Sometimes the planets and moons are way far away from where their orbits are drawn. Why?
To draw the orbits, Celestia only calculates 100 or so precise locations and then draws straight lines between them. If the orbiting body doesn’t happen to be close to one of those 100 points, then it won’t be very close to the line, either. The position of the orbiting body is calculated very accurately. The lines aren’t. Celestia only calculates a few points around the orbit in order to minimize the amount of computation needed between frames. The more calculations are required, the slower the frame rate is.
