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This question already has an answer here:

I recently became aware that the flat Earth theory still exists in the 21st century, and has colored the views of a friend of mine. Roughly speaking, the tenets are:

  • The Earth is a flat disk, with the south pole blown up into a circular "ice wall" where one would expect Antarctica to be.

                                        enter image description here

  • The sun and moon are either spheres or disks floating above the earth and moving in a spiral pattern with the seasons.

  • NASA is part of a conspiracy to conceal the truth, and sends us animations and faked photo shoots.

The Flat Earth Theory is a scientific theory, in the sense that it makes falsifiable predictions about the universe that can be seen to match observation (or not). What are some good arguments or backyard experiments that could convince a layman that in fact the Flat Earth Theory is false?

A similar question and useful related reading is What is the simplest way to prove the Earth is round?, but it focuses more on the theoretical and conceptually simple side, applicable to the proverbial "numskull cousin". This question is more focused towards convincing a doubtful scientific person using (preferably low-tech) experiments and observations.

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marked as duplicate by Kyle Kanos, user36790, Floris, Qmechanic♦ Jan 27 '16 at 21:14

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/26427/2451 and links therein. $\endgroup$ – Qmechanic♦ Jan 27 '16 at 17:10
  • $\begingroup$ A note, I do not consider this to be a duplicate. There is a mighty difference between "show the earth is not flat" and "show the earth is round." One requires disproving a hypothesis they have, while the other requires proving a hypothesis you have. Given that flat earthers often rely heavily on skepticism to bolster their argument, "the earth is not flat" is an argument you can make using that skepticism, but you cannot make the "the earth is round" argument using the same skepticism. $\endgroup$ – Cort Ammon Jan 27 '16 at 21:22
  • $\begingroup$ @CortAmmon You are correct, and I chose that particular wording for a reason. Any scientific theory should be disprovable in principle, and the flat earth theory meets this criterion. "The earth is round" is not an argument I care to make, although it is of course the leading alternative hypothesis so disbelieving one will probably lead you to the other. $\endgroup$ – Mario Carneiro Jan 27 '16 at 21:31
  • $\begingroup$ I disagree with the votes to reopen the question. An answer posted in this version would be acceptable in the other and similarly the other way. $\endgroup$ – Kyle Kanos Jan 30 '16 at 22:02
  • $\begingroup$ I'm late to the party, but I disagree with your statement that "The Flat Earth Theory is a scientific theory" since it is literally impossible to falsify. Try it. Any Globe Denier will dismiss any evidence against the flat earth nonsense. You literally cannot prove them wrong. $\endgroup$ – Corey Oct 15 '18 at 10:37

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I live close to Lake Erie and often see scenes like in this picture. Note that the bottom of the cargo ship cannot be seen due to the curvature of the Earth.

enter image description here

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    $\begingroup$ This image is clearly photoshopped by "the man"! :-) $\endgroup$ – CuriousOne Jan 27 '16 at 16:38
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    $\begingroup$ NASA dug a trench into the water to deceive us! $\endgroup$ – pfnuesel Jan 27 '16 at 16:38
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    $\begingroup$ You would think that is convincing evidence, but never underestimate the power of pseudo-scientific arguments when dealing with hard convictions. Related: wiki.tfes.org/Basic_Perspective $\endgroup$ – Nephente Jan 27 '16 at 16:41
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    $\begingroup$ Actually, this is an exceptionally bad example, because the mirroring of the bottom half of the cargo ship indicates that a mirage effect is occurring. In other words, light is curving away from the sea (even under standard physics), so it's no wonder you can't see the hull. $\endgroup$ – Mario Carneiro Jan 27 '16 at 16:51
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    $\begingroup$ Flat Earth Person - "That ship is sinking!" $\endgroup$ – camden_kid Jan 27 '16 at 16:57
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Related to the Andrea di Biagio answer. Here is a typical flight path for the Beunos Aires-Auckland route. The distance is approximately 10,300 km by the shortest route along a sphere. Direct flights are offered by Air New Zealand and take 11h40m - an average speed of 882 km/h.

