Who Discovered That The Earth Is Round?

A view of Earth from the Space Shuttle Discovery shows late afternoon sun on the Andes Mountains, with glare and heavy cloud illumination. (Bettmann/Getty Images)

Saying the Earth is round may feel like saying the sky is blue for as much common sense as it feels to us in the modern age, but before the advent of airplanes and space travel, there were no photographs to show us that the planet is indeed a globe. So how did people figure it out as far back as 500 B.C.E.?

The Other Pythagorean Theorem

Much of the acclaim for determining that the Earth is round falls to Pythagoras, best known for the famous theorem you definitely had to learn back in school. He devised the first recorded theory that the Earth was round, based on his observation of all the other round heavenly bodies that could be seen by the human eye. Later, Aristotle followed up this notion by pointing out that during a lunar eclipse, a round shadow is cast upon the moon. He also cited the curious shifting of stars when a person traveled far north or south, with some constellations changing or disappearing entirely.

Eratosthenes teaching in Alexandria by Bernardo Strozzi (1635). (Montreal Museum of Fine Arts/Wikimedia Commons)

Eratosthenes Of Cyrene

But these were all simply observations, with no real math or testable science to back them up. Enter Eratosthenes of Cyrene, a Greek mathematician, astronomer, and head librarian to the Library of Alexandria in Egypt. One day, he heard that a well in the city of Syene (present-day Aswan) cast no shadows on the summer solstice, as it was lit by the sun directly overhead. On the next summer solstice, he decided to measure the shadow cast off a stick when the sun was positioned in the exact center of the sky, which he determined was seven degrees.