Which Greek scientist concluded that Earth was round?

In Syene (now Aswan, Egypt), Eratosthenes noticed that the Sun's rays fell directly at noon on the summer solstice, producing no shadow. However, when he travelled to Alexandria (a nearby city), he noticed that the Sun's rays produced a shadow from a wall. Curious about the phenomenon, he calculated the angle between the object and the shadow it created, noted as, #z# (#7.2^@#), in the following picure.

Because of the Alternate Interior Angle Theorem, Eratosthenes also knew that angle #z# created by the wall in Alexandria was equal to the angle separating Alexandria and Syrene.

He also found the distance between Syene and Alexandria (5,000 stadia or about 500 miles/800 kilometres), which led him to create the following equation:

#360^@/7.2^@="circumference of Earth"/"distance from Alexandria to Syene"#

Since Eratosthenes knew the distance from Alexandria to Syene, all he had to do was solve for the circumference of the Earth. From his findings, he concluded that the Earth was round.