  Aberration and radiation pressure reflected by a moving mirror are examples
of the Klein and Poincar\'e models of hyperbolic geometry, respectively.
Reflection at a moving mirror produces a two-way Dopper shift. Its one-way
counterpart, aberration, has nothing to do with the radiation pressure on a
moving mirror, but, rather with the pressure on a completely absorbing surface.
Both pressures vanish when the angle of parallelism is reached. Two-way,
second-order Doppler shifts can be used to establish experimentally the
existence of an angle of parallelism.
