Are you sure the earth is round?


If you had to persuade a medieval peasant that the world was round, how would you do it? Why do you believe the world is round? And what does the American public in general think?

One of the hardest tasks I encountered as a professor was getting my students to recognize that all of their convictions – even assumptions as basic as “the world is round” or “the sun will come up tomorrow” – are built on a lifetime of accumulated experience. Sometimes the experience is direct: we’ve all seen the sun come up. But sometimes it’s not. We often underestimate how little direct evidence we have for our beliefs, and how much depends on trusted sources like parents, teachers, books, and (gasp) television.

San Francisco’s Exploratorium has just launched a really cool new web application, my evidence, which lets visitors explore the evidence behind their beliefs graphically, making a kind of concept map of the type of evidence, age at which it was acquired, and source. You can then compare your beliefs and evidence to those of other people, and clearly see similarities or differences in how you construct knowledge!

It’s interesting to look, for example, at a word cloud for “Humans cause global warming,” filtered by scientist contributors vs. non-scientist contributors. The evidence cited by non-scientists includes words like year, climate, more, human, scientist, people, and snow. For scientists, human, climate, carbon, dioxide, effect, and cycle are top words.

I’ve answered the round Earth question, so you’ll see “bioephemera” listed as a participant and can filter and read my responses. I found it hard to separate out the evidence for a round earth from the evidence for a spinning earth, or stuff about orbits, so some of my evidence is not dead on. And I could quibble with some of the semantics in the application interface.* But it was actually strangely fun to bang my head against the wall trying to remember why I’m convinced the earth is round. In the process, I suddenly remembered a pop-up book I once had about Skylab, that I haven’t thought about in years, camping trips with my parents, and some of my first plane flights. Overall, I feel good having critically examined my beliefs. And I highly recommend this to others, especially teachers!

So far, the exploratorium team has created only three beliefs to map out: “The Earth is round,” “Humans cause global warming,” and “Ghosts are real.” But David Beck, one of the creators, assured me that more questions will be added in the future – in fact, they’d like people to be able to submit their own. In order to do this, they need to build an active community of participants, so people can effectively compare their answers with others, especially with working scientists. This is pretty much the sort of thing the Scienceblogs crowd would be perfect for! It would help them out if you’d go over there and share your experience – or invite your students to participate. It’s very easy to create an account, and you can spend as much or little time as you like adding evidence.

Visit my evidence through the Exploratorium’s very nice Evidence website – go to “Map Your Knowledge” and launch “myevidence.” Or go directly here.

*I assure you, the exploratorium team behind this is very much aware how scientists feel about the word “believe”; the final design of the project was based on focus group testing with the public, and tries to balance these concerns with clarity for the general public.


  1. #1 John S. Wilkins
    October 15, 2008

    Medieval peasants would be well aware that the earth was round. It was widely known, and – for example – I have a very nice photo of a 12th century clock in Exeter cathedral with a round earth in the centre.

    The notion that anyone thought the earth was flat after around the 5th century, and then only a few religious nutbars like Cosmo Indicopleustes, was invented by Washington Irving in the early 19th century.

  2. #2 Mike
    October 15, 2008

    Good thought experiment:
    If the world were flat and you put a bright object on the desert floor (desert for no obstructions) and walked away, no matter how far you walked, the object would still be visible. You might need binoculars but you should be able to see it miles away.

    The facts show that the item will drop below the horizon at a point (depending on the height of the observer) determined by the curvature of the earth.

  3. #3 Who Cares
    October 15, 2008

    If you had to persuade a medieval peasant that the world was round, how would you do it?

    Not. The idea that the medieval people thought the world was flat is a myth.

    Why do you believe the world is round?

    Not so much believe as make the assumption that people performing fairly simple experiments to establish if the earth is round are not lying.

    That site looks neat but their program has problems it seems. I can’t get my answers in the right age and where brackets.

    Interesting though that it seems you are the only one of three so-far who has put in a why the earth doesn’t seem round comment. And all three seem to be based on the gut feeling of I can’t see it.

  4. #4 Romeo Vitelli
    October 15, 2008

    “So far, the exploratorium team has created only three beliefs to map out: “The Earth is round,” “Humans cause global warming,” and “Ghosts are real.””

    Only two of those beliefs can be empirically tested.

  5. #5 Flaky
    October 15, 2008

    Coincidentally, I’ve amused myself with the roundness of Earth question. I think that the question of how to convince a medieval peasant vs. why I believe is quite different. I can easily accept the roundness of Earth alone from gravitational equilibrium, which follows straigth from the quite intuitive Netwonian gravity. But I could probably only convince the peasant that the Earth is not flat by showing that things don’t fall off the Earth when they go below the horizon.

