Billiard maps of confocal ellipses commute: a geometric proof
We give a new, purely geometric proof of the classical fact that billiard maps in confocal ellipses commute. Existing proofs of this result rely on symplectic geometry and an invariant measure on the space of oriented lines; ours uses only elementary projective and Euclidean geometry. The argument rests on a main theorem describing how a billiard reflection can be constructed geometrically via tangents to confocal ellipses, from which the commutation property follows directly. This also yields, as a byproduct, an incidence result for tangents at four reflection points, revealing a hidden symmetry in the confocal billiard configuration. The main theorem recovers, via purely synthetic means, a fact previously established only by direct computation (Berman et al., 2024).