Eccentricity       0.24880766
   Inclination       17.14175
   AscendingNode    110.30347
   LongOfPericenter 224.06776
   MeanLongitude    238.92881
   }

   RotationPeriod  153.293904
   Obliquity      115.604 # to ecliptic plane: 122.53 to plane of orbit
   EquatorAscendingNode   228.335 # in ecliptic plane, from vernal equinox

   RotationOffset  -219.254

   Albedo            0.3
}

"Charon" "Sol/Pluto"
{
   Texture "charon.jpg"
   Radius   593

   InfoURL "http://www.nineplanets.org/pluto.html#Charon"

   EllipticalOrbit
   {
   Period         6.387246
   SemiMajorAxis  19636
   Eccentricity   0.0076
   Inclination    0.0 # 96.163 degrees relative to ecliptic
   MeanAnomaly    -39.254
   }
   
   RotationOffset  -39.254

   Albedo         0.3
}


Lay this into solarsys.ssc in place of the original data. Open Celestia and set the time to around 05:30UT on 17 Feb 1985. Go to Earth if you're not already there, and then go to Pluto. Haul your distance way back to flatten the perspective, and then zoom in by narrowing your FOV, so you end up with a view similar to what might be seen through an exceedingly powerful Earth-based telescope.
What you see is the very first Pluto-Charon mutual event to be recorded in its entirety, by Rick Binzel at McDonald Observatory in West Texas.

But if you look back at Earth, you'll find that you can't see Texas from Pluto at 05:30UT. What's going on? Four hours light travel time! Turn the time forward to 09:30UT, and there's Texas in view, as it would be when the eclipse light curve arrived at Earth - at 03:30 local time in Texas. (I don't know the exact time of Binzel's observations, but it seems like I've got a pretty good match for the real event.)

The fact that I've orientated the prime meridians of Pluto and Charon properly means that anyone who wants to paste on texture maps based on the real observed brightness data for Pluto and Charon can do so and see them displayed properly. (But bear in mind that if the edge meridians of your maps are anything other than zero degrees, you'll need to fiddle the RotationOffsets to correct the map orientations.)

Grant

[granthutchison]

Oops, sorry, had forgotten to log in when I made the last "Anonymous" post.

Grant

[Calculus]

Good Job Grant !
Can you check now the mutual events of Saturn's moons ?

[selden]

Some people are never satisfied....

[Guest]

Can you check now the mutual events of Saturn's moons ?

Well, I have noticed that the EquatorAscendingNode for Saturn seems to be a degree-and-a-half out. Converting the J2000 RA&dec coordinates of Saturn's north pole to eclipitic coordinates implies

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   EquatorAscendingNode  169.53


This corrects the dates of Earth's ring-plane crossings: compare the view of Saturn from Earth on 21 May 1995 using Celestia's default EquatorAscendingNode (which shows the rings slightly open), and using my revised version (which shows them pretty much disappeared as we view them edge on).
(It's nice, BTW, to watch the whole triple sequence of ring-plane crossings in 1995 and 1996 in fast time, both from Earth and from Saturn's equator beneath the rings.)

Unfortunately, I find that correcting Saturn's orientation seems to disrupt mutual events - the Rhea-on-Titan annular eclipse of 9 May 1995 happens more or less on schedule using the original Celestia value for Saturn's EquatorAscendingNode, but is a wide miss using my value. This must be something to do with the way the satellite CustomOrbits are defined relative to Saturn's orientation. But I'm not a C programmer, so I can't access this sort of stuff.

Grant

[Calculus]

Thanks again Grant.
Chris already knows there is some work to do about jupiter and saturn moons. I just hope he'll get enough time to spare on this matter (and others ) .

[chris]

Unfortunately, I find that correcting Saturn's orientation seems to disrupt mutual events - the Rhea-on-Titan annular eclipse of 9 May 1995 happens more or less on schedule using the original Celestia value for Saturn's EquatorAscendingNode, but is a wide miss using my value. This must be something to do with the way the satellite CustomOrbits are defined relative to Saturn's orientation. But I'm not a C programmer, so I can't access this sort of stuff.

This problem is probably a screwed up reference epoch . . . I think the calculations for Saturn's satellites use B1950 instead of J2000. I'll have a look tonight.

--Chris