During a total solar eclipse, the moon lines up in front of the sun so perfectly, it would put most posts from Reddit’s r/perfectfit community to shame.

The perfection of the alignment is a big part of an eclipse’s beauty. But why is it so perfect?

Why do the sun and moon line up so perfectly?

This won’t take long to explain because there’s a simple answer: they have almost the exact same angular diameter — a measure in cosmology for determining the size of objects in the sky. If you’re into math, you can calculate angular diameter using a mathematical formula similar to the one used in geometry for calculating circular chord length

You can also estimate angular diameter by stretching your arm all the way out in front of you and holding up one finger. The sun (to be measured only when you’re wearing your eclipse glasses) has an angular diameter of approximately half of your fingertip, meaning 0.5 degrees. The moon’s angular diameter varies a little more than the sun’s but it’s still always approximately half a fingertip, or 0.5 degrees — same as the sun.

There’s yet another, possibly simpler way of describing this, University of Arizona astronomer Chris Impey told Mashable. “The gist of the coincidence is that the sun is 400 times larger than the moon, and 400 times further away,” Impey said.

The path of the total solar eclipse on April 8, 2024.
Credit: NASA Science Visualization Studio

Is there a non-scientific explanation for the eclipse coincidence?

The technical answer is the only answer. There is no reason or deeper significance. It’s a coincidence. It’s a very fortuitous coincidence since the sight of total solar eclipses can be life-altering, and such eclipses have been necessary for some of the most important discoveries in history — such as the famous 1919 Eddington experiment that validated Einstein’s Theory of General Relativity. But they’re nonetheless just coincidences, and from an astronomical bean-counter’s standpoint there’s not much else to say about them.

But that doesn’t mean there’s nothing more to say about the insane good luck that allows us to witness them.

“We can certainly say that in the solar system, it’s unique and there’s nothing even close to it,” Impey said. For instance, Mars’ potato-shaped moon Phobos produces rather clumsy solar eclipses, including in 2024, and NASA’s Perseverance rover photographed it. The site of a lumpy, oblong shadow transforming the sun temporarily into a Muppet’s eye is undeniably cool, but Earth’s eclipses are elite by comparison.

Venus visible as a tiny dot on the sun during the planet's transit.

Venus “eclipsing” the sun, but really just transiting it.
Credit: SDO / NASA

“We can certainly say that in the solar system, it’s unique and there’s nothing even close to it.”

Other objects, like satellites and planes, pass in front of the sun all the time. For that matter, if you want to be annoying about it, you can create a one-person “eclipse” any time you want by putting on a hat. But very rarely are we Earthlings lucky enough to see heavenly bodies other than the moon eclipse the sun. It does happen, however, like when Venus and Mercury transit the sun, a breathtaking and rare event in which a planet is briefly visible as a tiny dot silhouetted against our star. But transits only serve to underscore the mind-blowing harmony of the sun and moon’s convergence during eclipse totality.

Are there total eclipses elsewhere in the universe?

“Probably somewhere out there, there are eclipses,” said Impey. “Planet-planet eclipses? Probably. A planet-moon eclipse like ours could happen too, but we have zero information to go on there,” he continued. The odds of a planet having a sun and moon with the same basic shape and the same angular diameter visible from a solid viewing surface isn’t something that can easily be calculated — there are just too many moving parts involved, and astronomers have not yet definitively discovered any exomoons, so we have to rely on the measurements within our own solar system — a very tiny dataset.

But there are ways of illustrating the coincidence that don’t require us to quantify how rare it is.

“Probably somewhere out there, there are eclipses.”

The diameter of the sun is about 865,370 miles, compared to the moon’s 2,159.1 miles. That’s a big differential: the number of miles three very dependable cars will drive before they stop working entirely, versus the number of miles someone can drive in three days. 

Mashable Light Speed

An amateur astronomer posting on the Nasaspaceflight.com web forum once crunched the numbers and analogized the sizes of the sun and moon to a yoga ball and a lentil. Now picture a grape called “Earth” floating in space. From the surface of this grape, looking out into the void, there’ll come a time when the aforementioned galactic yoga ball will be perfectly covered up by this one single solitary space lentil

And that’s more or less what happens during every total solar eclipse. 

Does the eclipse coincidence unlock a larger secret about what makes Earth unique?

Short answer: probably not.

Earth is overflowing with rare phenomena compared to other planets. It’s pretty wild, after all, that Earth happens to have a pleasant amount of gravity, a livable climate, liquid water, a magnetic field to protect us from solar radiation, and all the other things that make life possible. But that’s not a coincidence. It’s just an observation of the necessary conditions for life, from the standpoint of an alive being, currently here to enjoy them. 

