## Henk W. Broer

### Near-horizon celestial phenomena inspired by Minnaert and Wegener, Bernoulli and Hamilton

In geometric optics light rays are being defined with the Fermat Principle, asserting that light rays follow paths of shortest time. Later this was turned into the Hamilton Principle. It turns out that quite a number of optical phenomena in the atmosphere can be explained with a medium only consisting of two homogeneous layers. We'll discuss blank strips or zones in the setting sun, but also fata morganas. For smooth refraction index profiles the theory gets differential geometric, where light rays are formed by geodesics. This immediately gives rise to a dynamical system. In the simplest approximation the systems are integrable, but also perturbations are possible where chaos can emerge. A large number of these models can be realized in terms of a surface of revolution.