Regiomontanus, name adopted by Johann Müller, a celebrated German astronomer and mathematician, born at Königsberg, in Franconia; appointed professor of Astronomy in Vienna (1461); sojourned in Italy; settled in Nüremberg, where much of his best work was done; assisted Pope Sixtus IV. in reforming the Calendar; was made Bishop of Ratisbon; died at Rome; was regarded as the most learned astronomer of the time in Europe, and his works were of great value to Columbus and other early navigators (1436-1476).