Copernicus wasn’t the first person to theorize that the Earth revolved around the sun, but he was the first person to publish support for the theory that anyone paid attention to. In 1543, he published De revolutionibus orbium coelestium libri VI (“Six Books Concerning the Revolutions of the Heavenly Orbs”), which asserted the idea Philolaus and Hicetas, Greek philosophers stargazing in about 500 BC, first posited: that the Earth is round, and that it revolved around a “central fire” that holds the universe together. Even though the heliocentric model of the universe was first conceived long, long ago, it wasn’t until Galileo came along in the 1600s and proved it that people’s views began to shift. Of course, Galileo paid a price for his work: 381 years ago today, the Vatican declared him a heretic.

The heliocentric model had a few sticking points: that the earth wasn’t the center of the universe, of course, but also that the positions of the stars seemed never to change, regardless of Earth’s supposed orbit around the sun. It was in response to that second item that Claudius Ptolemy theorized that if Earth were fixed, and everything else revolved around it, then that would explain why the stars never moved. This made sense to folks, so the geocentric theory reigned for almost 1,500 years.