The Eclipse Coincidence Question

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.”..

3 Responses to “The Eclipse Coincidence Question”

  1. Anton Garrett Says:

    True or not it is a brilliant hypothesis. There is plenty on how our ancestors crawled out of the sea in Neil Shubin’s fine book Your Inner Fish.

  2. Phillip Helbig Says:

    I’ll have to read the paper, but it is well known that the day used to be much shorter and the Moon much closer, so the possibility of a total solar eclipse with corona visible (since the angular sizes are almost exactly the same) is possible only for a geologically short time. Yes, there might be reasons for a big Moon, but the precise coincidence of size needs a quantitative explanation if it is more than just coincidence.

    • Phillip Helbig Says:

      OK, I’ll have to read it in more detail. At first glance, though, it seems that he can explain that the sizes are within, say, 10 per cent, but a total eclipse requires more precision than that. The last time I checked, evidence was pretty firm for a shorter day in the past, and hence closer Moon, too large to allow one to see the corona.

      In other words, it is not just the coincidence in size, but also the fact that it happens now.

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