The Ancient Greeks (possibly Pythagoras or one of his followers) had first pointed out that world was spherical by the 500s b.c.e., though there was considerable disagreement as to its size. The idea that the world is flat is counterintuitive to anyone who has watched objects appear over the horizon, a phenonomenon that must have been frequently observed in the ancient Mediterranean. However, many peoples before the Greeks had characterized the earth as a flat disk. Historians do know that one Greek philosopher, Eratosthenes, calculated the circumference of the Earth to within one percent of modern measurements, and published his work around the 240s b.c.e. His calculation was considerably smaller than other contemporary measurements.
Pythagoras first passed by the idea that the world was in fact round and not flat.
His reasoning behind him was:
He saw harmony in the universe and sought to explain it. He reasoned that Earth and the other planets must be spheres, since the most harmonious geometric solid was a sphere.