Summary
Highlights
Initially, around 500 BC, most Greeks believed the Earth was flat. However, Pythagoras and his pupils were the first to propose a spherical Earth.
Around 340 BC, Aristotle provided several arguments. He noted that the North Star's position changed depending on the observer's location, appearing closer to the horizon near the equator. He also reasoned that if the sun and moon were spherical, the Earth likely was too. Finally, he observed that as ships sailed away, their hulls disappeared before their sails, suggesting the Earth's curvature.
Eratosthenes provided the most accurate calculation of the Earth's circumference during his time. While in Alexandria, he learned that at noon during the summer solstice, vertical objects in Syene cast no shadow, unlike in Alexandria where they did. He hypothesized that the sun's rays were parallel and the Earth was curved. By measuring the angle of the shadow in Alexandria (7.2 degrees difference from Syene), he calculated the Earth's circumference to be approximately 250,000 stadia (about 40,000 kilometers).
Between 500 and 430 BC, Anaxagoras supported Pythagoras's idea by observing the circular shadow the Earth cast on the moon during lunar eclipses.