Were it not for the Moon's tilted orbit, solar eclipses would occur at every New Moon, and be routine events. As it is, however, their frequency at any particular location is spectacularly irregular. The rhythms of many astronomy-related phenomena — the rising and setting of the Sun, the ebb and flow of the tides, the changing of the seasons — are hard-wired into nature. By contrast, solar eclipses are seemingly random events; they come, literally, out of a clear blue sky.
For fun, we can ask two questions. First, are the kind of solar eclipses we experience on Earth rare within the universe? Second, have solar eclipses had any impact on the development of nature?
In a forthcoming paper in Proceedings of the International Astronomical Union No. 367, I suggest the answer to both questions is maybe. And that raises a third question: What are the implications?
Coincidence?
Let's consider the first question. Fernando Ballesteros, an astronomer at the University of Valencia, points out the difficulty of determining whether something is a coincidence or not. He gives the example of the Titius-Bode law, which says that each planet is about twice as far from the Sun as the one before it. The law successfully predicted the discovery of Uranus and Ceres, but it broke down when Neptune was discovered in 1846.
"The fact there was this exception led us to think it was simply a numerical coincidence," says Ballesteros. "The debate was sterile for decades until the discovery of exoplanets allowed us to investigate whether the law is present in other planetary systems. The answer turned out to be that it was, although with other numerical factors, and therefore was not a coincidence."
So it might be that the remarkable geometry of the Earth-Moon-Sun system is simply the way things often are for rocky planets in the universe. Still, Ballesteros is not so sure — particularly as the arrangement of the three bodies is not fixed. Billions of years ago, the Moon was much closer, and thus larger in the sky, and it continues to drift away from the Earth today (at about 1.5 inches [3.8 centimeters] per year). Furthermore, the Sun is, very slowly, expanding, and this growth will accelerate in a few billion years' time.
In other words, the perfect solar eclipses we witness on Earth are — on a cosmic scale — available for a limited time only. "Therefore my intuition tells me it is a simple coincidence, although a very amazing one," says Ballesteros. "But who knows?"
The Dawn of Man
Our second question is whether these eclipses have had an effect on the development of nature. The quick answer is no. Solar eclipses cause short-term changes in levels of light, temperature, and humidity, and these can trigger striking reactions from plants and animals. But the environment quickly returns to its normal state at the end of totality or annularity.
Yet solar eclipses have a profound and lasting impact on humans. "I doubt if the effect of witnessing a total solar eclipse ever quite passes away," wrote Mabel Loomis Todd, an author and world traveler, in 1894. Francis Baily, the astronomer who gave his name to Baily's beads, expressed similar thoughts in a paper on the total solar eclipse of July 1842. "I can readily imagine that uncivilized nations may occasionally have become alarmed and terrified at such an object." (In fact, ancient myths around the world often tell of angry gods punishing humanity or hungry animals devouring the Sun.)
In my paper, I take Baily's speculation one step further, and wonder whether solar eclipses could have influenced the evolution of human cognition.
In general, our species developed in an environment of rhythmic regularity — even earthquakes and volcanoes are routine events in the places where they tend to happen. For evolution, however, regularity can be a trap. If novelty within the environment had remained below a certain threshold, humans might never have acquired a defining characteristic of our species: curiosity.
Solar eclipses, on the other hand, could have pumped novelty into the environment at a near optimal rate. If they occurred more routinely, they would have been less terrifying; if they occurred less frequently, there would not have been enough opportunities for them to have helped humans reach the watershed stage where we started to seek explanations for why things happen.
We began by telling stories of dragons eating the Sun. But, from an evolutionary perspective, once that watershed moment was reached, it was a relatively small leap to placing laser ranging retroreflectors on the surface of the Moon to measure its distance from the Earth within millimeters.
If this sounds like the famous scene transition in 2001: A Space Odyssey — from the spinning bone to the orbiting spacecraft — that's because science-fiction has a habit of getting there first when it comes to highly speculative ideas. In Stanley Kubrick and Arthur C. Clarke's version of human evolution, the development of curiosity was triggered by the monolith.