I stumbled on this image from here:
It's mentioned w.r.t. to the elliptical orbits of planets and how the focus-directrix property comes into play. It's an interesting POV but when was such a thing known about an ellipse's directrix? Does it predate Kepler or was it a modern discovery? Or a side effect of Dandelin's spheres which are relatively modern? Or something discovered as a result of studying projective geometry?