Looking at the map you present, the distance from Auckland to Buenos Aires looks to be about twice this by the most direct route. Therefore the plane would have to fly supersonically. But Jumbo jets and the like, cannot and do not fly supersonically.

polar route

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    $\begingroup$ That's great. I was trying to find a commercial flight path like this one since my friend mentioned commercial flights, but I couldn't find any paths that weren't contrived between cities in the middle of nowhere. $\endgroup$ – Mario Carneiro Jan 27 '16 at 18:52
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    $\begingroup$ Not really a backyard experiment in my opinion. $\endgroup$ – Alexander Jan 30 '16 at 11:39
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    $\begingroup$ @Alexander Huh? I have wifi in my backyard and was able to find this information in less than 5 minutes. The question asks for arguments or backyard experiments. $\endgroup$ – Rob Jeffries Jan 30 '16 at 13:27
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Foucault Pendulum is a great example.

The original purpose of this experiment was to prove that earth rotates relative to the stars and not the other way around, yet nevertheless it proves that the earth rotates in a way which contradicts the "flat earth" theorem.

This experiment can be easily recreated at home, if you don't live close enough to equator.

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    $\begingroup$ To be more precise: Under the round earth theory, a pendulum at the north pole precesses once every 24 hours, and this gradually increases as you get closer to the equator (with period $24\csc(\theta)$ hours for polar angle $\theta$), and it precesses the other direction in the south. In flat earth, it is a 24 hour period everywhere (or none at all if you don't think the earth rotates). This is a really nice idea! $\endgroup$ – Mario Carneiro Jan 27 '16 at 18:24
  • $\begingroup$ If flat earthers don't understand that the earth is flat, Foucault Pendulum will be si-fi for them. $\endgroup$ – Yashas Jul 19 '17 at 4:18
  • $\begingroup$ Foucault pendulum is a bad backyard experiment: although one can be careful enough to make the suspension symmetric, there'll still be intrinsic precession, which will dwarf the Foucault precession — unless you make the pendulum very long (tens of meters), similarly to the one in the Panthéon. For the discussion of this precession (and attempts to minimize it), see e.g. A Short Foucault Pendulum Free of Ellipsoidal Precession $\endgroup$ – Ruslan Oct 15 '18 at 14:26
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You need three flat Earth theorists on different continents (or otherwise well separated) all in on a (video) conference call, and have them measure the azimuth and elevation angles of the sun at the same time. The set of angles that they report will be inconsistent with their locations being on a flat disk laid out as in the map in the question.

This is a more modern way of doing Eratosthenes's experiment. Here we need three observation points to eliminate a possible objection of the conspiracy theorists: the sun actually isn't all that far away. (Eratosthenes used the fact that the rays from the sun are all parallel -- I'm not giving the flat Earthers that much credit).

This is easy to spell out if you pick the times/places such that

1) For one person the sun is directly overhead,

2) Another person is due north of person 1,

3) Another person is due east (on the same line of latitiude) as person 1.

If you assume that the sun is at the location consistent with observations (1) and (2), person (3) cannot see the sun just being to his/her west given the geographic layout in the original question.

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  • $\begingroup$ I don't think you need observer (2) at all. With the standard flat earth theory, if you can stay exactly on the same parallel between (1) and (3) (accepting that this at least is measured correctly by modern cartographers), then (3) should see the sun with a northward deviation. $\endgroup$ – Mario Carneiro Jan 27 '16 at 18:37
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    $\begingroup$ @MarioCarneiro I see your point, but I also know that there exist situations where a pair of observers could make observations that are compatible with the flat Earth (+near sun) idea, thus specifying 3 observers as the general protocol is safe. $\endgroup$ – Dave Jan 27 '16 at 18:43
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If the sun is a disk (or sphere) that is a certain distance above the earth, close enough that you get it at different angles above the horizon depending on where you stand, then it has to look like a different size to different people. If it looks the same size, then you cannot explain that it's at a different height above the horizon. See this diagram:

enter image description here

If they can explain at what height $h$ the sun is "affixed to the sky above" to give rise to the observation that the angle of the sun changes as you move in the North-South direction, then it immediately follows that the sun must appear to have a different size in the sky. Which a simple sextant measurement will tell you is not true.

