From Carl Sagan:
Eratosthenes’s only tools were sticks, eyes, feet and brains. Plus a zest for experimenting. With those tools, he correctly deduced the circumference of the Earth, to high precision. With an error of only a few percent. That’s pretty good figuring for 2,200 years ago.”
Seriously, Greek scientist Eratosthenes did not only figure Earth was round over 2200 years ago, he was also able to calculate the circumference of the planet with a very small margin of error using rudimentary means. Fast forward to 2023, some people still believe our planet is flat.