A bloke with a stick in 290BC could work this out.... but give a conspiracy theorist a computer in the information age and they are stumped by it.
Eratosthenes erected a pole in Alexandria, and on the summer solstice he observed that it cast a shadow, proving that the Sun was not directly overhead but slightly south. Recognizing the curvature of the Earth and knowing the distance between the two cities enabled Eratosthenes to calculate the planet's circumference.
