Hi PF, According to Kepler's first Law any planet in orbit of the Sun describes an elliptical orbit. I don't know why I can't go on wikipedia since days. But if I remember well I've seen that when there are 2 bodies like 2 stars for example then the center of mass will remain quiet (since there's no external force) while the 2 bodies would rotate around it. So I'm confused and don't understand why the motion of a satellite should be an ellipse. Probably due to conservation of angular momentum. But then I don't understand why the 2 bodies are rotating around their barycenter. Or maybe it's a combination of the 2 situation. I'm all confused. Could you please explain to me what's happening?