In previous posts in our solar eclipse series (here, here, here, here, here, here, here, and here) we touched on safe viewing methods, the mechanics of solar eclipses, and a couple of surprising coincidences. We also explained why the solar eclipses we enjoy from earth’s surface are the best in the solar system.

The solar eclipse coincidences have been noted by astronomers, but most have treated them as mere coincidences. Some scientists are troubled by them. The popular British science writer and astronomer John Gribbin comments on solar eclipses in Alone in the Universe: Why our Planet is Unique:

Just now the Moon is about 400 times smaller than the Sun, but the Sun is 400 times farther away than the Moon, so that they look the same size on the sky. At the present moment of cosmic time, during an eclipse, the disc of the Moon almost exactly covers the disc of the Sun. In the past the Moon would have looked much bigger and would have completely obscured the Sun during eclipses; in the future, the Moon will look much smaller from Earth and a ring of sunlight will be visible even during an eclipse. Nobody has been able to think of a reason why intelligent beings capable of noticing this oddity should have evolved on Earth just at the time that the coincidence was there to be noticed. It worries me, but most people seem to accept it as just one of those things.

Yet other scientists consider these coincidences as pointing to a deeper truth. In 1999 an astronomer proposed a kind of anthropic “explanation” for these coincidences in a paper published in Astronomy & Geophysics.

The basic idea is that meeting the requirements for the habitability of the Earth for observers makes it more likely that solar eclipses are possible. First, the requirement that a terrestrial planet be located within the Circumstellar Habitable Zone (CHZ) of a G spectral type main sequence star sets the range of possible angular sizes that the star would appear in the sky. Second, the presence of a large moon enhances the planet’s habitability in several ways, including the tidal mixing of nutrients from the continents to the oceans, help driving the deep ocean currents, and stabilization of the planet’s rotation axis tilt (leading to a more stable climate). Thus, the players for a solar eclipse are on stage while the audience is watching in their comfortable theater.

Still, the extreme closeness of the angular sizes of the sun and moon seems to require addition tuning. Dave Waltham might have provided the final piece of the puzzle in a study published in Astrobiology in 2004 (and later updated in a 2011 paper in the same journal). He noted just how sensitive the stability of earth’s rotation axis is to the moon’s mass. Had its mass been just a tiny bit larger, earth’s rotation would have been slowed too much from the lunar tides, leading to a less stable rotation axis tilt.

Since there appears to be a physical basis for the solar eclipse coincidences, does this not remove the need for a design explanation? Not at all! It seems surprising on the chance hypothesis that the universe would be setup in a way that the most habitable locations would also be the best places to observe total solar eclipses. But this makes sense on the hypothesis that the universe is designed so that observers can enjoy total solar eclipses.

Thus, solar eclipses became the first example of the Privileged Planet thesis (published in book form in 2004), that the universe is designed for scientific observation and discovery.

Photo credit: National Park Service.