Celestia Forums

Why do some of the planets/ orbitting objects sometimes appear nowhere near their orbit path (Pressing O)?

Just curious.. plus why can't the orbit path be rendered as passing through the planet? When looking at a satellite (moon/spacecraft/planet) parallel to it's orbit (so the orbit line goes left right, or up down through your screen) everything looks fine, but when you try and rotate around the object, the line isn't rendered through it.. it's like it's rendered around it, so if you try and look at avarious other angles (ex: through the planet'd orbit, following it) it looks as though the orbit is all out of whack. Is there some limitation that the orbit line can't pierce the planet? Or is this because of some asthetic reason?

First post here, great program.

AA,

My understanding is that the visual orbit tracks are only computed once, using 100 points along the orbit. When Celestia draws a track, it draws straight lines between those points which are in the field of view. If the visible objects happen not to be near one of the precalculated locations, the tracks can be quite a distance away.

The problem is that the orbits of the planets are calculated extremely precisely, using hundreds of terms in a numerical expansion. The time to do those calculations adds up quickly. It'd reduce the frame rate by an unacceptable amount if all 100 points of all of the visible orbit tracks were recalculated for each frame.

Chris has posted to the Forum that he's considering ways to improve this situation (e.g. recalculating only the track of the selected object), but I suspect the modifications to Celestia won't happen soon.

Thanks for the response, and it makes sense (and I can see it too if I move out far enough). I don't know what other people are running.. but my rig is probably powrful enough to calculate more than just that... (I get an almost rock-steady 85fps with full AA/pixel-shaders/vertex-shaders on) maybe there could be a varible somewhere to choose how many times you'd like the orbit path calculated? Just thinking outloud.

Still wondering why (even when you're at a point in the orbit where the orbit of the planet matches up with the orbit line) that the orbit line can't/isn't rendered as piercing the planet tho' I suppose that it would look silly for the orbit path to look like it was passing in and out of the planet itself (if you were up close enough).

I think simplest way to handle orbit deviation is to get planet's actual position into one of those 100 orbit calculation points. In that way orbit is drawn ewery time througt the planet.

That's not quite what I meant... The orbit deviates, but on those 100-points, or however many there are, at the right angles the planet is most definitely passing through the orbit line perfectly; and then it begans to deviate until it hits the next line (well, the planet is in the right orbit, the line displaying the orbit is incorrect [so I've been told]). The other thing I was mentioning is that the line isn't rendered through the planet, it never pierces the planet, it's like it's rendered behind it.

The way to see this is... look from above, at the circular (near circular, whatever) orbit of a planet. Everything should look fine, then zoom up real close, and get behind the planet's orbit, so it's as if you're following the planet in a space ship, or suspended above the planet. Then try and move the camera (hold right click and rotate around the planet) so you're directly in the path of the orbit, and the line will violently shift around the planet since the orbit doesn't render as piercing through the planet... but it try's to render behind the planet, so the planet is visible at all times.

Maybe this isn't importatnt, but that's what I was the other thing I was talking about.

AA,

Unfortunately, you're seeing an effect that's due to a more general limitation:
Celestia does not implement collision detection or any of the related multi-body effects.

When objects interpenetrate, for example, You'll see one object entirely in front of the other object. You won't see just the parts that are outside the surfaces. Which object is drawn in front will change with your viewpoint.

Hopefully in some future version...

A FEATURE PROPOSAL:

A new option, "Show only orbits I want" could be implemented which would allow us (Celestia users) to.... well, guess.
The Mars is most fascinating planet, at least for us and we want to see the obits of it's satelittes WITHOUT Mars Orbit.