



You also have to somehow get the radiation of the sun to be directional. That’s many more ad hoc assumption. But even with those assumptions, the size of the sun will change during the day more than we observe. And no one has ever successfully predicted solar eclipses on a flat earth, or calculated the observed motions of the planets.



The bottom line is: it’s not easy to improve on today’s scientific standard. It was for good reasons that the hypothesis of a flat earth was abandoned more than two thousand years ago.



Some people suggested to me that flat earthers do not actually believe the earth is flat, they are just mocking people who take scientific evidence on trust. And that, let us to be honest, is something we all do to some extent every now and then. And it is probably the case that some flat earthers are indeed just pretending. But I find it exceedingly implausible they are all just faking it.





To begin with, they would all have to be excellent actors. Just look at some of the videos on YouTube. Also, they’re putting quite some time and, in some cases, money behind their conviction. And that’s while most of them full well know coming out as flat earther will make others doubt their sanity. All that makes it unlikely they are just in for the fun.



Now, you may want to discard flat earthers as conspiracy theorists, which some fraction of them arguably are. But I think that would be somewhat unfair to most of them. To understand why, it helps to have a look at the history of the flat earth society.



The flat earth society goes back to an Englishman by name Samuel Rowbotham, who lived in the 19th century. He was a medical doctor who believed he had proved that the earth is flat and then complained for the rest of his life that the supposed scientific authorities ignored him. He referred to his methodology as “Zeteticism” after the Greek word zeteo, “to seek”.





“The world looks flat, the bottoms of clouds are flat, the movement of the Sun; these are all examples of your senses telling you that we do not live on a spherical heliocentric world. This is using what’s called an empirical approach, or an approach that relies on information from your senses. “ That flat earthers insist on evidence from your own senses only really is key to understanding their problem; I will come back to this. But first, let me tell you the rest of their history.



After Rowbotham’s death in 1884, the flat earth idea was carried forward by another British guy, Samuel Shenton, who once explained to a journalist: “No man knows the ultimate shape of the earth, but that portion we life on is definitely flat. No one will ever know what the whole complexity is like, I suppose, because it goes beyond his sphere of observation, investigation and comprehension.” Again, note the emphasis on personally collected evidence. In 1954, Shenton created the International Flat Earth Society. Few people cared. He died in 1971.



After his death, the Flat Earth Society was taken over by the US-American Charles Johnson. But even after the advent of the internet, flat earthers did not attract much attention. Johnson died in 2001, at which point the flat Earth society had 3500 or so members. The job then fell to another American, Daniel Shenton, who is not related to the earlier Shenton but whose logic falls right in line. He said in “I don't think there is solid proof. I'm not intentionally being stubborn about it, but I feel our senses tell us these things, and it would take an extraordinarily level of evidence to counteract those. How many people have actually investigated it? Have you?” Shenton had the idea to set up a wiki page for the flat earth community. Still no one cared. But in 2016, everything changed.



What happened in 2016 is that a few devout flat earthers put up videos, here, on YouTube. And that really got things going, by way of recruiting new believers. These videos have meanwhile been watched by millions of people. And that had consequences: By “Zeteticism” he meant an extreme version of the philosophy of empiricism. Rowbotham’s philosophy, which is still the philosophy of flat earthers today, is that if you want to understand nature, you should only rely on information from your own senses. You can for example read on the website of the flat earth society:That flat earthers insist on evidence from your own senses only really is key to understanding their problem; I will come back to this. But first, let me tell you the rest of their history.After Rowbotham’s death in 1884, the flat earth idea was carried forward by another British guy, Samuel Shenton, who once explained to a journalist:Again, note the emphasis on personally collected evidence. In 1954, Shenton created the International Flat Earth Society. Few people cared. He died in 1971.After his death, the Flat Earth Society was taken over by the US-American Charles Johnson. But even after the advent of the internet, flat earthers did not attract much attention. Johnson died in 2001, at which point the flat Earth society had 3500 or so members. The job then fell to another American, Daniel Shenton, who is not related to the earlier Shenton but whose logic falls right in line. He said in an interview with the Guardian in 2010 Shenton had the idea to set up a wiki page for the flat earth community. Still no one cared. But in 2016, everything changed.What happened in 2016 is that a few devout flat earthers put up videos, here, on YouTube. And that really got things going, by way of recruiting new believers. These videos have meanwhile been watched by millions of people. And that had consequences: In a 2018 poll in the United States, two percent of the respondents said they believe the earth is flat, while another 5 percent are not quite sure.





