Aristarchus: A Sun-Centred Cosmos, Known from One Sentence

Tonight, a man who put the sun in the middle of the universe, eighteen centuries before Copernicus: Aristarchus of Samos.

Then why does everybody still say the sun goes round the Earth?

Because almost nobody believed him, and for good reasons, as you will see. And because his book about it is lost. We know his idea from one passage in Archimedes.

That, and a few lines in Plutarch. Let me show you.

Aristarchus was born on the island of Samos and probably worked for a time in Alexandria. Only one of his books survives: On the Sizes and Distances of the Sun and Moon. Strangely, it is written as if the Earth stands still.

In it, he uses geometry to compare the distances of the sun and the moon. When the moon is exactly half lit, the angle between the moon and the sun, seen from the Earth, tells you how much farther away the sun is. He measured the angle as 87 degrees and concluded that the sun is 18 to 20 times farther away than the moon, and many times larger than the Earth.