Hi all. I have been working, off and on, on a Solar System simulator (in MS Excel) for many months. I am using the Runge-Kutta-Fehlberg 8/9 8th-order integrator and JPL's initial conditions to try to reproduce an accurate model of planetary motions. This, of course, involves modeling the relativistic effects of gravity. And therein lies my problem... using a couple of different post-Newtonian models of relativity, I seem to be getting exactly twice the effect I should be. In other words, Mercury's perihelion apparently advances by exactly twice the famous 43 arcsecond per century rate. I have done a lot of checking... Newtonian-only effects are modeled perfectly vs analytical. I ran some test cases of two-body motion and compared the perihelion advance to analytical... exactly 2x. Obviously, I seem to be applying these models incorrectly. And the fact that a couple of different post-Newtonian models give me exactly the same result points to something I am doing wrong, rather than the models themselves. I am, however, baffled about what that something might be. Help?