Eratosthenes, an ancient Greek philosopher, was the first human being in recorded history to calcualte the circumference of a planet (specifically, the Earth), and he did so with remarkable accuracy.
Basically, he had two sticks stuck vertically upright in the sand in two towns, one in Alexandria, the other in Swenet. When the sun was directly overhead, casting no shadows from the stick in Swenet, he noted there was a shadow cast by the other stick in Alexandria at the same time.
Measuring the distance between the two sticks (by basically having some dude walk from one town to the other and count his steps) and measuring the length of the shadow cast and angle of the light casting the shadow, he basically figured out not only that the Earth had to be round, but he went on to calculate it to an accuracy of 99% of the modern day accepted circumference of the Earth.
All this using a couple of sticks, some dude walking and counting his steps, and a little maths.
Since this probably blew your mind like it did mine, I'll give you a minute to clean up brain matter off your screen.
He was also the first to calculate the tilt of the Earth's axis, again with remarkable accuracy; and it is possible he also may have calcualted the Earth's distance from the Sun to an accuracy of 99% compared to that of today's accepted distance.
He did all this around 200 years before Jesus walked the Earth, or a solid 1,700 years before man circumnavigated the planet and removed all doubt of the Earth being flat.
Or, you know, he was all wrong and it's all a bunch of lies.