So are the sun and moon’s angular diameters another one of these precursors to a livable planet? We do, after all, need our sun at right about its current orbital distance, or we likely wouldn’t have the temperatures we need to survive, but in some other reality, a cooler, closer sun, or a hotter, more distant sun could probably provide the conditions we need. That’s why astronomers looking for exoplanets beyond our solar system in the “Goldilocks Zone” of habitability are often attracted to planets that orbit much closer to their stars than we do ours.

And as for the moon, while it’s true that its tidal effects are crucial to the lives of, say, mollusks, and the moon’s light is pretty important for, um, dung beetles (yes, really), it’s doubtful the moon’s mere existence in our orbit — let alone its angular diameter — is a necessary precursor for life on Earth. Granted, an entirely moonless Earth is a drastically different counterfactual world, and to suppose that humans might not have evolved in such a reality is far from an outlandish hypothesis. But even if we grant that a moon is necessary for all life, habitability almost certainly has nothing to do with our moon’s specific angular diameter. 

A total solar eclipse viewed in 2017.

A total solar eclipse viewed in 2017.
Credit: NASA / Gopalswamy

In fact, while there is at least one such hypothesis out there about the moon being crucial to the original formation of DNA from Scottish molecular biologist Richard Lathe, this moon-centric Genesis story relies on conditions that are no longer present on Earth anyway. Impey cautions that moon-centric explanations of life “are not compelling arguments,” but they are illustrative, and are worth dwelling on briefly, so please stay with me.

As all mollusks know, the moon’s gravity “pulls up” on bodies of water here on Earth, creating our tides. Lathe theorizes that the DNA molecules that eventually evolved into us humans originally formed in the violent churning of turbulent seas in which the tides were constantly in dramatic, hour-by-hour flux. That’s because the moon was much closer to Earth back in those primordial days, and its gravity would have routinely created vertigo-inducing tsunami-like tides as it orbited. In fact, the moon at one time would have been a vast disc that dominated the sky like one of the UFOs from the movie Independence Day. That means a total solar eclipse back then would have lasted hours and hours, and would have looked like an enormous moon utterly engulfing the puny sun.  

Not just a coincidence of geometry, but also of time

Regardless of whether or not we technically “need” the moon, the fact that the moon’s presence in our sky was once gargantuan leads us to the melancholy fact that the current state of these celestial objects — as with the states of all celestial objects — is transitory. The moon’s angular diameter was once immense because it was closer to us. It was, and still is, moving away from us at a rate of about an inch per year. Its angular diameter will shrink down further and further, and it’ll become smaller and smaller in the sky. 

In other words, the astonishing aesthetic unity of a total solar eclipse on Earth isn’t just a cosmic coincidence; it’s a temporary cosmic coincidence, and you happen to be around to witness and comprehend its unlikelihood — to the extent that anyone can comprehend such a thing.

If the Lathe hypothesis is right, and an enormous lunar disc in the sky was necessary for the start of life on Earth, perhaps the moon at approximately its current size in the sky is a necessary precondition for such a stable ecosystem that we can develop agriculture, math, and eventually the astronomical know-how required to grasp the sheer immensity of our good fortune when we see an eclipse. 

There’s a total solar eclipse somewhere on Earth every year or two, but seeing one is tricky — you have to anticipate the event, and possibly travel to a locale inside the narrow path of totality. As Impey explains, this only underscores the historical coincidence that makes eclipse-viewing a potential hobby. “Until the modern age of travel, an average person would be unlikely to ever see an eclipse in their lifetime in the place they live, or a place they could get to easily,” he said.

“Until the modern age of travel, an average person would be unlikely to ever see an eclipse in their lifetime in the place they live, or a place they could get to easily.”

In a 1980 Rolling Stone interview, the legendary astronomer Carl Sagan brushed off the idea that humanity can ever fully understand the cosmos. “We are bathing in mystery and confusion on many subjects, and I think that will always be our destiny,” Sagan said, and added that our universe, “will always be much richer than our ability to understand it.” 

Perfectly straddling the line between the richness of our universe and our ability to understand it is the phenomenon of a total solar eclipse. Since humans are the only known arbiters of beauty, a solar eclipse right here on Earth might be the most beautiful cosmic coincidence that can — or will — ever be observed anywhere, at any time, in the entire universe. Can scientists explain and quantify the phenomena that give rise to this coincidence? Absolutely. Can they tell you how beautiful that coincidence is and how lucky you are to be here for it? Nope.

You just have to witness that for yourself.

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