I keep forgetting the formula, so here it is for posterity.

In PHP, it looks like this:

$foci = round (sqrt (pow ($major_axis, 2) - pow ($minor_axis, 2)) ,4);

And here's a form to prove it:

** Place nails 1.7321 units apart, string is 2 units long. **

For a nice explanation of elliptical math and nomenclature, see

http://en.wikipedia.org/wiki/Ellipse

In PHP, it looks like this:

$foci = round (sqrt (pow ($major_axis, 2) - pow ($minor_axis, 2)) ,4);

And here's a form to prove it:

Major axis

Minor axis

Minor axis

For a nice explanation of elliptical math and nomenclature, see

http://en.wikipedia.org/wiki/Ellipse