Just a thought experiment, let’s say the earth is flat, looks like many professionals would know, including
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Pilots/ATC/sea captain
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Land surveyor and civil engineers
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Astronomers and Geologists, at least the one dealing with experimental data
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Militaries, at least radar operator and artillery
Just these categories are already a lot of persons, I assume I miss a lot of others. So how many person does it make in total ? 10 % of the population ?
Mathematicians. On a sphere, you can make a triangle that consists of three 90 degree angles, adding up to 270. A flat triangle will require that all angles add up to 180, i.e. 60, 60, 60.
Edit: I’m not a mathematician, and yet I know this. So this one could probably include anyone who has ever had a math teacher that covered spherical geometry.
I’m not sure that works though. For it to have all three angles at 90 degrees, wouldn’t it need to cover a full 1/8th of the planet? Flat earthers would at lest understand that much. They would just deny anyone had measured that accurately. After all, anyone could go up in a spaceship to see the true shape of the earth, but only bigger governments and a few shitty billionaires have the means to do so.