  6. #6 Heuristics
    October 15, 2008

    >If you had to persuade a medieval peasant that the world was round
    >We often underestimate how little direct evidence we have for our beliefs

    You might do well in realising how little evidence you have for the myth that people generally thought the world was flat during the midieval days.

    Great blog btw, I like it alot.

  7. #7 Jessica Palmer
    October 15, 2008

    John and Heuristics, I’m not assuming that all medieval peasants thought the earth was flat, although I’m sure some of them did, just as some people today do as well. My point is that they didn’t experience a lot of the most convincing evidence we have today – namely photos from space, airplanes, video of astronauts, highly accurate maps and GPS, and instant worldwide communications. When I see that the majority of my own experience with the concept is based in technology – which was a very interesting outcome of doing the exercise – I begin to wonder how a person without access to any of that was able to intuit the roundness of the Earth.

    In a way, the readiness of the medieval mind to accept the “auctoritee” of learned sources might have helped to make the case based only on teachers’ wisdom and written text – but it’s hard to see how any but the most educated and well-traveled could have had much personal experience indicative of roundness (you’ll note I didn’t say “merchant”). In sum, I think it’s really interesting to imagine what evidence they used to build their knowledge about the roundness of the Earth, and how that differs from the evidence used for the same belief today.

    Actually, I challenge either of you, assuming you remember your history of science better than I do, to go create a username like “Piers Plowman” and list all the evidence that a medieval peasant would use to prove the earth is round! That would be extremely cool!

    In any case, I had no luck whatsoever in 2006 convincing a classroom of college kids that the moon’s orbit is synchronized with Earth such that we only ever see one side of it. (Apparently they were all too young to remember Pink Floyd). So don’t underestimate the scientific ignorance of individuals in any age. . .

  8. #8 Jessica Palmer
    October 15, 2008

    “Only two of those beliefs can be empirically tested.”

    Indeed, Romeo. That’s why it will (hopefully) be an interesting experience for people to compare the kind of evidence they have for that belief with the evidence they have for a conclusion based in science.

    My take on it? The ghosts question is a lot less inflammatory than the evolution/creation question which is likely to eventually show up there, so might be a better way to beta-test a non-scientific belief. The only problem is that so few people believe in ghosts. . . I’d have opted for astrology myself!

  9. #9 Heuristics
    October 15, 2008

    Evidence for the roundness of the earth would have been readily available for anyone living in a town close to the ocean as the masts of ships appear before the boat do on the horizon.

  10. #10 Jim Thomerson
    October 15, 2008

    I recall this from when I had a history of biolgy course in 1961, so the details may be fuzzy. A fundamentalist preacher posted a reward for anyone who could demonstrate that the earth was round. Alfred Russel Wallace knew a way. He asked his friend Lyell if he should proceed. Lyell advised him to go ahead.

    Wallace knew of a long canal with stagnant standing water. He and the preacher (and observers?) went there and placed three stakes, one at each end and the other at the middle. The distances from the water surface to a mark on the stakes was carefully measured to be the same for all three stakes. So, viewing all three stakes, from one end, with a transit, when the two end marks were lined up, the middle mark was higher.

    The preacher accused Wallace of fraud and refused to pay. There were lawsuits and fistfights, but I don’t recall how it turned out. What struck me was Lyell, who told Darwin to stand back from the fray, advised Wallace to go ahead.

    According to my surveying book, curvature of the earth is 0.6 ft/mile. Can I just show my surveying book to the peasant?

  11. #11 Jessica Palmer
    October 15, 2008

    Indeed, Heuristics, that’s an observation I included on my map at myevidence, and one of the few that require no advanced technology (beyond the ability to build tall ships). However, not everyone lives within sight of an ocean (I’ve known college students who have never seen it). Also, I couldn’t see it myself until I got corrective lenses (which I would not have were I a medieval peasant). And it’s easy to imagine a flat-earther dismissing the ship’s retreat as an optical distortion, like a mirage. What other evidence would you put forth as a medieval peasant? I’m very interested to hear!

    Jim, haven’t you watched “Time Bandits” or “Back to the Future 2?” Keep the surveying book away from the peasants! Back away slowly!!

  12. #12 Richard Simons
    October 15, 2008

    A piece of evidence for the roundness of Earth used by one of the Greeks (I can’t remember who) was the shape of Earth’s shadow on the moon during an eclipse.

