February 19, 1473. Nicolaus Copernicus was born on this date, 540 years ago. Copernicus was a Renaissance astronomer and mathematician. He lived at a time when people believed Earth lay enclosed within crystal spheres at the center of the universe. Can you picture the leap of imagination required for him to conceive of a sun-centered universe? The publication of Copernicus’ book – De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) – just before his death in 1543, set the stage for all of modern astronomy. Today, people speak of his work as the Copernican Revolution.