Copernicus studied at Bologna University during 1496–1501, where he became the assistant of Domenico Maria Novara da Ferrara. He is known to have studied the "Epitome in Almagestum Ptolemei" by Peuerbach and Regiomontanus (printed in Venice in 1496) and to have performed observations of lunar motions on 9 March 1497. Copernicus went on to develop an explicitly heliocentric model of planetary motion, at first written in his short work "Commentariolus" some time before 1514, circulated in a limited number of copies among his acquaintances. He continued to refine his system until publishing his larger work, "De revolutionibus orbium coelestium" (1543), which contained detailed diagrams and tables.