I think what you see there is not necessarily an indication of the simulation happening in 3-d space[1]. It is however evidence of [precession](http://en.wikipe

I think what you see there is not necessarily an indication of the simulation happening in 3-d space[1]. It is however evidence of [precession](http://en.wikipedia.org/wiki/Precession http://en.wikipedia.org/wiki/Precession), which should not happen in a 2-body problem. If no other heavy body was close to the ones shown in your screenshot, this should actually not happen. Since the simulation does, however, only have a finite precision, this could very well be an artifact of the simulation (happens often if you have near-collisions, since the change of velocity there happens rapidly and is hard to describe correctly with a fixed-length time step). [1] In fact if all initial positions and velocities lie in the same 2d plane, all trajectories would also lie in this plane. Since the interface only allows one to set such initial conditions, I am fairly confident that the simulation is 2d only.