<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 calculated the circumference of the earth by observing the angle of the sun as it appeared at two different locations on Earth. He noticed that the angle of the sun appeared to be the same at two different locations, but the length of the shadow was different, indicating that the earth must be curved.