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  • $\begingroup$ My worry with this argument is that there is also a competing effect where the moon, say, appears larger when it is close to the horizon (I forget the source of the effect, I think it is light bending through the atmosphere). So a flat earther will probably explain this as a perspective effect. $\endgroup$ – Mario Carneiro Jan 27 '16 at 18:27
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    $\begingroup$ Well - the moon appears larger when compared to objects on the horizon, but when you measure it (with a sextant), it's the same size. The only way in which its apparent size changes is as the sun sets: the thermal gradients in the atmosphere can act as a lens that compresses the sun in the vertical direction - by a small amount that can be calculated. See this image ; Or you can believe this answer - don't. $\endgroup$ – Floris Jan 27 '16 at 18:37
  • $\begingroup$ Original post in which first linked image from previous comment appeared is here - but I ran out of space in the last comment. $\endgroup$ – Floris Jan 27 '16 at 18:38
  • $\begingroup$ This is excellent. The moon and sextant would be very good. I suspect a flat Earther might come back with "I'm not good enough to do the math myself but this book has all the math done correctly that shows Earth is flat." $\endgroup$ – Todd Wilcox Jan 27 '16 at 19:34
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    $\begingroup$ @Floris It's written in the 1800s, so the language is a bit weird, but that's just a long-winded way of saying "it is an optical illusion, so don't just naively trust your eyes here", which I think is a perfectly valid thing (round earthers use the exact same argument quite commonly, for instance with mirages or the sun's distortion near the horizon). $\endgroup$ – Mario Carneiro Jan 27 '16 at 22:08
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Airplanes. Note that dozens of flights cross every ocean and the equator in all directions every day. If the Earth were a disk, it would actually be impossible to do that.

For example, if the border of the disk were more or less where the borders of a planisphere are, it would be impossible to cross the Pacific. How would your friend explain how flights from LA to Tokyo only take 11h if it had to cross the whole of the US, the atlantic, and the whole of Eurasia?

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  • $\begingroup$ In the flat earth theory, the north pole is the center, something like this map. So for flights whose great-circle paths take them northward, there is little deviation from expected. The problem is that there aren't too many flights that angle southward, since no one bothers to go directly from cape horn to cape of good hope, for example. $\endgroup$ – Mario Carneiro Jan 27 '16 at 16:45
  • $\begingroup$ @MarioCarneiro That's actually surprisingly clever of them! Well I guess the difference in airplane flight times could easily be manipulated then, but what about cargo ships? I'll have to think about it! $\endgroup$ – Andrea Jan 27 '16 at 16:48
  • $\begingroup$ @MarioCarneiro so according to flat-earthers, what happens to planes that reach the edge? $\endgroup$ – Ameet Sharma Jan 27 '16 at 16:50
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    $\begingroup$ @AndreaDiBiagio Actually the cargo ship idea might work, i.e. if you tabulated the travel times of ships from Chile or Ecuador to somewhere in east Africa or the middle east (that go straight around the capes) against comparable voyages in the north, you might be able to get a feel for the actual distance (which should be three or four times longer in the flat earth theory). $\endgroup$ – Mario Carneiro Jan 27 '16 at 17:23
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    $\begingroup$ Flying time between Sydney and Santiago is 13 hours and 10 minutes only: qantas.com/travel/airlines/media-releases/mar-2012/5380/global/… $\endgroup$ – Johannes Jan 27 '16 at 18:42
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Can you convince your friend that time zones are for real? If he will believe that it is nighttime in China when it is daytime in the US, then his disk theory can't be correct.

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    $\begingroup$ Assume you'd really want to believe, doesn't that wiki.tfes.org/… sound convincing...? $\endgroup$ – Nephente Jan 27 '16 at 16:46
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    $\begingroup$ I don't think so either, but they claim sunrise- and -set are perspective effects. For a layperson biased towards these ideas, such insane statements sound like real science. $\endgroup$ – Nephente Jan 27 '16 at 17:01
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    $\begingroup$ @James The sun illuminates a circular portion of the world according to what is below it. He actually owns a book "Zetetic Astronomy" by a scientist from the late 1800s that describes the theory in detail (and is frequently cited in the FES page). I will look up the exact argument. $\endgroup$ – Mario Carneiro Jan 27 '16 at 17:03
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    $\begingroup$ Time zones are being created by remotely controlling all the clocks on the planet with the GPS systems. Contrails are also used for that purpose. $\endgroup$ – CuriousOne Jan 27 '16 at 17:19
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    $\begingroup$ @MarioCarneiro: It's called "humor", I believe. In this case it serves a purpose: how do you "prove" that contrails are not controlling clocks? $\endgroup$ – CuriousOne Jan 27 '16 at 17:29
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Take an arrow which you put tangent to the earth (assume it to be a perfect sphere). Now, travel around in such a way that

1) you enclose some finite area,

2) do not rotate the arrow locally.

