William IV of Hesse-Kassel (24 June 153225 August 1592), also called William the Wise, was the first Landgrave of the Landgraviate of Hesse-Kassel (or Hesse-Cassel). He was the founder of the oldest line, which survives to this day.
Life
Landgrave
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 among the four sons from 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. 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.
William was a pioneer in astronomical research, and perhaps owes his most lasting fame to his discoveries in this branch of study. Most of the mechanical contrivances which made instruments of Tycho Brahe so superior to those of his contemporaries were adopted in Kassel 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'. William also collected botanical books, including those by Dodanus, Lobelius and Clusius.