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 ?
Since flat earth nonsense is just rebranded creationism, the only being who really needs to be part of the conspiracy is satan.
Then anyone who tells you that the earth is round is either deceived by satan, or is a satan worshiper.
This makes it super easy to dismiss anything that would hint at reality, like ships sailing over the horizon.