Nicolaus Copernicus (19 February 1473 – 24 May 1543) was a Polish astronomer. People know Copernicus for his ideas about the sun and the earth. His main idea was that our world is "heliocentric" ("helios" = sun). His theory was that the sun is in the middle of the solar system, and the planets go around it. This was published in his book, "De revolutionibus orbium coelestium" (On the Revolutions of the Heavenly Spheres) in the year that he died.