Hello, Let's consider a small 3 planet system. One of the planet's is fixed and is a lot heavier than the others. The acelleration is mP2 * aP2 = - FP3 - FP1 From with the velocity is derived with allows to calculate the positions of the planet's in a simulator program. After running the simulation, The orbit of one of the "moving" planets isn't fixed - some variation occur after each loop. What can cause this? The other planet (that's not fixed) has an elliptic fixed orbit. FP = Gravitational Force of Planet Thanks.