  13. #13 kraant
    October 16, 2008

    A ship descending below the horizon…

    It’s pretty convincing.

  14. #14 Riggald
    October 16, 2008

    The way the horizon moves further away the farther up a mountain you go – available to people who have mountains instead of ocean in their backyard.

  15. #15 maxamillion
    October 16, 2008

    Evidence for the roundness of the earth would have been readily available for anyone living in a town close to the ocean as the masts of ships appear before the boat do on the horizon.

    Posted by: Heuristics | October 15, 2008 3:55 PM

    You would think that was the case.
    But not for this Astronomer

  16. #16 John S. Wilkins
    October 16, 2008

    Wallace was, I think, Lyell’s surrogate for attacking the ignorant middle class, being middle class himself (Wallace, that is). Poor Wallace needed the money but, like many honest men, could not conceive that either his honesty would be called into question, or that the nutbar in question was dishonest enough to do so. Those of us who have debated creationists long enough know well enough…

  17. #17 Jessica Palmer
    October 16, 2008

    Wow, Maxamillion! I didn’t expect there to actually be such a perfect example of someone dismissing the ocean observation, but there you go! Astonishing.

    That “researcher” has some pretty thick glasses – maybe like me, he’s never seen a distant ship very clearly?

    Richard, I got the roundness of the earth’s shadow on the moon – but hypothetically the earth could cast a round shadow and be a flat disc or ellipse. That’s why I allotted it less volume in my knowledge map at myevidence.

  18. #18 Tony Sidaway
    October 16, 2008

    I’m reasonably sure I’m seen a curved shadow on the moon at a lunar eclipse. The sun sets at a different time when I move a few hundred miles to the north. These fit with a curved earth. The interesting thing is that these facts, and others such as the way a ship’s top-sails are visible first over the horizon if you watch for it with a telescope, form an accumulation of evidence that make it difficult to argue for various other scenario than a round earth. There is no conclusive evidence.

  19. #19 Abel Pharmboy
    October 16, 2008

    A terrific post on a terrific topic. Thanks for the tip on ‘my evidence.’

    Having a small child has really made me revisit my assumptions and get back to the basics of all the things I take for granted. Having to back way up for to answer a curious six-year-old about science and nature is a challenge and has actually improved my university teaching skills. I think also that is why Janet Stemwedel’s Friday Sprog Blogging at Adventures in Ethics and Science is so successful.

  20. #20 PeteC
    October 16, 2008

    Many of these (the ship fading from view, the canal & sticks) only demonstrate that the earth is locally curved, which any ignorant peasant might accept without conceding the truth of the highly counterintuitive claim that there are people walking on the other side of the spherical earth whose feet are closer to you than their heads are, but they don’t fall off.

    (Also the shadow of the earth on the moon could come from a round disk.)

  21. #21 Jeremy
    October 17, 2008

    Most people find it impossible to prove that the Earth is round. Certainly things like the view of the Earth from flights is not sufficient to accurately discern that it is flat. The only 2 convincing methods I’ve seen that can be genuinely tested personally and don’t rely on what other people say are taking photos of the stars with an open shutter (so you see them “moving” in circles”) or making a number of recordings with a sun dial at different places around the world as close to the same time as possible. I’m unsure how far apart each reading has to be to be able to see significant enough differences to maintain a round Earth, but readings either side of the equator would be most ideal.

    Ignoring Occam’s Razor, it’s easy to come up with flat Earth consistent theories for most other claims.

  22. #22 Comrade PhysioProf
    October 17, 2008

    Holy fucknoly, BioE! I didn’t know you are also a philosopher! Do you knit?

  23. #23 Larry Ayers
    October 17, 2008

    Great post and comment thread, Jessica! That’s one perk of writing for a blog — interesting folks stop by and a conversation can develop.

  24. #24 Richard
    October 17, 2008

    The curvature of the horizon is measurably the same in all directions.

    The shadow of Earth on the moon during a lunar eclipse is consistent with either a flat pancake viewed perfectly head-on, or a sphere. Not consistent with a torus.

    Not to mention, hey, all the images of Earth from space, from the moon, and all the circumnavigational sea and air voyages.

    Also the offset of the stars with latitude and longitude, calibrated with an honest clock and calendar. The way they change with position is consistent only with a near-perfect spheroid. Not too oblate, not too prolate.

    Etc, etc. It is trivial to verify that Earth is a sphere, even if you Trust No One. You can even measure its circumference in the same way that Eratosthenes did.

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