William IV, Landgrave of Hesse-Kassel

William was born in Kassel, the eldest son of Landgrave Philip the Magnanimous and Christine of Saxony. After his father's death in 1567, the Landgraviate of Hesse was divided between the four sons out of the late Landgrave of Hesse's first marriage, and William received the portion around the capital Kassel, the Landgraviate of Hesse-Kassel.

William took a leading part in safeguarding the Lutheran Reformation, and was indefatigable in his endeavours to unite the different sections of Protestantism against Catholicism.[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]

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 of Venus 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 of right 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 in Historia coelestis (Augsburg, 1666) by Albert Curtz, and a number of other observations are to be found in Coeli et siderum in eo errantium observationes Hassiacae (Leiden, 1618), edited by Willebrord Snell.[1]

R. Wolf, in his Astronomische 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]