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**Henk W. Broer**

- Professor Henk W. Broer is a professor of Dynamical Systems at the Johann Bernoulli Institute (JBI) of Groningen University.
- His main research interest is with transitions between and co-existence of periodic, multi-periodic and chaotic dynamics.
- He is an acting scientific director of the JBI and also a member of the Royal Netherlands Academy of Arts and Sciences (KNAW).
- Apart from research and teaching he focusses on the popularization of mathematics and mathematical physics.

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**Plenary talk: Near-horizon celestial phenomena, a study in geometric optics**

Near horizon phenomena, like blank strips in the setting sun or Fata Morganas, can be largely understood in terms of geometric optics. Here the existence of a warm layer above the observer in which light rays can be either refracted of reflected, already suffices to explain many of the phenomena; a reasoning which goes back to Alfred Wegener in the 1920s.

We review this theory, where first the atmosphere is divided into discrete, optically homogeneous layers, and where later the atmosphere is considered as a continuous, isotropic medium. Thus the theory gets embedded in differential geometry, where light rays are geodesics. A simplifying assumption is that the refraction index only depends on the elevation above the surface of the earth.

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