Note that [math]a[/math] is always the semimajor axis. In this figure, the ellipse remains centered at the origin. The black points indicate the foci of the ellipse and [math]c_e[/math] is the distance from the origin to a focus. Drag the blue points to see how the foci are affected.

[i]This applet was developed for use with [url=https://www.pearson.com/en-us/subject-catalog/p/interactive-calculus-early-transcendentals-single-variable/P200000009666]Interactive Calculus[/url], published by Pearson.[/i][br]