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.
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.