Definition:Dandelin Spheres/Ellipse

Definition
Let $\CC$ be a double napped right circular cone with apex $O$. Let $\PP$ be a plane which intersects $\CC$ to form an ellipse.

The Dandelin spheres $\SS$ and $\SS'$ appear as follows:


 * Dandelin-spheres-ellipse.png

Also see

 * Dandelin's Theorem