The eclipse of 2024: why are we lucky enough to have these on Earth?

A total solar eclipse showing the "diamond ring" effect

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!


  1. OverlappingMagisteria says

    Though if space travel is cheap, you could see an eclipse whenever you like. You just have to travel to the right spot to put the moon between you and the Sun. Currently, we have to wait for that spot to happen to come here to Earth, but you could chase it around instead. Not sure if an eclipse is better or worse without an atmosphere.

  2. moarscienceplz says

    ” we have no basis for proclaiming how common Earthlike planets are. ”
    It doesn’t matter. Even if M-class planets (to get all Star Trekkie for a moment) numbered exactly 1 in the universe, it still wouldn’t prove Earth was created for our benefit. The mildew in my shower stall might worship me as their loving provider, but in reality the shower was not built for them, I don’t want them there, and I intend to exterminate them when I get around to it.

  3. Prax says

    @moarscienceplz #2,

    And we will never know the number of Earthlike planets in the universe, because we don’t know how far the universe extends beyond the observable horizon, and we don’t know whether there’s a multiverse or how big it is.

    And none of it bears on whether there’s a god, because nobody knows how many Earthlike planets a hypothetical god wants or how it would arrange to produce them. Maybe it just wants one such planet because Earth is all that really matters and the rest of the universe is background scenery. Maybe it wants eleventy-zillion Earthlike planets so life can evolve all over the place. Maybe it doesn’t care, because it’s a god and it can make hamsters thrive inside the core of the Sun if that suits its purposes.

    As usual, the hypothesis of an omni-God is scientifically useless because it has no testable implications.

  4. sonofrojblake says

    If we ever make contact with extraterrestrial life, I like to imagine, Earth could be a major tourist draw for this reason

    It is a major plot point in the sf novel “Transition”, by Iain Banks (no M in the UK editions, bafflingly, although if you’re unfortunate enough to be in the US you do get the “M”), that they are already here, and for precisely that reason. While this is possibly not the only planet in the galaxy that can support life, and possibly not the only planet in the galaxy that can support intelligent life, it is pretty close to certain that it’s the only such planet in the galaxy where you can, from the surface, observe a total eclipse like this. Thus if you’re looking for aliens visiting earth, pay attention to blacked out cars or confusingly-chartered yachts in the path of totality, and wonder who the occupants are and how far they’ve come…

  5. lpetrich says

    We also came along just in time, because in about a billion years, there will be no total eclipses, only annular ones, where there is a ring of uneclipsed Sun surrounding the Moon. Annular eclipses now happen around as often as total ones, with some eclipses being hybrid: annular at their beginnings and ends and total in the middle. I’ll call them central eclipses for convenience.

    Why sometimes total and sometimes annular? Because the orbits of the Earth and the Moon are not quite circular. The Moon’s orbit has an average eccentricity of 0.0549, while the Earth’s orbit has an eccentricity of 0.0167 at this time, and over timescales of 50,000 – 200,000 years, it varies between less than that and as high as 0.058, with a rough average of 0.03.

    Why no more eclipses after a billion years? The Earth pulls the Moon’s tides forward, pulling the Moon outward. It is currently receding at 3.78 cm/yr or 1.49 in/yr.

    As the Moon goes farther and farther away, central eclipses will be more and more likely annular ones and less and less likely total ones, until the only time of a total eclipse is when the Moon is at its farthest, the Earth at its closest, and the Earth’s eccentricity at its largest. After that, no more.

    This also means that earlier in our planet’s history, all central eclipses were total. The first annular eclipse was thus around 500 – 750 million years ago, around the base of the Cambrian Period.

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