A mathematician, astrologer and astronomer who made great contributions to the development of the Copernican system. After studying at the University of Vienna he was appointed to the faculty, thereby becoming a colleague of Peurbach, with whom he had studied astronomy. The two men became fast friends and worked closely together as observers of the heavens. Among Regiomontanus' most important achievements was a portable sundial built for Pope Paul II, a pioneering work on arithmetic and algebra, and his 'Epitome of Ptolemy's Almagest'. Copernicus referred to this work as an influence on his own ideas about the solar system.