William IV
Portrait by Kaspar van der Borcht, 1577
Landgrave of Hesse-Kassel
Reign	31 March 1569 – 25 August 1592
Successor	Maurice
Born	(1532-06-24)24 June 1532 Kassel,Landgraviate of Hesse,Holy Roman Empire
Died	25 August 1592(1592-08-25) (aged 60) Kassel,Landgraviate of Hesse-Kassel, Holy Roman Empire
Spouse	Sabine of Württemberg ​ ​ (m. 1566; died 1581)​
Issue More...	Anna Maria, Countess of Nassau-Weilburg Hedwig, Countess of Schaumburg Princess Sofie Maurice, Landgrave of Hesse-Kassel Christine, Duchess of Saxe-Eisenach
House	Hesse(by birth) Hesse-Kassel(founder)
Father	Philip I, Landgrave of Hesse
Mother	Christine of Saxony

William IV of Hesse-Kassel (24 June 1532 – 25 August 1592), also calledWilliam the Wise, was the firstLandgrave of theLandgraviate of Hesse-Kassel (or Hesse-Cassel). He was the founder of the oldest line, which survives to this day.

William was born inKassel, the eldest son of LandgravePhilip the Magnanimous andChristine of Saxony. After his father's death in 1567, theLandgraviate of Hesse was divided among the four sons from the late Landgrave of Hesse's first marriage, and William received the portion around the capitalKassel, theLandgraviate of Hesse-Kassel.

William took a leading part in safeguarding theLutheranReformation, and was indefatigable in his endeavours to unite the different sections ofProtestantism againstCatholicism.^[1] However, he was reluctant to use military force in this conflict.

As an administrator he displayed rare energy, issuing numerous ordinances, appointing expert officials, and in particular ordering his slender finances. By a law of primogeniture he secured his Landgraviate's land against such testamentary divisions as had diminished his father's estate.^[1]

William is most notable for hispatronage of the arts and sciences. As a youth he had cultivated close connections with scholars and as a ruler he kept up this connection. His interest in astronomy may have been inspired byPetrus Apianus'sAstronomicum Caesareum.^[2]

William was a pioneer inastronomical research, and perhaps owes his most lasting fame to his discoveries in this branch of study. Most of the mechanical contrivances which made instruments ofTycho Brahe so superior to those of his contemporaries were adopted inKassel about 1584. From then on, the observations made in Hesse-Kassel seem to have been about as accurate as those of Tycho. However, the resulting longitudes were 6' too great in consequence of the adopted solar parallax of 3'.^[1]

The principal product of the astronomical observations was the Hessian star catalogue, a catalogue of about a thousand stars. The locations were determined by the methods usually employed in the 16th century, connecting a fundamental star by means ofVenus with the sun, and thus finding its longitude and latitude, while other stars could at any time be referred to the fundamental star. It should be noticed that clocks, on which Tycho depended very little, were used at Kassel for finding the difference ofright ascension between Venus and the sun before sunset. Tycho preferred observing the angular distance between the sun and Venus when the latter was visible in the daytime.^[1]

The Hessian star catalogue was published inHistoria coelestis (Augsburg, 1666) byAlbert Curtz, and a number of other observations are to be found inCoeli et siderum in eo errantium observationes Hassiacae (Leiden, 1618), edited byWillebrord Snell.^[1]

R. Wolf, in hisAstronomische Mittheilungen, No. 45 (Vierteljahrsschrift der naturforschenden Gesellschaft in Zurich, 1878), has given a resume of the manuscripts still preserved at Kassel, which throw much light on the methods adopted in the observations and reductions.^[1]

William was married toSabine of Württemberg, daughter ofChristoph, Duke of Württemberg.
They had the following children:

1. Anna Maria of Hesse-Kassel (27 January 1567 – 21 November 1626), married on 8 June 1589 toLouis II, Count of Nassau-Weilburg
2. Hedwig of Hesse-Kassel (30 June 1569 – 7 July 1644), married on 11 September 1597 toErnst of Schaumburg
3. Agnes of Hesse-Kassel (30 June 1569 – 5 September 1569)
4. Sofie of Hesse-Kassel (10 June 1571 – 18 January 1616)
5. Maurice, Landgrave of Hesse-Kassel (25 May 1572 – 15 March 1632), succeeded as landgrave on William IV's death in 1592.
6. Sabine of Hesse-Kassel (12 May 1573 – 29 November 1573)
7. Sidonie of Hesse-Kassel (29 June 1574 – 4 April 1575)
8. Christian of Hesse-Kassel (14 October 1575 – 9 November 1578)
9. Elisabeth of Hesse-Kassel (11 May 1577 – 25 November 1578)
10. Christine (19 October 1578 – 19 August 1658), married on 14 May 1598 toJohn Ernst, Duke of Saxe-Eisenach
11. Juliane, born and died 9 February 1581

In addition William had a few illegitimate children. Most significant and favored among these was Philipp von Cornberg (1553–1616), William's son by Elisabeth Wallenstein. Philipp was ennobled by his father and became the ancestor of the current Barons von Cornberg.

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• Biographical data and references at The Galileo Project

• Landgraves of Hesse-Kassel
• 16th-century German astronomers
• Scientists from Kassel
• People from the Landgraviate of Hesse-Kassel
• 1532 births
• 1592 deaths
• Landgraves of Hesse
• German Lutherans

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