WILLIAM IV. (1532-1592), landgrave of Hesse, well known as an astronomer, son of Philip the Magnanimous, was born at Cassel on 14th June 1532. During his father's captivity after the battle of INIfildberg (1547) lie carried on the government in his name for five years, and succeeded him on his death in 1567. At an early age lie became interested in astronomy ; and in 1561 he built an observatory at Cassel, where observations were regularly made, first by himself, afterwards by Rothmann and Biirgi. The last-named was not only a very skilful mechanic (it seems probable that he applied the pendulum to clocks long before Huygens did) but an original mathematician, who independently invented logarithms. William died on 25th August 1592.
Most of the mechanical contrivances which made Tycho Bribe's instruments so superior to those of his contemporaries were adopted at Cassel about 1584, and from that time the observations made there seem to have been about as accurate as Tycho's ; but the resulting longitudes were 6' too great in consequence of the adopted solar parallax of 3'. The principal fruit of the observations was a catalogue of about a thousand stars, the places of which 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 he referred to the fundamental star. It should be noticed that clocks (on which Tycho Brahe depended very little) were used at Cassel for finding the difference of right ascension between Venus and the sun before sunset ; Tycho preferred observing the angular distance between the sun and Venns when the latter was visible in the day time. The Hessian star catalogue was published in Lucius Barettus's Historia Ca?lcstis (Augsburg, 1668), and a number of other observations are to be found in Ccoli et ideru >n, in. co Errantima Observationes Hassiacx, Leyden, 1618, edited by SNELLIUS (q.v.). R. Wolf, in his "Astronomische Mittlicilungen," No. 45 (Vierteljahrssehrift d. naturfo•sehcoden Gesellschaft Zurich, 1878), has given a résumé of the manuscripts still preserved at Cassel, which throw much light on the methods adopted in the observations and reductions.