Before we discuss the implication of this meme, I’d like to say that there’s something about the top image that seems a bit…off, and I’m not talking about the flatness of the horizon. To be honest, I don’t know what it is, but some aspect of the size and texture of the clouds in that image seems incongruent with the altitude from which the photograph was apparently taken. Now I’m not an expert in photography or meteorology, and the top image could be completely legit. It just rubs me as wrong in a way I can’t quite put my finger on. If any readers have the same impression that I have, and if they can do a better job of saying what’s odd about the image, I’d love to hear about it in the comments.
Anyway, Earth is a spheroid, but it’s a really big spheroid. The diameter of Earth, which was first calculated with surprising accuracy by Erastosthenes more than 22 centuries ago, is about 12,742 kilometers (7,917.5 miles). Compared to the size of Earth, we humans are pretty puny. Small wonder, then, that the curvature of this enormous ball is not visible to us on the ground.
At what altitude should the curvature of Earth become apparent? Actually, that question does not have a simple answer, as it depends on many complex factors. Some people claim to have seen the curvature of Earth from the tops of tall mountains, although optics experts dismiss this as wishful thinking. Photographs showing the same are probably the result of barrel distortions, which can happen when the camera is aimed above or below the horizon.
But just as Globe Earthers are susceptible to perception errors when claiming that they see the curvature from a low altitude, Flat Earthers are also susceptible to errors of reasoning when they claim that no curvature is visible from any altitude. It is theoretically possible, albeit difficult, to detect the curvature from a commercial airliner at 35,000 feet (10.67 kilometers). Complicating the issue is the fact that the horizon is seldom free from obscuring haze or clouds, and even if it is clear, you need a fairly wide field of view to appreciate the subtle curvature. Commercial airline windows simply do not provide a wide enough vista for passengers to perceive Earth’s curvature, which is why most people who have flown in commercial airliners do not remember having seen the curve, if they were looking for it at all.
David K Lynch wrote an excellent short paper in 2008 titled Visually discerning the curvature of the Earth (PDF). It is well worth the few minutes it takes to read it, as Lynch, perhaps without meaning to, answers many Flat Earthers’ complaints about why the curvature of Earth is not apparent in situations where they think it should be.
According to most high-altitude pilots, the curvature of Earth’s horizon becomes obvious at an altitude of about 50,000 feet (15.24 kilometers) and is prominent by the time you reach 60,000 feet (18.29 kilometers). That is well above the height of any mountain, but still within the range of humanity’s highest flying aircraft, to say nothing of our spaceships. (Oh right, the spaceships are all lies!) To put it another way, many, many people have had the opportunity to witness Earth’s curved horizon since the dawn of manned flight. No pilot or passenger who has been sufficiently high enough to see the obviousness of Earth’s curvature has returned to the ground convinced that Earth is flat.
Still, Flat Earthers love to provide photographs and videos, purportedly taken from high-altitude weather balloons, showing a perfectly flat horizon. The irony of this tactic is that if you show them a photo which clearly does depict the curvature of Earth, they dismiss it out of hand as being a fake (in the case of satellite photography) or as the result of lens distortions. How peculiar is it that the phony evidence gate seems to swing in only one direction?
Some Flat Earthers claim that they could be convinced if they were personally taken to an altitude from which Earth’s curvature is evident. That isn’t likely to happen (a fact which Flat Earthers must be aware of). The Concorde, the only commercial airliner which routinely soared to altitudes from which Earth’s curvature was unassailable, is gone, and all other aircraft that routinely cruise those lofty skyways are scientific or military aircraft. And unless a certain Flat Earther is also the CEO of a Fortune 500 company, he isn’t likely to purchase a spaceship ticket anytime soon. It looks like we are doomed to hear Flat Earthers’ perpetual chant – Show me the curvature! – for the foreseeable future.