Why does the orbit track sometimes 'vibrate'

[Dave-In-Texas]

i've noticed that occasionally an orbit track that should be stable, meaning no thrust is occurring and no other obvious things are happening to cause fluctations, will jiggle madly. the peri and apo apsis markers will swing back and forth rapidly and things like surface velocity are jumping around. is there any consensus why this happens? i'm running the 32bit version for the usual reasons..

[sal_vager]

Floating points, basically

The further from the root of the active vessel, the greater the inaccuracies, also orbits that are very near to circular will have their Ap and Pe move due to this.

[Nathair]

The further from the root of the active vessel, the greater the inaccuracies

What further from the root of the active vessel? The "control from here" point?

[Anquietas314]

What further from the root of the active vessel? The "control from here" point?

Other parts of your vessel. If you notice, larger vessels tend to cause your orbit to shift temporarily when they're rotating . The "root" is just the "first" part on the vessel, or the part you assigned using the re-root tool.

[Nathair]

Other parts of your vessel.

So it's just length; distance from root to extremity?

[Tex_NL]

So it's just length; distance from root to extremity?

Yes. Orbits are calculated from the root but your craft rotates around the center of mass. The further those are apart the bigger the oscillations.

[Anquietas314]

So it's just length; distance from root to extremity?

Well, no, not quite. The offset's stored as a vector of three separate floats, each of which has its own floating point errors. The larger the magnitude ("distance") of that vector, the larger each of the floats is, which means the errors are larger, and so have a larger impact on the physics. Then of course there's the relative rotations, converting all that back to the world coordinates, etc. It's obviously a bit more complicated than that, but let's really not get into the technical issues of floating point numbers. We'd be here for weeks if not months.

[Nathair]

Yes. Orbits are calculated from the root but your craft rotates around the center of mass. The further those are apart the bigger the oscillations.

Thanks, that makes a lot more sense (or at least it conforms with what I've always thought was happening.)

[Mesons]

You may have noticed that the oscillations cease when you are time warping. The jiggle comes from rotations and flexing of joints from your vessel, because:

Yes. Orbits are calculated from the root but your craft rotates around the center of mass. The further those are apart the bigger the oscillations.

This means that the velocity used to calculate the orbit is varying as your vessel jiggles and wobbles as it flies. Time warping stops the physics calculations causing these small movements, so the velocity used for orbit calculation is the same as the velocity of the center of mass.

[Dave-In-Texas]

Thanks guys! I am using a very short craft in my recent save, only 7.4m x 4 x 4; I radially attached 4 science juniors each with a fuel tank underneath no struts just a simple technique i learned from a scott manley video :-p unsure if this makes my ships more wobbly but since i've done this from like day 1 and seen this issue every time eventually, it makes sense.

I was also in a nearly circular orbit in the recent instance that made me log in and ask the question

edit: i had a circular orbit at 20k within a few meters and the nodes were nuts as expected. I changed the apoapse up by 4k abnd things calmed down a lot, with only the last 2 digits bouncing around.

Edited January 3, 2015 by Dave-In-Texas