<human>:  Who was the first person to discover that the earth was round?
<bot>: It was the Greek philosopher Eratosthenes, who lived in the third century BCE. He is most famous for his estimate of the circumference of the earth. He found that the sun sets at the same time in Syene, Egypt and in Alexandria, Egypt. This meant that the sun must be further away from the earth in Syene than in Alexandria, and therefore the earth must be larger in circumference than in diameter.