TIL that there is no formula for the exact perimeter of an ellipse, but that you could make an arbitrarily close approximation of a formula for the perimeter of a *given* ellipse, and that such a formula would depend on the sum of an infinite series.

TIL that this is technically how we have a formula for the perimeter of a circle, but don't notice the messiness because the circle's "axes" are of equal length and thus a single sum of an infinite series works. We call it "pi" to keep it unmessy.