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 ?
Anybody who has ever flown far enough Eastwards or Westwards and saw how the time of day changed either much faster or slower than the time their clock said it was.
(Which can’t be mathematically explained for a flat Earth but is a pretty simple effect to explain with a round Earth)