What is the other focus in an elliptical orbit?

[ShadowDatsas]

Hello.
I've read in many books that the orbits of the planets of our Solar system are ellipses.
With the one Focus as the Sun.
Well, what is the other one? I mean every ellipse has two focuses...

 

[Hippasos]

This article describes planetary motion in flat 2D plane:

http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion

I guess there is generalization to 3D as well somewhere...

Or is that really necessary?

 

[ShadowDatsas]

Yay! It's good enough to understand the half!
Thanks, now I see that there isn't something at the second focus. (like a mass or a something)
Thanks.

 

[D H]

I guess there is generalization to 3D as well somewhere...

Or is that really necessary?

It isn't. Gravity is a central force. The velocity vector can always be represented as a sum of two vectors, one parallel to and the other normal to this displacement between the two bodies. The force between the two bodies is directed against the displacement vector between the two bodies. The bodies will always on the plane defined by the initial displacement and velocity vectors.

Planets do not really follow elliptical orbits. The orbits are instead approximately elliptical and the planes slowly (very slowly) wobble. The planets (particularly Jupiter) perturb the orbits of the other planets.