Nicolaus Copernicus was a Renaissance-era mathematician and astronomer from Poland. He is best known for his heliocentric theory, which proposed that the Sun, rather than the Earth, was the center of the universe. This theory revolutionized the understanding of the cosmos and laid the foundation for modern astronomy. Copernicus' work challenged the prevailing geocentric model and paved the way for future scientific advancements in the field of astronomy. His most famous work, "De revolutionibus orbium coelestium" (On the Revolutions of the Celestial Spheres), was published just before his death in 1543. Copernicus' contributions to science have had a lasting impact on our understanding of the universe.