Coming back to your starting point you will notice the following: Although you did not rotate your arrow locally, you will end up with an arrow which is rotated compared to the one you started with. The reason is: Due to the finite curvature of the sphere, you will gain a geometrical phase. The latter is the angle about which the arrow was rotated.

For more information read about: Parallel Transport

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    $\begingroup$ This is clever, but hardly a "backyard" experiment, given how large an area you should inclose! Plus all the maths! You might as well replicate Eratosthenes' experiment! en.wikipedia.org/wiki/Eratosthenes#/media/… $\endgroup$ – Andrea Jan 27 '16 at 16:51
  • $\begingroup$ Couldn't you get that same behavior using a different metric connection...? ;-) I haven't done the math and maybe I'm missing a basic fact of differential geometry, but it sounds to me like Gauss, Riemann and consorts are in on the conspiracy. $\endgroup$ – Nephente Jan 27 '16 at 17:14
  • $\begingroup$ With a vector ${\bf A}$ and an infinitesimal area $df^{\kappa\xi}$, one finds $\oint \delta A_\nu=-\frac{1}{2}R^\mu_{\nu\kappa\xi}A_\mu\,df^{\kappa\xi}$. Here, $\delta {\bf A}$ is the parallel transport and $R^\mu_{\nu\kappa\xi}$ the curvature tensor. Taking the "wrong" curvature tensor can give you a different result than the experiment! $\endgroup$ – pawel_winzig Jan 27 '16 at 17:28
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    $\begingroup$ Haha, as a mathematician I appreciate this approach, but you would need an awfully sensitive arrow. $\endgroup$ – Mario Carneiro Jan 27 '16 at 17:28
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    $\begingroup$ @AndreaDiBiagio: [Devil's advocate] I don't think Eratosthenes' experiment would help. The difference in shadow angles could be explained on a flat Earth with a relatively nearby Sun. $\endgroup$ – James Jan 27 '16 at 18:11
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If you believe that anyone lives on antarctica at all, or really anywhere south of the antarctic circle, then you can use the "midnight sun" effect, which should not happen in the flat earth theory (because the sun moves around the "antarctica ring" during northern winter). Surveillance cameras will also work for this, if eyewitness accounts are not believable.

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Try https://www.insecam.org/ .Take a flat map of earth and make them go to the cities and watch the light and the online clocks. Nothing but a sphere fits the data.

If the sun is going around a disk there would be night almost simultaneously over the flat disk, whereas the cameras will show progressive changes and night for half of their flat earth.

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  • $\begingroup$ You will have to be more precise about the proposed experiment here. Flat earthers don't think night and day don't happen, but they disagree about the cause. $\endgroup$ – Mario Carneiro Jan 27 '16 at 18:13
  • $\begingroup$ In a flat earth one cannot fit a sun going around a flat disk with the time table that will come up from the cameras. Take the capital cities of the countries near the equator , and near the poles, or longitude lines. Only a sphere will fit. $\endgroup$ – anna v Jan 27 '16 at 18:25
  • $\begingroup$ The flat Earth argument is that the sun is actually small, dim, and very close. As it moves around the disk of the Earth, it moves too far away to provide light to the "night" areas as it moves closer to other areas where it is providing "daylight". $\endgroup$ – Todd Wilcox Jan 27 '16 at 19:25
  • $\begingroup$ @ToddWilcox Then these people cannot add two and two and not worth the trouble, if they follow the cameras and are still talking of such a sun. $\endgroup$ – anna v Jan 27 '16 at 19:43
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    $\begingroup$ @annav I think they are not worth the trouble and that this question would be best migrated to Philosophy, since I can't see anything from Physics having any sway over a flat Earther. Anyone who has any trust and understanding in Physics would not be a flat Earther in the first place. $\endgroup$ – Todd Wilcox Jan 27 '16 at 19:46
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Have them pull out their smart phone and open up Google maps and touch the icon to zoom to their current location. Remind them that what they just did is only made possible by satellites in orbit using clocks specially tuned according to general relativity to maintain accurate synchronization with clocks on the surface (i.e., in the smart phone). Welcome them to believe in a flat Earth as long as they remember they are lucky that everyone involved in making GPS work believes it's an oblate spheroid.

