I'm voting for Eratosthenes, the Greek scholar (circa 276 -195 BC) who measured the circumference of the world by observing the angle of the shadow cast by a vertical stick at noon, and applying Euclid's theorem of equal angles formed by a straight line intersecting two parallel lines. This angle was measured on a particular day in Alexandria which was about 5,000 stades directly north of Syene (now called Aswan). A stade is the length of a Greek stadium.
Now, on that sunny day, while there was a shadow in Alexandria at noon, a strange phenomenon was taking place down in Syene. There were absolutely no shadows. Poor Eratosthenes, no one ever told him about winter and summer solstices.
But solstice or no solstice, Eratosthenes surmised, from empirical observations, that this phenomenon was because the surface of the earth was curved. Therefore he reasoned that if the rays from the sun striking two vertical sticks, one at Alexandria and the other at Syrene, are parallel; then according to Euclid's theorem, the angle of the shadow at Alexandria must be identical to the angle where the imaginary lines of the two sticks on the surface of the earth meet. In a flash, Eratosthenes has deflated the rubbish of a flat earth being held up on the aching shoulders of Atlas and has pin-pointed the centre of the earth having no hi-tech instruments.
And by simple 6-grade arithmetic, he calculates the angle of the shadow at Alexandria with the known distance (5,000 stades) from Syene where there is no shadow, and completes it to make a full 360 degrees circle. The postscript is, the actual circumference of the earth is 24,900 miles and Eratosthenes, with his crude calculation is within a few percentage off.
Hey, let's give this guy a posthumous Nobel prize for the most elegant math equation ever!