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The angular size of the Sun and Moon and the emergence of Devonian tetrapods

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The very similar angular sizes of the Sun and Moon as subtended at the Earth is generally regarded as coincidental. In fact, it is a simple mathematical consequence of even roughly comparable lunar and solar tidal amplitudes. I will present a case for why the latter may have been a biological imperative. The key point is that comparable tidal amplitudes sharing very close, but distinct, Doppler-shifted forcing frequencies leads to strongly modulated equilibrium tides. These tides should be viewed in the context of reconstructions of Late Devonian continental land masses which suggest a broad western opening to the Iapetus Ocean that tapers to a more narrow, shallow-sea strait between Laurasia and Gondawana. The combination of this geography with strongly modulated tidal forcing may well have been conducive to a particularly rich, penetrative network of shallow tidal pools at the epoch when tetrapods were making their amphibian transition. A. S. Romer’s classic vision of stranded aquatic tetrapods forced to scramble seaward by evaporating inland pools may be supported not by arid conditions (as once envisaged), but by the ancient astronmomical observation that the Sun and Moon have the same angular size when viewed from the Earth.

This talk is part of the Institute of Astronomy Extra Talks series.

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