Have them travel to a dark spot in the northern hemisphere and observe and partially catalog the night sky. They should make notes on the big and little dipper, Orion, Cassiopea, and other major constellations. Have them travel to a dark spot in the southern hemisphere and repeat the process, noting that none of the familar constellations are visible but all new ones like the Southern Cross are visible. Have them return to the north one more time to verify that it is their travel that made the sky look different. Have them attempt to explain this with a flat Earth.

Many refutations on the Flat Earth Wiki rely on the "vanishing point" of perspective. The assertion is that things don't disappear from sight because of a curved Earth, but because of them "sinking to the vanishing point". Commision a helicopter ride on a calm day. As the helicopter rises, point out that while perspective makes the helipad seem smaller with distance, it doesn't move laterally through the field of view.

Do they really think that a secret known to more than 500 people can be kept for more than 50 years? https://en.wikipedia.org/wiki/List_of_space_travelers_by_name

If the Earth is flat, the entire American space program was/is fake. If that's the case, why would the Russians, Chinese, Iranians, North Koreans, etc, continue to say nothing about the American space program being fake? Why would they all strive to create their own space programs and ballistic missile systems? Perhaps some nations, like Russia, might also have a vested interest in the conspiracy, but not Iran. Not North Korea. Not Venezuela.

In history, people who have made demonstrably true realizations and discoveries that are inconvenient or contrary to those in power have been persecuted. Why are no flat Earthers persecuted? Why does the Catholic Church not support flat Earthers? Why does everyone pretty much ignore them?

I think a flat Earther might have said it best:

...even cults seem to all be ball earth.

Source

Go with your friend and re-create all of the experiments from the seminal flat Earth book, starting here: http://www.sacred-texts.com/earth/za/za06.htm and using this math for calculating what should be visible: https://en.wikipedia.org/wiki/Horizon

I'm pretty sure your friend will prove to themselves that the alleged results of these experiments from Earth Not a Globe are actually falsely reported.

Alternatively (as in experiment 7) the alleged expected result for a round Earth from the experiment is falsely reported. For example, the author wrote that if the Earth were round, the horizon should seem to curve downwards left-to-right. Anyone considering a globe for only a few minutes could see that makes no sense. Suppose the round Earth expectation were true. Would not the necessarily fake Moon landing footage have showed such a curvature? Especially since the wrong-headed thinking that creates the expectation of curvature would further suggest the curvature expected would be greater for a smaller round body like the Moon. Yet the curvature was not faked in moon photos for the simple reason that we should not see curvature of the horizon on a round body as suggested by experiment 7. See: http://science.nationalgeographic.com/science/photos/moon-exploration-gallery/#/tranquility-base_1255_600x450.jpg

Really, just reading this book should be enough to do it. Chapter III begins in especially hilarious fashion by supposedly using science to prove the Earth does not rotate while instead actually showing the author knows nothing of science: http://www.sacred-texts.com/earth/za/za21.htm

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    $\begingroup$ That book... The person can't use the excuse that they don't know that stuff moves in parabolas, because they actually explained why it wasn't a parabola. And the person stated their theory, and tested it in a way which could work with both theories, but didn't actually test whether their theory was right by moving the cannon. :roll: $\endgroup$ – wizzwizz4 Jan 27 '16 at 21:23
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    $\begingroup$ The language of this post is not entirely objective, which tends to exacerbate matters. It's great if you can show that some of the experiments in the book are falsely reported (and a specific example would be nice here), but ridiculing the theory only weakens your position. Also, "arguments by popularity" are clearly not useful, since both sides are aware of this and it's not relevant. $\endgroup$ – Mario Carneiro Jan 27 '16 at 22:18
  • $\begingroup$ What exactly shows that the author "knows nothing of science" in sacred-texts.com/earth/za/za21.htm ? The page is long and I'm not sure what I am looking for. $\endgroup$ – Mario Carneiro Jan 27 '16 at 22:27
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    $\begingroup$ @MarioCarneiro First paragraph: momentum is not a force. If he's using some other definition of "force", then he can't use the same word to describe gravity and momentum. Fourth paragraph: forces are not "expended". The assertion that a ball thrown upwards on a moving ship will travel diagonally upwards along with the ship and then straight down landing behind the ship is ludicrous and can be proven false simply by trying it out. The ship traveling and the Earth rotating cannot be directly compared, because the air moves along with the Earth but not with the ship. And so on. $\endgroup$ – Todd Wilcox Jan 27 '16 at 23:41

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