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Rh journey. Leaving his family at home until he had decided where he would finally settle down, he went first to Cassel to make the acquaintance of the distinguished astronomer, Landgrave Wilhelm IV. of Hesse. Wilhelm was born in 1532, and was the son of Landgrave Philip the Magnanimous, one of the most determined champions of the Reformation, who, after the disastrous battle of Mühlberg (1547), had surrendered to the Emperor, and had been kept a close prisoner for five years, during which anxious time his dominions had been governed by Wilhelm. When Philip became free in 1552, Wilhelm gladly turned back to the learned occupations, to which he had already for some years been devoted. By accident he came across the curious work of Peter Apianus, Astronomicum Cæsareum, in which the orbits of the planets are represented by movable circles of cardboard, and he became so much interested in the subject, that he had circles of copper made for the same purpose. Having afterwards studied Purbach's planetary theory and the other principal works of the time, he became, like Tycho, convinced of the necessity of making systematic observations, as he found considerable errors in the existing star catalogues. In 1561 he built a tower on the Zwehrer Thor at Cassel, of which the top could be turned round to any part of the sky, and here he observed regularly up to 1567, when the death of his father and his own consequent accession to the government of his dominions gave him less leisure for scientific occupations. As yet he had not any astronomer