Archive for coincidence

The Eclipse Coincidence Question

Posted in The Universe and Stuff with tags , , , , on March 22, 2015 by telescoper

The day before last week’s (partial) solar eclipse I posted an item in which I mentioned the apparent coincidence that makes total eclipses possible, namely that the Moon and Sun have very similar angular sizes when seen from Earth.

In the interest of balance I thought I would direct you to a paper by Steve Balbus that develops a detailed argument to the contrary along the lines I described briefly in my earlier post. I am not entirely convinced but do read it and make up your own mind:

Here is the abstract:

The nearly equal lunar and solar angular sizes as subtended at the Earth is generally regarded as a coincidence. This is, however, an incidental consequence of the tidal forces from these bodies being comparable. Comparable magnitudes implies strong temporal modulation, as the forcing frequencies are nearly but not precisely equal. We suggest that on the basis of paleogeographic reconstructions, in the Devonian period, when the first tetrapods appeared on land, a large tidal range would accompany these modulated tides. This would have been conducive to the formation of a network of isolated tidal pools, lending support to A.S. Romer’s classic idea that the evaporation of shallow pools was an evolutionary impetus for the development of chiridian limbs in aquatic tetrapodomorphs. Romer saw this as the reason for the existence of limbs, but strong selection pressure for terrestrial navigation would have been present even if the limbs were aquatic in origin. Since even a modest difference in the Moon’s angular size relative to the Sun’s would lead to a qualitatively different tidal modulation, the fact that we live on a planet with a Sun and Moon of close apparent size is not entirely coincidental: it may have an anthropic basis.

I don’t know if it’s a coincidence or not but I always follow the advice given by my role model, Agatha Christie’s Miss Marple in Nemesis: “Any coincidence is worth noticing. You can throw it away later if it is only a coincidence.”..