An anthropic explanation for the nearly equal angular diameters of the Sun and Moon.

The very similar angular sizes of the Sun and Moon as subtended at the Earth is generally portrayed as coincidental. In fact, close angular size agreement is a direct and inevitable mathematical consequence of even roughly comparable lunar and solar tidal amplitudes. I will argue that the latter was a biological imperative for the evolution of land vertebrates and can be understood on the basis of anthropic arguments. Comparable tidal amplitudes from two astronomical sources, with close but distinct frequencies, leads to strongly modulated forcing: in essence spring and neap tides. This appearance of this surely very rare tidal pattern must be understood in the context of paleogeography and biology of the Late Devonian period. Two great land masses were separated by a broad opening tapering to a very narrow, shallow-sea strait. The combination of this geography and modulated tidal forces would have been conducive to forming a rich inland network of shallow but transient (and therefore isolating) tidal pools at an epoch when fishy tetrapods were evolving and acquiring land navigational skills. I will discuss the recent fossil evidence showing that important transitional species lived in habitats strongly influenced by intermittent tides. It may be that any planet capable of harbouring a contemplative species displays a moon in its sky very close in angular diameter to that of its sun.