How to draw an ellipse with a pencil, a string, and two nails
I keep forgetting the formula, so here it is for posterity.
, it looks like this:
$foci = round
($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