Encyclopædia Britannica, Ninth Edition/Regiomontanus
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REGIOMONTANUS (1436-1476). The real name of this astronomer was Johann Müller, but from his birthplace, Königsberg, a small town in Franconia, he called himself Joh. de Monteregio. The name Regiomontanus occur for the first time on the title page of his Scripta, published in 1544, but he has since become best known by it. He was born in June 1436 and became the pupil of Purbach at the university of Vienna, and jointly with him endeavoured, with such imperfect instruments as they could onstruct, to test the accuracy of the Alphonsine tables of the motions of the planets. After Purbach's death Regiomontanus finished and published his Epitome in Ptolemæi Almagestum, but, having in the meantime become acquainted with Cardinal Bessarion, who was anxious to spread the knowledge of the Greek literature among the Western nations, he proceeded with him to Italy in 1462, and for the following eight years devoted a great deal of time to the study of the Greek language and to collecting Greek manuscripts. He returned from Italy in 1471 and settled at Nuremberg, at that time one of the chief centres of German industry and literary life. Here he became associated with Bernhard Walther (1430-1504), a wealthy patrician and an enthusiastic astronomer. An observatory was erected, and the finest instruments the skilful artisans of Nuremberg could make were regularly used by the two friends for observing the heavens. Clocks driven by weights were here used for the first time for scientific purposes, the influence of refraction in altering the apparent places of the stars better appreciated, Venus substituted for the moon as a connecting link between observations of the sun and of stars, and other improvements introduced in practical astronomy. Regiomontanus also published a number of calendars and ephemerides, which induced Pope Sixtus IV. to summon him to Rome to assist in reforming the confused calendar. He died very shortly after his arrival in Rome, July 6, 1476.
In 1464 Regiomontanus finished a work on trigonometry, which science had made considerable progress among the Arabians but had to be reinvented in Europe. The work was, however, never printed till 1533 (De Triangulis libri quinque), probably because the author, after introducing the use of tangents, had wished to re-write his book, but was prevented from doing so by his early death. In his Tabulæ Directionum (Nuremberg, 1475) there is a table of tangents (tabula fecunda). His instruments and observations at Nuremberg are described in a posthumous work, — Scripta clarissimi mathematici Joh. Regiomontani (Nuremberg, 1544). The ephemerides and calendars were published partly in German (Magister Johann von Kunsperk's teutscher Kalender), partly in Latin (Ephemerides Astronomicæ, Nuremberg, 1473 or 1475, for the years 1475-1506; Kalendarium Novum, Nuremberg 1474, re-issued many times and translated into German and Italian). The German geographer Martin Behem made these calendars known among the Spanish and Portuguese navigators, and they became of the greatest importance in guiding Columbus, Diaz, Vasco da Gama, and many others over the trackless ocean. The life of Regiomontanus was written by Gassendi (The Hague, 1654); among modern works see Regiomontanus, ein geistiger Vorläufer des Copernicus, by Zeigler (Dresden, 1874), Die Vorgeschichte der Gregorianischen Kalenderreform, by Kaltenbrunner (Vienna, 1876), and Rudolph Wolfs Geschichte der Astronomie (Munich, 1877).