Dear Rolf, Yes, this is indeed the answer that I was hoping for:
The midpoints M1 and M2 of parallel chords lie on a (conjugate) diameter. And its midpoint M is the centre of the ellipse. The proof is trivial. You just have to consider what the figure looks like if you project the ellipse (with the two chords) into a circle. You don't even need to know how to achieve this projection. You just need to know that it can be done! Very best wishes Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
