What did Nicolaus Copernicus discover in 1543?
Nothing. He died in 1543.
The revolutionary theory Nicolaus Copernicus published in 1543 was the Heliocentric model of the solar system. Published in his book On the Revolutions of the Heavenly Bodies, Copernicus proposed that the Sun, not the Earth, was the center of the Solar System. The book wasn't published until Copernicus was on his deathbed because of the outrage he expected his theory to cause.