Reliable numbers are hard to come by, but we are meanwhile probably talking about more than ten-thousand people in the developed world who reject science that was settled by the middle ages. Let that sink in for a moment.



How does someone end up rejecting something as scientifically well-established as the fact that the earth is round?





“Like most of the speakers at the event CNN spoke to, he was convinced after he decided he couldn’t prove the Earth’s roundness.” I want to leave aside here that, of course, you cannot strictly speaking prove any empirical fact; you can only prove mathematical identities, so more precisely we should speak of seeking evidence that disfavors the hypothesis that the earth is flat. Of which there is plenty, starting with the historical evidence about how stellar constellations shift if you travel, how the length of shadows changes, to Newton’s 1/R2 force law that is the law for a sphere, not a disk, not to mention Einstein and gravitational redshift and the perihelion precession of mercury, and so on, and so forth.



The problem that flat earthers have is that they cannot do most of these observations themselves. So if you buy the idea that it’s only your personally collected evidence that you should accept, then it seems you cannot refute the idea that the earth is round, and so flat earthers philosophy forbids them to accept scientific fact. There is not only one reason, of course. Some flat earthers find the idea is appealing for religious reasons, others are of the crowd who think NASA is evil, space a fake, and the moon landing didn’t happen. But mostly it’s because they think they are merely being rational skeptics. They have not themselves been able to prove the earth is round, so they believe they are only reasonable when they request evidence. CNN for example reports from a flat earth conference: I want to leave aside here that, of course, you cannot strictly speaking prove any empirical fact; you can only prove mathematical identities, so more precisely we should speak of seeking evidence that disfavors the hypothesis that the earth is flat. Of which there is plenty, starting with the historical evidence about how stellar constellations shift if you travel, how the length of shadows changes, to Newton’s 1/force law that is the law for a sphere, not a disk, not to mention Einstein and gravitational redshift and the perihelion precession of mercury, and so on, and so forth.The problem that flat earthers have is that they cannot do most of these observations themselves. So if you buy the idea that it’s only your personally collected evidence that you should accept, then it seems you cannot refute the idea that the earth is round, and so flat earthers philosophy forbids them to accept scientific fact.





Needless to say, if you want to hold on to your convictions it helps if you refuse to do observations that could speak against them. There are actually many ways to convince yourself that the earth is round which are not that technically difficult. Buy a telescope and try to explain the motions of the moons of Jupiter, for example.



So what’s wrong with flat earther’s attitude? Isn’t it asking for evidence exactly what rational thinkers should do? Sure, evidence is key to scientific progress, but flat earthers’ philosophical approach by which they reject certain types of evidence is inconsistent and, ultimately, logically wrong.





See, the only evidence anyone ever has of anything is evidence you collect with your own senses. Except, as Descartes pointed out, evidence of your own ability to think, but this is not relevant here. Relevant is that the distinction which flat earthers are trying to draw between different types of evidence does not exist.



All evidence you have is sensory input. If you hear an explanation of someone else’s experiment, if you read a paper laying out someone else’s argument, that’s your own sensory input. A distinction which does exist, however, is that some of our sensory input requires very little decoding, while some requires a lot. Flat earthers’ problem is that they refuse to decode difficult sensory input.



A good example for the need to decode sensory input by conscious thought are optical illusions. Your brain tries to interpret visual input in ways that sometimes gives a misleading result as in this example. You almost certainly think square A is darker than square B. It is not.





To understand your sensory input correctly you need to draw on other information, in this case your knowledge about optical illusions. Your brain interprets this image as if it was a natural, 3-dimensional scene, and therefore calculates back to the original color of the squares taking into account what appears to be a shadow. This is the wrong interpretation if you want to know the actual color of pixels on the screen. The lesson is, if you do not think about your sensory input, if you do not properly decode it, you arrive at a wrong conclusion.



Flat earthers similarly arrive at the wrong conclusion by failing to decode evidence, indeed by simply ignoring a lot of evidence that their own senses deliver. This is evidence about how society and science works.



