Our ancient ancestors *knew* the world was flat, and they could *prove* it.
This is actually NOT true. Anyone watching a ship's sails dip over the horizon *knew* that wasn't the case.
Aristotle estimated the diameter to be 400,000 stadia (about 45,500 miles) but Eratosthenes in 276 BC actually used a bit better math (and experimentation using shadows at noon, some trigonometry and so on) to compute it at 252,000 stadia (or about 24,662 miles, which is close to 24,901, the actual number).
Columbus - and the Renaissance era Europeans - KNEW the Earth was round, but they had no idea there was a whole CONTINENT between them, and Asia - just - a REALLY HUGE OCEAN. If there was no New World - using the estimates they had - it would be a trip of about 18,000 miles. WAY too far to sail THAT way. Columbus thought Eratosthenes was wrong - he believed another guy (Marinus of Tyre, whose math was TERRIBLE and did NO experiments) whose estimates were 1/4 of the correct number. AND - he doubted HIM.
GENERALLY - Europeans, Arabs and others sailing back then believed the Euro-Asian-African landmass was half the Earth - and water was the rest.
It IS true that - further back in history - they believed the Earth stood on the shoulders of elephants standing on a gigantic turtle. But this is the difference between SCIENCE - and superstition.
I do follow Occam's Razor for most things - because it is true most of the time that really, really complex answers are created to support a predetermined belief rather than science. The best historical example I can think of is the concoction of "epicyles" when observing the apparent retrograde motion of the planets. Since people "KNEW" the Earth was the center of the universe, the only explanation for their apparent backward motion was they followed these ridiculous Spirograph like mini circles within their orbits. Over time, they added MORE and MORE nested epicycles to explain it further. Once they understood the pre-supposition of the Earth was wrong - it was clear that the simplest answer was, they revolve about the Sun at different speeds in a simple ellipse.
Like the abandonment of "the ether" in space, it's what happens when you follow the science - instead of starting with assumptions and looking to confirm them.
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Regarding "aliens" - it might be slightly more believable if it weren't for the belief that they were all humanoid - two eyes, NOSE with two slits for nostrils, two arms and legs and walk upright - like people. There's no logic that all advanced life forms would somehow - look a lot like US.