Definition:Dandelin Spheres/Parabola

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

Then it is possible to construct one sphere $\SS$ so that it has ring-contact with $\CC$ such that $\PP$ is tangent to both $\SS$.


 * Dandelin-spheres-parabola.png

Also see

 * Dandelin's Theorem