One year ago today, I posted a Challenge to Flat Earthers on this blog. I proposed an experiment that could photograph the curvature of the Earth directly without having to worry about camera distortions, which is what Flat Earthers usually point to to explain away such images. If you hold a ruler against the horizon in view of the camera, it will give you an absolute standard for what is straight. With a ruler to measure against, you don’t have to be at the edge of space, but only up in an airplane to see a little bit of curvature in the horizon.

I was going to put my proverbial money where my mouth is and do this experiment myself the next time I flew, but then 2020 hit, so I haven’t flown since then–nor have I have any takers on the challenge.

However, this is about a different topic. I admitted in that post that there is one other thing that could cause an apparent curvature in the horizon: refraction. Refraction is the bending of light through…anything, really, but in this case, it’s mostly to do with air. In various weather conditions, air can bend light so that distant objects appear higher or lower than they should be. This is the basis for a various phenomena that are colloquially called mirages.^[1] Refraction can also make the horizon appear lower than it really is, and thus the edge of a flat disk would look more curved that it should be.

More generally, one thing Flat Earthers like to say, possibly the clearest^[2] positive evidence to support their claims,^[3] is that they can take photographs that show distant objects that should be behind the horizon.

Take this photo of the Chicago skyline, for example. It taken from the other side of Lake Michigan, 60 miles (100 km) away. If you do the math, the skyline should be behind the horizon on a round Earth. What’s going on?

The answer to that is, of course, refraction. Light from Chicago bends around the curve of the Earth to reach the other side of the lake. Flat Earthers may say that’s just a cop-out, but there’s a problem with that: Flat Earthers also invoke refraction.

The most obvious problem with the Flat Earth idea is that it doesn’t explain why the Sun sets. Some Flat Earthers invoke perspective and vanishing points to say that the Sun (which they believe is small and close to Earth’s surface) gets too far away to see. But that doesn’t make sense. The Sun is extremely bright, and even if it gets far away, it should never actually go below the horizon in this model.

Edit: some Flat Earthers also believe that the Sun is like a spotlight that doesn’t illuminate the whole disk. This would allow it to disappear, but it still wouldn’t actually go below the horizon.

To solve this, some Flat Earthers say the Sun appears to go below the horizon because of refraction. The light rays bend up and go over our heads so that we can’t see them.

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And here is where I found the rebuttal to the refraction argument. I considered another challenge involving repeating the photograph in different seasons and weather conditions to show how refraction changes over time. Or one involving photos taken over longer stretches of water. But the fact that Flat Earthers also invoke refraction complicates this. I couldn’t think of a good way to prove that a particular photo could only be taken on a Round Earth (or, counterfactually, that it couldn’t be taken on a Round Earth).

Instead, there’s a simpler solution: Flat Earthers invoke refraction in the opposite direction from the Round Earth model, and this goes against the laws of physics as we understand them.

Here’s how it works. On a Round Earth, sunlight is bent down around the curve of the Earth.^[4] This allows some of it to reach Earth’s surface behind where the horizon should be, but light coming in higher up doesn’t keep curving. It misses Earth completely and keeps going.

But on a Flat Earth, to get the same effect, the sunlight has to curve up. Light from the small, nearby sun bends away from the ground and up into the sky. Much of it is coming in too steep and hits the ground anyway. In those areas, it’s daytime. Light coming in at a shallow angle curves enough to miss the ground and goes up over the heads of anyone who’s farther away, creating “night.”

This would seem to explain the sunset on a Flat Earth, but it’s missing one critical piece of the puzzle: why does the light bend up?

The amount that air bends light depends on its density. This is something that’s observable in a laboratory, so it’s not “just a theory.” This means at lower air pressures, the light bends less, and on Earth, air pressure decreases as you go up in altitude.^[5] We may not agree on why it does (many Flat Earthers don’t believe in the vacuum of space), but we know it does because we can measure it from airplanes.

Thus, light coming in at an angle from a high altitude (as it does in both models) will first encounter thin air that causes it to bend slightly toward the ground. Then, it will encounter denser air and bend more, which leads it into even denser air, and so on. In other words, we expect the light to curve down. And in fact, this is what we see on the Round Earth. In most weather conditions, at sunset, the Sun appears half a degree higher than it really is. The light bends down, so it’s coming in to our eyes at a steeper angle and appears to come from a higher location.

But on a Flat Earth, we need the sunlight to curve up in all weather conditions in order to produce night. This is not supported by the physics because the upper atmosphere would have to be denser than the lower atmosphere (in violation of buoyancy, another buzzword Flat Earthers like).

This is why “refraction” is not an adequate rebuttal to my original Flat Earth Challenge. (And the challenge is still open. If anyone has to opportunity to photograph the curvature of the Earth from an airplane, leave a comment.)

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^[1] Technically, a “mirage” refers specifically to a duplicate image, but it can be any kind of distortion in colloquial parlance.

^[2] Clearest, not truest.

^[3] As opposed to their alleged debunking of round Earth models.

^[4] Or more precisely, towards Earth’s surface, not towards the bottom of the picture.

^[5] It also gets colder, and cold air is denser, bending light more. Near the surface, this is what causes mirages, and light can indeed bend up. But on the scale of the whole atmosphere, which gets much, much thinner at high altitudes, pressure wins.