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Weird circular orbit problem

cowcow · 2019-07-21T02:18:44

I am having a strange circular orbit problem. I set up the mass of the Sun, plus Mercury's initial orbit position and velocity: https://github.com/sjhalayka/mercury_orbit_glut/blob/a0213a84badf238618c19bc7eb08d1754a84cb9b/main.h#L50 I use these parameters to calculate Mercury's path through space, via Euler integration: https://github.com/sjhalayka/mercury_orbit_glut/blob/a0213a84badf238618c19bc7eb08d1754a84cb9b/main.cpp#L24 The problem is, if I set Mercury's initial velocity to be sqrt(G*sun_mass/distance), then there is still some eccentricity -- the distance and velocity vector length aren't constant, like you'd expect for a perfectly circular orbit. And the discrepancy is not small, the orbit velocity length varies by 100s of metres per second, so it's not a precision issue. Anyone have any idea why my code produces unwanted results? Thanks for your time and attention.