Wilhelm IV (Hessen-Kassel)

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Wilhelm IV of Hessen-Kassel

Wilhelm IV of Hessen-Kassel , called the Wise , (* June 24, 1532 in Kassel ; † August 25, 1592 ibid) from the House of Hesse was the first Landgrave of Hesse to Kassel and founder of the Hesse-Kassel line from 1567 to 1592 .

Wilhelm was a patron of the natural sciences and a competent and respected astronomer himself . He had the first observatory in Central Europe set up in Kassel and equipped it with the most modern instruments. He also sponsored the astronomer Tycho Brahe . From 1579 Jost Bürgi was one of Wilhelm's employees . Wilhelm also built up a collection of valuable clocks and scientific instruments of all kinds.


Hereditary Prince Years

Wilhelm was the eldest son of Landgrave Philip I of Hesse from his marriage to Christine (1505–1549), daughter of Duke Georg of Saxony . At the age of eight, his upbringing was overseen by his father, who had him carefully trained. The focus was on the Latin theological field. Learning the Greek language and turning to the sciences came later.

After the outbreak of the Schmalkaldic War , Wilhelm was sent to Strasbourg , lived there in the house of Johann Winter von Andernach and learned French. In 1547 his father brought him back to Kassel and handed over government affairs to him and his mother, only to be taken prisoner by the emperor . Wilhelm and his mother were supported during this reign by a Regency Council, which consisted of Heinrich Lersner , Rudolf Schenk zu Schweinsberg , Wilhelm von Schachten and Simon Bing . Wilhelm opposed the Augsburg Interim and took part in a campaign against the Kaiser at the head of the Hessian troops. After the Treaty of Passau , Wilhelm's father returned from captivity, but Wilhelm continued to participate in the government and was represented by Wilhelm in the election of Archduke Maximilian in Frankfurt in 1562.

During negotiations with Duke Christoph von Württemberg , Wilhelm met his daughter Sabine (1549–1581), whom he married on February 11, 1566 in Marburg .

After Wilhelm vehemently protested against the elevation of his half-siblings, whom he called the "Ishmaelites", from the marriage between his father and Margarethe von der Saale, to the Imperial Count of Nidda , the father changed his previous will, in which Wilhelm de facto became the sole ruler of the landgraviate Hessen was determined on April 6, 1562 and divided the country under Wilhelm and his brothers. Wilhelm received Niederhessen , with the royal seat of Kassel , most of the former county of Ziegenhain and the Hessian part of the Schmalkalden rule .

Landgrave of Hessen-Kassel

The brothers confirmed the stipulations of the will and came to a settlement on May 28, 1568 in the Ziegenhain agreement that regulated the division of the country. Wilhelm became the founder of the Kassel line of the Hessian princely house. His three younger brothers founded the Hessen-Marburg , Hessen-Rheinfels and Hessen-Darmstadt lines .

This division meant that Hesse lost its supremacy in the battle for the Reformation , which passed to the Orange and the Electors of the Palatinate. However, Wilhelm was able to successfully trade between the Electors Friedrich III. mediate of the Palatinate and August of Saxony and thus promote Protestantism. In 1552 he was a signatory to the Treaty of Chambord . However, the union of Lutherans and Calvinists in the empire, which he strived for throughout his life , remained an unfulfilled wish of the landgrave.

Wilhelm increased his rule after the death of nobleman Dietrich IV von Plesse (1571) and Count Otto VIII von Hoya († 1582), Friedrich II. Von Diepholz († 1585) and Georg Ernst von Henneberg († 1583). From the possession of the latter he received in particular the Henneberg part of the Schmalkalden rule with the city of Schmalkalden , which he made his secondary residence and where he had the Wilhelmsburg Castle built after him . After the death of his brother Philipp von Hessen-Rheinfels, he also received most of the former County of Katzenelnbogen . In the Merlau Treaty of September 8, 1583, he reached an agreement with the Archbishop of Mainz, Wolfgang von Dalberg, on long-simmering border conflicts between Kurmainz and the Landgraviate, with almost all of the remaining Mainz properties in North Hesse finally falling to the Landgraviate, while Hessen-Kassel made his claims in Eichsfeld gave up.


