Celestia Forums

I very frequently have this weird looking orbit for moons. I already reported this problem before (especially for the Sun), but here it is again :



Can't something be done to remove this horrible discontinuity ? I know the moon's orbit isn't a nice periodic orbit like an ellipse, but I can already hear the naive students asking what this is, and the teacher losing its time explaining it's just a rendering limitation in Celestia

While the moon orbit is calculated with some sophisticated algorithm, isn't possible to just show a "mean" elliptical orbit at the same time ?

A vivid illustration of a "Quantum Jump"...

Cham wrote:I very frequently have this weird looking orbit for moons.

...

Can't something be done to remove this horrible discontinuity ? I know the moon's orbit isn't a nice periodic orbit like an ellipse, but I can already hear the naive students asking what this is, and the teacher losing its time explaining it's just a rendering limitation in Celestia :x

While the moon orbit is calculated with some sophisticated algorithm, isn't possible to just show a "mean" elliptical orbit at the same time ?

Would it be acceptable if Celestia simply omitted the line segment connecting the endpoints of the orbit? In other words, if the orbit were left with a gap?

- Hank

I think Celestia should draw the "mean elliptical orbit", as defined in the SSC, instead of trying to show the true, calculated orbit. Of course, Celestia should still place the moon on the right - calculated - track, and only draw the elliptical orbit. Times to times, the moon would appear to not be placed on the elliptical orbit : this could actually be a feature, since it could show how the real motion differ from an elliptical one.

Cham wrote:I think Celestia should draw the "mean elliptical orbit", as defined in the SSC, instead of trying to show the true, calculated orbit. Of course, Celestia should still place the moon on the right - calculated - track, and only draw the elliptical orbit. Times to times, the moon would appear to not be placed on the elliptical orbit : this could actually be a feature, since it could show how the real motion differ from an elliptical one.

If there's a custom orbit, Celestia ignores the mean elliptical orbit that's defined in the ssc file. These elements are just there as a sort of documentation, mostly. However, Celestia already has a function to convert from a position and velocity to Keplerian elements. This could be used to compute an elliptical orbit path for the Moon and other bodies with orbits described by semi-analytic series (e.g. VSOP87 for the major planets.) The elliptical orbit path could be computed much more quickly than directly evaluation the series at hundreds of sample times, so this approach has advantages beyond just eliminating the discontinuity.

--Chris

chris wrote:The elliptical orbit path could be computed much more quickly than directly evaluation the series at hundreds of sample times, so this approach has advantages beyond just eliminating the discontinuity.

How frequently would the orbit path points be updated?

- Hank

Hmmm.

Just thinking...

If velocity and position are all that are needed (and are available: I suspect velocity might not be, depending on the ephemeris) and if it's faster than evaluating vsop87, spice or jpl ephemeride locations, while I think one still wouldn't want to recalculate all of the orbit paths every frame, I think it would be reasonable to consider always recalculating the paths of the currently selected, followed and/or tracked bodies. I suspect this might eliminate the intermittant drawing of orbit paths in front of the outer planets, for example.

It's not obvious to me if this change would have a chance of eliminating the gaps sometimes seen in orbit paths on some Macs. That'd depend on the algorithm used to actually draw the paths, I suspect.

selden wrote:It's not obvious to me if this change would have a chance of eliminating the gaps sometimes seen in orbit paths on some Macs.

The orbit gap problem is something I see often too, and it seems to be related to depth buffer precision as Chris would so often say. I don't know however if it's due to an actual bug where distant orbit segments are being accidentally placed in a near depth bucket, etc.

Cham,

I was answering a private e'mail which asked why the Moon varied so much from its orbital path in Celestia and it suddenly occurred to me to wonder why you don't use the discontinuities in the Lunar and Solar orbits as teaching opportunities. Don't you teach your students about precession?

Selden,

yes, I'm teaching all this (it's important to explain the eclipses variations). But we could do it also if the moon orbit was shown as a simple elliptical path, while the moon still following its complex path (without showing it). Since the moon wont appear exactly on the elliptical path (which would just be a "mean path"), the teacher could explain why it is this way, in the classroom.

Cham wrote:Selden,

yes, I'm teaching all this (it's important to explain the eclipses variations). But we could do it also if the moon orbit was shown as a simple elliptical path, while the moon still following its complex path (without showing it). Since the moon wont appear exactly on the elliptical path (which would just be a "mean path"), the teacher could explain why it is this way, in the classroom.

A I understand it (which I'll admit may be poorly), the moon's mean orbit changes with time (fairly rapidly) due to precession of the argument of the periapsis and the longitude of the ascending node. So it can't be displayed with a fixed elliptical orbit. It may be possible to hack something with the new BodyFrame feature, but I think it would take some doing.

- Hank