Wilhelm IV, Landgrave of Hesse

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(b. Kassel, Germany, 24 June 1532; d. Kassel, 25 August 1592)


A contemporary of Peter Apian, Copernicus, Tycho Brahe, and Kepler, Wilhelm lived in the age of the greatest astronomical revolution since antiquity. He played scarcely any role in the resulting debates, however, for he was concerned primarily with the refining of the techniques of astronomical observation. At the court in Kassel he was tutored, chiefly in mathematics, by Rumold Mercator, the son of the geographer Gerardus Mercator. He became interested in astronomy after reading Apian’s Astronomicum Caesareum, a splendid book although it was conceived within the conceptual framework of the old geocentric view. Wilhelm’s manuscript copy of this work, which is still extant, contains handwritten planetary tables that obviously were drawn up at his request by Andreas Schöner. On the model of Apian’s system of movable cardboard disks, Wilhelm devised an arrangement of metal plates that made possible the construction of the Wilhelmsuhr (1560–1561). This mechanical astronomical clock was so precise that the ephemerides could be read directly from it. Similar clocks were later produced in great quantities.

Satisfying the love of display then flourishing in most princely courts was not, however, Wilhelm’s goal; his main goal was to further the study of astronomy. In the course of his own astronomical observations, he noted the great differences between the true positions of the stars and those calculated on the basis of Ptolemaic theory. He therefore decided to establish a new star catalog derived from actual observations, a project not realized since the time of Hipparchus. Wilhelm began making observations at his private observatory in Kassel and continued until 1567, when he became landgrave. Tycho Brahe, who was in Kassel for a few days in 1575, urged him to hire assistants to carry on the work. Accordingly, Wilhelm invited Christoph Rothmann to come to his court as mathematician and observer, and Joost Bürgi as mechanic.

Both these men did work of considerable scientific distinction while at Kassel. Bürgi constructed globe clocks, pendulum clocks, and mechanical computing devices. Later he became known as Kepler’s friend at the court of Rudolf II in Prague. Rothmann was an industrious observer and computer and a resolute supporter of the Copernican world view, as can be seen from his correspondence with Tycho Brahe. The accuracy of the observations made by Wilhelm and Rothmann is astonishing. Their determination of the latitude of Kassel (51°19’) required a correction of only 10” in the heyday of astronomical geography at the beginning of the nineteenth century.

Only a small part of Wilhelm’s project of the Hessian star catalog was realized. Ultimately it included 179 of the 1,032 stars originally planned. (Wilhelm’s observations furnished the data for 58 of these and Rothmann’s for 121.) Wilhelm nevertheless deserves great credit for undertaking a program that became one of the major tasks of observational astronomy in the following decades, and without which the later development of celestial mechanics would have been inconceivable. He also introduced a new method for determining stellar positions: with his azimuthal quadrant he could determine the moment when a given star reaches a certain altitude. This process, in a slightly altered form, later developed into the basic method for determining stellar positions. Its superiority became evident, however, only with the development of much more accurate clocks than were available to Wilhelm. It was for this reason that Tycho Brahe criticized it.

Many of the instruments and clocks made for Wilhelm’s Kassel observatory are preserved at the Astronomisch-Physikalische Kabinett in Kassel.


I. Original Works. Tycho Brahe published selections of his scientific correspondence with Wilhelm IV and Rothmann in Tychonis Brahe Dani epistolarum astronomicarum. . . (Uraniborg, 1596). Portions of Wilhelm’s political correspondence was printed in various other publications. A list of this material was prepared by W. Ribbeck for the article in Allgemeine deutsche Biographie, XLIII (Leipzig, 1898), 39. MS material can also be found under the heading “Landgräfliche Personalia Wilhelm IV Astronomica” in the state archive at Marburg.

II. Secondary Literature. See P. A. Kirchvogel, “Landgraf Wilhelm IV von Hessen und sein astronomisches Automatenwerk,” in Index zur Geschichte der Medizin, Naturwissenschaften und Technik,1 (1953), 12–18; “Wilhelm IV, Tycho Brahe and Eberhard Baldewein–the Missing Instruments of the Kassel Observatory,” in Vistas in Astronomy,9 (1968), 109–121, which contains an extensive bibliography; F. Krafft, “Tycho Brahe,” in Die Grossen der Weltgeschichte, V (Zurich, 1974), 297–345; B. Sticker. “Landgraf Wilhelm IV und die Anfänge der modernen astronomischen Messkunst,” in Sudhoffs Archiv,40 (1956), 15–25: and “Die wissenschaftlichen Bestrebungen des Landgrafen Wilhelm IV,” in Zeitschrift des Vereins für Hessische Geschichte und Landeskunde,67 (1956), 130–137; R. Wolf, Geschichte der Astronomie, (Munich, 1877), 266 ff.; and F. X. von Zach, “Landgraf Wilhelm IV,” in Monatliche Correspondenz zur Beförderung der Erdund Himmelskunde,12 (1805), 267–302.

Dieter B. Herrmann

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Wilhelm IV, Landgrave of Hesse

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