Like lots of people, I’m planning to admire the total eclipse next month.
I’ll be heading north, into upstate New York. The path of totality will travel along an arc from Niagara Falls to Plattsburgh. I’m hoping to catch it near the Vermont state line. I’ve got the special protective eyewear, I’ve made hotel reservations, and I’m planning to leave well in advance to beat the traffic. Now I’m just hoping Mother Nature cooperates.
All us eclipse-chasers are at the mercy of the weather. If it’s cloudy that day, the whole trip will be for nothing. But it’s a gamble I’m willing to take for an almost once-in-a-lifetime opportunity.
Total eclipses aren’t all that rare, per se. One can be seen from somewhere on Earth about every eighteen months. However, this is the last one that will be visible from my neck of the woods for decades. If I don’t get to see it, it’s going to be a much bigger and more expensive proposition to try to catch another one a few years down the line.
What “rare Earth” gets right
Creationists love talking about the “rare Earth” idea: the argument that Earth is specially and uniquely fine-tuned to support life. It orbits in the habitable zone, not too close or too far from the sun, which is a stable star without massive flares. We have a regular day-night cycle, a mostly stable axial tilt, a magnetic field that screens out cosmic radiation, and so on. The creationists claim that this is evidence of God’s special favor.
The fallacy of the rare-Earth argument is that it’s an inference based on incomplete data. Just as you can’t compute the probability of a particular hand of cards unless you know what’s in the deck, we have no basis for proclaiming how common Earthlike planets are. Our sample size is too limited (although it’s growing all the time).
We also don’t know if life can exist on planets that are unlike Earth. We have no idea whether life could thrive with different chemistries, or under different conditions, than we’re used to. Could there be silicon-based life in the mantle of superhot planets, or aquatic life on ice worlds that uses ammonia or methane instead of water, or hydrogen-powered life soaring through the skies of gas-giant planets, or plasma-based life drifting in the coronas of stars? It might turn out that an Earthlike planet is one way, but not the only or even the best way, to support a biosphere.
However, the rare-Earth argument seems to be correct on just this one point. By – as far as we know – a complete coincidence, the Moon is 400 times smaller than the Sun, but also 400 times closer to Earth. The numbers cancel out perfectly for the Moon to perfectly block the Sun’s disc but not its corona, giving rise to the spectacular sight of fiery streamers haloing a disc of black shadow.
A unique moment in cosmic time
If the numbers didn’t mesh so precisely, we wouldn’t get to enjoy this spectacle. The Moon either wouldn’t block the Sun’s entire disc (so we’d only see partial and annular eclipses, not total ones), or it would block the whole Sun, including the corona.
And in fact, it’s only at this moment in cosmic history that we can appreciate this sight – because the Moon is slowly moving further away from the Earth. In 600 million years, give or take, there won’t be total eclipses anymore. If there are still humans (or other intelligent beings) around by then, their view of the sky will be poorer for it.
Of course, this doesn’t help the creationists’ fine-tuning argument, because total eclipses aren’t necessary for life. They just look cool. Still, this kind of favorable coincidence must be rare. There’s a real sense in which the Earth, in this era, may indeed be uniquely privileged among planets in the galaxy.
If we ever make contact with extraterrestrial life, I like to imagine, Earth could be a major tourist draw for this reason. The prospect of seeing a total eclipse is something special, a spectacle that almost no other planet can offer. If you think Airbnb prices are out of control now, just wait!