The long and short radii a and b of an ellipse lie on the hyperbola a² – b² = f², where f is half the distance between the foci.

Your mouse points to one of the foci, and the left and right arrow keys change the ratio a/b.

You may need to click the display before it will pay attention to your keyboard input.

Here's the source code.

Written in Processing, and licensed under a Creative Commons Attribution 4.0 International License.