Whether we are scientists or not, we all constantly use this evidence to navigate life. And I am sure flat earthers are no exception. Just consider going to the supermarket and buying canned soup. Do you have evidence that what’s in the can is edible? Probably not. For one, the can’s closed. And if you are anything like me, you probably have no idea how or where or by whom it was produced. Why then are you not afraid of eating canned soup? Isn’t this entirely irrational?



No, because you do have evidence that canned soup is edible. You know how the legal system in your country works, you know that there are regulations on what can be sold as food in a supermarket, you know that if what’s in that can was harmful to you, then a lot of people along the food chain would be punished for their mistake, and they don’t want that. Your trust in canned food is an entirely reasonable inference from evidence, evidence that you collected with your own sense, because what else could you possibly have collected it with?



Now let’s come back to flat earthers. Most of you don’t have a physics degree and chances are that after learning in school how we know the earth is round you didn’t think much about it ever again. By and large you are probably confident it’s correct because what you learned in school was plausible, and you know it is widely taught to children, and you know that your government strives to give children in your country a scientifically accurate education. So you have good reason to think the knowledge you were taught is backed by solid scientific evidence.



There is no appealing to authority here. You have totally yourself collected all this evidence about how society works. You have also yourself collected lots of evidence that science works. Any airplane, any laptop, any pair of glasses is evidence that science works. It’s evidence that the system works. It’s evidence for how the whole world works.



So, if you cannot recall just what experiments demonstrate that the earth is not flat, or if you cannot immediately figure out what’s wrong with flat earther’s arguments, there’s no shame in rejecting their claims, because your rejection is based on evidence, evidence that science works.



What’s wrong is that flat earthers’ claim they are leading a scientific argument. But there is no scientific argument about whether the earth is flat. This argument was settled long ago. Instead, flat earthers’ argument is about whether you should trust evidence that other people have collected before you. And it’s an important argument because this trust is essential for society and science to progress. The only alternative we have is that each and every one of us has to start over from scratch with birth. You see, flat earthers would eventually figure out the earth is round. But it might take them a thousand years until they’ve reinvented modern science.



This is why I think scientists should take flat earthers’ philosophical problem seriously. It’s a problem that any scientifically advanced society must address. It is not possible for each and every one of us to redo all experiments in the history of science. It therefore becomes increasingly important that scientists provide evidence for how science works, so that people who cannot follow the research itself can instead rely on evidence that the system produces correct and useful descriptions of nature.



To me, therefore, flat earthers, are a warning sign that scientists should take seriously. The more difficult scientific experiments and arguments are to follow for non-experts, the more care we must take to explain how we lead those arguments.

I, as many people in science communication, am fascinated with flat earthers. Here you have a group of people steadfastly rejecting evidence that’s right in their face. Today, I want to tell you why I nevertheless think flat earthers are neither stupid nor anti-scientific. Most of them, anyway. More importantly, I also want to explain why you should not be embarrassed if you can’t remember how we know that the earth is round.But first I have to tell you what flat earthers actually believe and how they got there. The most popular flat earth model is that of a disk where the North pole is in the middle and the south pole is an ice wall on the edge of the disk. But not all flat earthers sign up to this. An alternative is the so-called bipolar model where both poles are on the disk, surrounded by water that’s held by a rim of something, maybe ice or rocks. And a minority of flat earthers believe that earth is really an infinite plane.They mostly agree though that gravity does not exist, and that the observations we normally attribute to gravity come instead from the upward acceleration of the flat earth. As a consequence, the apparent gravitational acceleration is the same everywhere on earth. I explained last week that this is in conflict with evidence – we know that the gravitational acceleration is most definitely not the same everywhere on earth.The idea that gravity is due to upward acceleration also causes other problems. For example, you have to assume that the moon and the sun accelerate along with the flat earth so we don’t just run into them. That’s an ad-hoc assumption which disfavors the flat earth hypothesis against models where the orbits of the moon and the sun can be calculated from the gravitational law.But that’s not the only problem. You also have to get the moon and the sun to somehow circle around over the disk to explain day and night and the phases of the moon. To get the day-night cycle to be noticeable, you have to shrink the sun and move it closer to the earth.