Wilhelm IV had the following children from his marriage:

Before his marriage, Wilhelm had children born out of wedlock with Elisabeth Wallenstein:

  • Christine (1552? -); ⚭ 1570 Nikolaus von Gaugreben
  • Philipp Wilhelm (1553-1616); ⚭ 1. 1582 Anna Christine von Falcken († 1602); ⚭ 2. 1603 Christine von Boineburg (1582–1632)
  • Wilhelm († 1564)

Astronomical activity

Scientific training

Wilhelm's interest in astronomy seems to have awakened Peter Apian's “Astronomicum Caesareum” in particular, which still worked in the Ptolemaic system and was introduced to him by his teacher Rumold Mercator , the son of the cartographer Gerhard Mercator . Wilhelm also studied the current textbooks by Peuerbach and Regiomontan . The mathematician and astronomer Andreas Schöner (1528–1590), who has been working in Kassel since 1558 , calculated the planetary movements for Wilhelm for the years 1560 to 1600 on a Ptolemaic basis. The method of representation proposed by Apian using paper disks may be seen as the basis for Wilhelm's later metal circular disk systems. One example is the automatic mechanism (the "Wilhelmsuhr") completed around 1561, which made it possible to read off the geocentric longitudes and latitudes of Mercury, Venus, Mars, Jupiter, Saturn and the moon on several dials.

The first modern observatory in Europe

Around 1560, Wilhelm had two three-story extensions built at the south-west and south-east ends of the Kassel Palace. From the terraces one had an unobstructed view of the Fulda valley. This is how the first permanently established modern observatory in Europe came into being (for comparison: Paris 1667, Greenwich 1675), if Bernhard Walther's renovation of his private house in Nuremberg is not to be regarded as such. The Kassel Castle burned down in 1811 and was completely destroyed.

Astronomical work

The Landgrave commented on pressing problems of his time, such as the calendar reform. Both for this project and to substantiate the Copernican theory , however, many new and exact measurements were required. There were many deviations of the star locations from the Ptolemaic predictions. Like few others before him, Wilhelm set about making new measurements in order to compile a corrected list of stars. The Protestant Wilhelm thus proved to be a child of his Reformation century. Naturalists across Europe began to review ancient teachings and observe nature directly.

Until he took office in 1567, Wilhelm took an active part in the measurements for the star catalog. From 1584 the mathematician and astronomer Christoph Rothmann (around 1555 to around 1600) continued this work. The catalog, based on Ptolemy, ultimately comprised 1025 stars, was not completed until after Wilhelm's death. For this purpose, a total of 383 stars have been redefined over the years. Wilhelm actively supported the Gregorian calendar reform in October 1582. His dual role as a researcher on the Princely Chair seems to have brought him the role of moderator between astronomers and secular rulers.

In 1575 the 14 years younger Dane Tycho Brahe visited the Landgrave at his castle in Kassel for ten days to exchange ideas. Wilhelm already had 20 years of experience as an astronomer. Among other things, the visit is thanks to Wilhelm's recommendation to the Danish king for the establishment of Tychos observatory on the island of Ven . In the following years Tycho tried to increase the accuracy with ever larger instruments (for example quadrants up to 2.50 meters radius), while Wilhelm had his mechanics build precision instruments of small dimensions out of metal. Eberhard Baldewein (approx. 1525–1593) worked for him from 1568 , and from 1579 the Swiss watchmaker Jost Bürgi , who later worked for Johannes Kepler in Prague. Wilhelm maintained a lively correspondence with Kepler.

Paul Wittich was also one of his employees .

The Kassel instruments

The accuracy of almost all instruments in the Kassel observatory was enormous. The geographical position of the Kassel Castle, for example, could be determined with an inaccuracy of only ten arc seconds. The most famous pieces come from Jost Bürgi. He was self-taught and constructed excellent planetary and globe clockworks , later he was also known for his logarithm tables. Bürgi built pendulum clocks with a second hand, which was very unusual for that time. It was these clocks that enabled Wilhelm to abandon the sky surveying method that had been practiced since ancient times.

