I am trying to write a simple java program to model the motion of planets according to kepler's second law. I have everything working...except I can't find how to find an equation relating position and time. I have x = a cos(t) y = b sin(t) where a and b are the semimajor and semiminor axises respectively, but those equations are for uniform velocity which planets do not exhibit. I've googled around and looked in a few books but I can not find any such equation. I can find the equation for velocity...but I need a model for the actual position. Does anyone know of any such model or have any suggestions?