I was recently studying elliptical orbits and the precession of the perihelion of Mercury. I remembered from my pre-calc class that all ellipses have two focal points. In this case the obvious would be the Sun as on focal point for all the planets, but where would the second be? I have a feeling it is something simillar to the Lagrangian points in that there is no concentration of anything, just a point in coordinate space relative to the repective bodies, i.e. L1 for Earth-Sun. Of course there would be a different focal point for each orbit. How might this point be affected if at all, by the presence of another body such as a moon. I don't know if this is a valid question or not due to the space being 3 dimensional and the material to which I am refering was dealing with 2 dimensional ellipses, but I had no idea how to word that in google or the search. Thanks in advance everyone.