How did the early astronomer Copernicus study the stars? - Answers

Other people recorded their observations of the stars and planets with astrolabes. Ptolemy came up with a (very complicated) mathematical system that would predict the location of planets over time. His basic, geocentric idea was modified over the centuries, but only became more complicated as observations became more exact. Copernicus took these observations that others had written down, and proposed a heliocentric system that he (correctly) noted was far simpler in making accurate predictions.