In 1585 Bürgi began work on a celestial globe, which was only completed after the landgrave's death. The celestial globe offers a representation of the starry sky, with the earth in the center, so the star positions on the surface are mirrored. The observation horizon, meridian and ecliptic are clearly visible. The movement of the mechanical globe is done by the mechanics housed inside the sphere, the globe rotates once on a sidereal day . On the horizon ring there is an annual calendar, by setting the Easter date once, even moving holidays were correctly displayed.

New method of sky surveying

With the method for determining the star positions, unchanged since antiquity, the star catalog of Ptolemy Almagest had already been created and Tycho Brahe also worked in this way. The method is based on the measurement of the mutual angular distances from star to star with angle measuring instruments such as armillary spheres or sextants or quadrants . This is based on the known coordinate of a fixed star .

Wilhelm, however, worked with a predecessor of today's theodolite . With an azimuthal quadrant and a height circle, he could measure the height of a star in a certain azimuth . These variables change every minute, but with the help of a precise clock, the position of the star in equatorial coordinates , i.e. in right ascension and declination , can be calculated from altitude, azimuth and local sidereal time . In contrast, in the traditional system, positions were determined by measuring several relative angles with the aid of spherical trigonometry , i.e. by calculating spherical triangles. Which of the two methods is more accurate depends on the accuracy of the devices used and this was Tycho's criticism. Because although Bürgi's watches were first-rate chronometers, their inaccuracy was still high compared to the long-established angle measurement methods. Wilhelm's new method of sky surveying was not taken up again until a hundred years later by John Flamsteed .


The moon crater Wilhelm is named after him.


  • Walther Ribbeck:  Wilhelm IV, Landgrave of Hesse . In: Allgemeine Deutsche Biographie (ADB). Volume 43, Duncker & Humblot, Leipzig 1898, pp. 32-39.
  • F. Albrecht: The observatory of the Landgrave of Hesse Wilhelm IV in Kassel . In: The universe. Illustrated magazine for astronomy and related fields 2 (1902), pp. 229-237, 251-254.
  • Jürgen Hamel : The astronomical research in Kassel under Wilhelm IV. With a partial edition of the German translation of the main work of Copernicus around 1586 (Acta Historica Astronomiae; 2). Thun: German 1998.
  • Paul A. Kirchvogel: Tycho Brahe as an astronomical friend of Landgrave Wilhelm IV of Hessen-Kassel . In: Sudhoff's archive. Volume 61, 1977, pp. 165-172.
  • Paul A. Kirchvogel: Wilhelm IV, Tycho Brahe, and Eberhard Baldewein - the Missing Instruments of the Kassel Observatory . In: Vistas in Astronomy 9 (1968), pp. 109-121.
  • John H. Leopold: Astronomers, Stars, Devices. Landgrave Wilhelm IV and his self-moving globes . Lucerne: Fremersdorf, 1986.
  • Ludolf von Mackensen: The first observatory in Europe with its instruments and clocks. 400 years of Jost Bürgi in Kassel. Exhibition catalog ed. v. State Museum in Kassel. Callwey, Munich 1979.
  • Sabine Salloch: The Hessian medical system under Landgrave Wilhelm IV and Moritz the scholar. Role and work of the princely personal physicians. Dissertation Marburg 2006.
  • Senta Schulz: Wilhelm IV Landgrave of Hessen-Kassel (1532–1592). Dissertation Leipzig 1941.
  • Bernhard Sticker: Landgrave Wilhelm IV and the beginnings of modern astronomical measuring art. In: Sudhoff's archive for the history of medicine and the natural sciences. Volume 40, 1956, pp. 15-25.

Individual evidence

  1. Christoph Rothmanns Handbuch der Astronomie from 1589 , Christoph Rothmann, Miguel A. Granada, Jürgen Hamel, Ludolf von Mackensen , Verlag Harri Deutsch 2003.

Web links

predecessor Office successor
Philip I. Landgrave of Hessen-Kassel