Johannes Müller von Königsberg (June 6, 1436 - July 6, 1476), Latin name Regiomontanus, was an important mathematician, astronomer, and astrologer of the 15th century. He was born in Königsberg village Unfinden in Franconia.

He is also called Johannes Müller, der Königsberger. His Latin full name is Joannes de Regio monte, which shortens to Regiomontanus (translation of "Königsberg" or "King's Mountain").

A son of a miller, child prodigy, at eleven years of age, he became a student at Leipzig, Saxony university. Three years later he continued his studies at the Alma Mater Rudolfina in Vienna, Austria. There he became a pupil and friend of Georg von Peurbach . In 1457 he graduated with the degree "magister artium" and held lectures in optics and antique literature. In the same year he built an astrolabium for Maximilian I, in 1465 a portable sundial for pope Paul II. The work with Peurbach brought him to the writings of Nikolaus von Kues (or Cusanus ) who had a heliocentric approach. However, Regiomontanus remained geocentric following Ptolemy's work. After Peurbach died he continued his translation of Ptolemy's Almagest which was initiated by Johannes Bessarion. From 1461-65 Regiomontanus lived and worked at Cardinal Bessarion's house in Rome. He wrote De Triangulis omnimodus (1464) and Epytoma in almagesti ptolemei. De Triangulis was one of the first textbooks, presenting the current state of trigonometry including a kind of repetitorium (list question for recapitulating each chapter). In it, he wrote:

You, who wish to study great and wonderful things, who wonder about the movement of the stars, must read these theorems about triangles. Knowing these ideas will open the door to all of astronomy and to certain geometric problems.

In the Epytoma he commented the translation and indicated inaccuracies. Later, Nicolaus Copernicus referred to this book as an influenced on his own work. In 1467, he went from Rome to work at the court of Matthias Corvinus of Hungary. He calculated and made extensive astronomical tables. He also built astronomical instruments.

In 1471 Johannes Müller moved to the Free City of Nuremberg in Franconia, which at that time was one of the most important places of learning, publication, commerce, artistry etc of the empire. Regiomontanus remains known for building the first astronomical observatory of Germany, perhaps of Europe, at Nuremberg. He made and printed many astronomical charts.

In 1475 he went to work with Pope Sixtus IV in Rome on calendar reform. While in Rome, Müller died mysteriously; some say of the plague, others (more feasibly) of assassination. It was July 6, 1476, and he had just turned forty exactly one month earlier.

Domenico Maria Novara da Ferrara, the teacher of Nicolaus Copernicus, referred to Regiomontanus as having been his teacher.

Johannes Müller was internationally famous already during his lifetime. He was a very active writer. Despite his plans for writing four times the volume he actually did get to finish, he left us a number of works.

It is not true that after his death he simply became known for the place of his birth, Königsberg or in Latin: Regiomontanus. In fact, the story is much more interesting: It happens that in the time of Müller it was common for scholars to author their works under Latin pennames . Copernicus did likewise and that is why we do not know him as Zepernik, as he was really called. By the same token, the only reason we know Regiomontanus by this place-name is that we can only know him, as is true for everyone sooner or later, by the very words he wrote.

Regiomontanus and Astrology

One biographer detected a decline in Regiomontanus' interest in astrology over the course of his life, and came close to asserting that he rejected it altogether. But more recent commentators have suggested that occasional expressions of skepticism about astrological prognostications reflect a disquiet about the procedural rigour of the art, not its underlying principles. It seems plausible that, like certain other astronomers, Regiomontanus concentrated his efforts on mathematical astronomy because he felt that astrology could not be placed on a sound footing until the celestial motions could be modeled accurately.

In his youth, Regiomontanus cast horoscopes (or natal charts) for famous patrons. His Tabulae directionum, completed during his time in Hungary, were designed for astrological use, and contained a discussion of the different ways of determining the astrological houses. The calendars for 1475-1531 which he printed at Nuremberg contained only a limited quantity of astrological information, namely the method of finding times for blood-letting according to the position of the Moon; subsequent editors added a quantity of additional material. But perhaps the works most indicative of Regiomontanus' hopes for an empirically-sound astrology were his almanacs or ephemerides, produced first in Vienna for his own benefit, and printed for the years 1475-1506 in Nuremberg. Weather predictions and observations were juxtaposed by Regiomontanus in his manuscript almanacs, and the form of the printed text enabled scholars to enter their own weather observations in order to likewise check astrological predictions; extant copies reveal that several did so.

Regiomontanus did not live to produce the special commentary to the ephemerides which he promised would reveal what advantages these almanacs hold for the multifaceted activities of physicians, for human births and telling the future, for weather forecasting, for the start of employment, and for a host of other activities, although this lack was again rectified by subsequent editors. Nevertheless this announcement suggests that Regiomontanus was either as convinced of the validity and utility of astrology as his contemporaries, or willing to put aside his misgivings to achieve commercial success.

External links

• Adam Mosley, Regiomontanus Biography http://www.hps.cam.ac.uk/starry/regiomontanus.html , web site at the Department of History and Philosophy of Science of the University of Cambridge (1999).