It seems to me that Kepler's Laws necessitate some definition of an ellipse in terms of a coordinate system. I am wondering whether Kepler's Laws mathematically defined what an ellipse is, or if he used an already defined shape in his laws.
The reason I got curious about this in the first place was that I realized that Descartes may not have invented the Cartesian coordinate system by the time that Kepler discovered these laws and I have only seen proofs of Kepler's Laws in a Cartesian system.