Sun's orbital path weird ! (Celestia 1.5.0)

[Cham]

I just noticed this, with the latest CVS Celestia 1.5.0 :



I can already hear my students asking "Sir !? What is this ? Do the sun really follow THAT path ??", with some worrying on their large opened eyes...

And there's something similar (but less critical) on the Moon's orbital path. This needs to be fixed !

[selden]

Cham,

Except for the connection from the endpoint of the orbital path back to the beginning of the path, yes, the sun *does* follow that path! The sun's orbit around the system's barycenter does not follow exactly the same path every time because of the motions of Jupiter and other planets. Celestia shows only one revolution.

In other words, I think the fix should be to leave the end points disconnected and not to try to make it look like it's a continuous loop.

[Malenfant]

The screenshot looks suspiciously like the sun is orbiting a point that is itself orbiting a stationary barycentre. So you get a spiral pattern, not a closed orbit (though it would close eventually). Is that correct?

[Cham]

I removed completelly Runar's addon about the sun. But the orbit is still there with its weird and unacceptable shape. Moving in time can even show a stupid configuration :



This is really unacceptable.

[chris]

The screenshot looks suspiciously like the sun is orbiting a point that is itself orbiting a stationary barycentre. So you get a spiral pattern, not a closed orbit (though it would close eventually). Is that correct?

Yes, that's correct . . . I'm not sure what should be done about the Sun's orbit, since it's not really periodic. I could just omit the segment connecting the beginning and ending points, as Selden suggests. That would be an easy improvement. The problem with continuously updating the orbit to some window centered about the current simulation time is that the orbit calculations are quite expensive.

--Chris

[Cham]

Chris,

I suggest two solutions : one easy to code and the other probably very hard to code. I let you decide which one is the easy solution

1- simply remove the drawing of that orbit, in the special case of the Sun.

2- make the path partially drawn and dynamically drawing itself while the sun move on it, a bit like a ghost (sorry, I'm having some troubles expressing this in English). The path could be shown like an animated trail, erasing itself in real time continuously while the Sun moves forward in front of that trail. The trail-path should be drawn from the Sun's center, and not on the side (like on my last picture above, which is just nonsense).

[ElChristou]

If we cannot simulate the orbit, why not create a kind of ??? ?

[Cham]

If we cannot simulate the orbit, why not create a kind of ??? ?

Yes, this is a good compromise. Third solution.

[chris]

If we cannot simulate the orbit, why not create a kind of ??? ?

I don't understand what you mean. Infinity?

--Chris

[Cham]

He means a closed curve of a Lissajou type, or knot type, as an approximation for the representation of the real path followd by the Sun.

[chris]

He means a closed curve of a Lissajou type, or knot type, as an approximation for the representation of the real path followd by the Sun.

The problem is that Sun's orbit doesn't ever repeat itself exactly.

--Chris

[Cham]

He means a closed curve of a Lissajou type, or knot type, as an approximation for the representation of the real path followd by the Sun.

The problem is that Sun's orbit doesn't ever repeat itself exactly.

--Chris

Yes, of course, but the closed curve idea is just an approximate representation. IMO, if the real curve (in Celestia) doesn't close itself, we should hide the path, or use a ghost trail like the one I've described. The actual representation is very crude, and can mislead a lot of people, especially if the curve is entirely outside the Sun (as shown on my last picture).

[ElChristou]

And what about scripting an orbit in continuous change?

[Cham]

The ghost trail idea can be expanded :

Lets draw HALF of the orbital path, with a dash curve. The path moves with the Sun and never detach itself of it.

[Cham]

Chris,

since the new position of the sun, Celestia start with the camera extremelly close to the sun. I get a full white screen, before it goes to the Earth and editing the start.cel script don't change this behavior. Why ? Having a white screen at startup is very annoying !

[chris]

Chris,

since the new position of the sun, Celestia start with the camera extremelly close to the sun. I get a full white screen, before it goes to the Earth and editing the start.cel script don't change this behavior. Why ? Having a white screen at startup is very annoying !

Celestia has always set the starting position to be the origin; now that the Sun is very near the origin, it fills the whole screen when you start. I agree that it's jarring to have a glaring white screen at start up. I'm trying to come up with the best workaround.

--Chris

[ElChristou]

No way to Celestia start with and offset of 1/2 UA for example?

[chris]

No way to Celestia start with and offset of 1/2 UA for example?

I just checked in a fix for a bug that caused problems when you attempted to do a goto object command with a duration of zero. With that working, it's not possible to create a start script to set the starting location wherever you like.

--Chris

[ElChristou]

I just check this famous Sun orbit, and really it seems to not correspond in anything so don't make much sense... why not just eliminate it? (in this state it is useless or not?)

[revent]

In the middle of messing with other things, I've come up with a workaround/solution for the strange way the Sun's orbit is shown.

This /is/ the Sun's actual orbit around the barycenter for the period from 1599 to 2201, from the latest JPL ephemeris. Wierd, huh.



What I did here is extract the Sun's orbit from the DE415 ephemeris (which is more recent than what HORIZONS uses, actually) into a separate SPICE kernel (about 3.7 Meg) and created a SSC to replace the sun with it. Here it's showing the full timespan from the ephemeris, but you can make it show a shorter timespan by defining a Period in the SpiceOrbit. Celestia is still (I think) connecting the ends of the orbit, but who can tell in that spaghetti?

The downside to doing this (at least with only DE415 data) is that at the ends of the ephemeris the Sun stops moving. You can fix that problem easily enough by leaving in the CustomOrbit declaration for VSOP87, but then Celestia goes back to drawing the VSOP orbit even when it's using the SPICE kernel. The real fix to the Sun stopping would be to use a longer ephemeris, or merge frames from one onto the ends, but I haven't bothered to do it. I mostly look at times near the present.

(As a side note, shouldn't SPICE orbits have the highest priority when it comes to orbit rendering, since they're the most precise?)

If anyone wants to use this to fix the Sun orbit problem until Chris comes up with a better solution, I'd be more than happy to email it or post it on the Motherlode. I'll even make it longer it anyone even cares (it's a pretty trivial task).

FYI, I'm in the process of creating a version of solarsys.ssc that uses the DE415 ephemeris, which is how this came up. I'm planning on merging the other ephemerides that are needed for the major bodies in the default distro (MAR063, JUP230, PLU013, etc) as a addon, and then creating a couple more addons for the leftover moons.

As another rambling side note , I saw a post from Selden a while back where he made a comment about Jupiter being located about 200km from where Horizons puts it. I think the source of that last little discrepancy is that VSOP87 is providing the location of the Jupiter System Barycenter, not Jupiter. Jupiter averages about 165km for the barycenter, which is pretty similar to what he noticed. I should know for certain once I get out to Jupiter with DE415.