Flat vs. Round – How can people prove it to themselves?

I am what you might call a “round-earth”-er, and I was surprised to see this recent resurgence of people claiming that the earth is flat. I started reading more about this and inevitably ended up on the site of the Flat Earth Society (as well as aplanetruth.info). As a disclaimer, I still think the earth is round, but I do find the skeptical nature of flat-earthers interesting.

There’s this excellent article in the Atlantic by Lizzie Wade called “In Defense of Flat Earthers“. She references a book called “Physics on the Fringe” by Margaret Wertheim that goes into many similar ideas. The people who promote these ideas are mostly just skeptics that want to be able to prove things to themselves. They share a common concern that too many people take what they are told in science class for granted. I generally have much more faith in scientists, but I can still relate to this notion a little.

There’s a lot of articles that claim to have “proof” of a flat or round earth, like this one. Most of the “proofs” for a flat earth are absurd, but most of the “proofs” for a round earth have subtle implicit assumptions. In other cases people offer ways to prove for yourself that involve a huge undertaking, such as walking a 1000 miles in a straight line.

This all got me trying to think of a way any normal person could make their own measurements and make a judgement for themselves. Because the flat-earth ideas revolve around a NASA conspiracy I wanted a way that didn’t rely on the existence of satellites, so no GPS or imagery from space. I came up with 3 ways to to do this, but the third one is the only reasonable one for anyone with limited funds.

  1. Measure the Moon – In an episode of the Big Bang Theory they use a laser to bounce some photons off the moon and calculate the distance. Obviously getting a high power laser is not trivial, but they are used in all sorts of applications so I’m sure you could acquire one if you wanted to. This wouldn’t prove the earth is round, but it would certainly be problematic for a central part of the flat-earth theory in which the sun and moon are only 3000 miles above the surface.
  2. Reflected Radiation – A similar idea to the first would be to measure the spectrum of radiation coming from the moon when it is full and when it is nearly new. The idea would be to show that the difference in the spectra is consistent with the idea that the full moon is reflecting light from the sun. The arrangement of the sun/earth/moon needed to make this happen doesn’t work with the flat-earth model in which the son and moon are always floating above a flat disk.
  3. Coriolis – Although some flat-earthers think the coriolis effect does not exist, it is trivial to demonstrate how this works with any fluid moving within a rotating frame of reference. Once you have have proven this to yourself, you could take basic measurements of wind for several days while a low pressure system is moving over you. Obviously it would be best to do this in a flat region or over the ocean. If you did this in a mid latitude location in the northern and southern hemisphere you would notice that the wind moves in opposite directions around the area of low pressure. This could not happen on a disk-shaped earth unless you crossed onto the underside of the disk.

I realize I’m wasting my time here, but it’s still interesting to think about!

4 thoughts on “Flat vs. Round – How can people prove it to themselves?

  1. dws

    Great blog and interesting read.

    You can prove to yourself that the Earth is curved if you have four miles of level water (e.g. a lake or canal), low refraction, and a good sight, as Alfred Wallace did in 1870:


    Such an experiment would probably require getting the consent of local landowners, but it would probably be simpler than walking 1,000 miles in a straight line.

    1. Walter Post author

      Thanks, and thanks for the link, which is a fascinating story! I don’t have a telescope, but I would actually consider trying this out just for the fun of it.

      1. ConG

        Well plenty of people are doing the Bedford experiment and proving that it’s not got the curvatures, and things do not go out of distance like they should…..


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