Nicolaus Copernicus

Nicolaus Copernicus was born in 1473 in the Polish city of Toruń. He trained in canon law, medicine, and mathematics at universities in Kraków, Bologna, and Padua, and spent most of his adult life as a church administrator in the cathedral chapter at Frombork. Astronomy was a private obsession that he worked on for decades before publishing.

His book De revolutionibus orbium coelestium appeared in 1543, the year he died. In it he proposed that the Sun, not the Earth, sits at the center of the universe, with the Earth and the other planets moving around it. The idea was not entirely new — some Greek astronomers had suggested it — but Copernicus was the first to work out a full mathematical model. He kept one ancient assumption that would turn out to be wrong: that the orbits are circles. To match the observed motions, he still needed epicycles, though fewer than Ptolemy.

The heliocentric picture took another century to win out. Galileo's telescope, Kepler's ellipses, and Newton's gravity did the heavy lifting. But the starting point — the move that made the rest possible — was Copernicus's decision to let the Earth be just another planet.