Another Greek mathematician, Eratosthenes, proved that the earth was spherical. He, like Leonardo da Vinci, was a versatile man, and he was an astronomer. Math He is also an expert in geography and philosophy. Not only did he prove that the earth was round, but he could also measure the circumference of the earth using sticks and the shadow of the sun. Arotos Thanes was so eager to create a world map that he first needed to know the size of the earth.