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Rh other letters can, however, compare in importance with the lengthy essays exchanged between Hveen and Cassel, which give a most instructive picture of the revolution in practical astronomy effected by Tycho. The dedication to Landgrave Maurice alludes to the origin of Tycho's acquaintance with Landgrave Wilhelm, the renewal of it through Rantzov in 1585, praises the Landgrave for not having studied astronomy in books but in the heavens, and quotes from a funeral oration in which the hope had been expressed that the correspondence of the deceased with Tycho Brahe might be published, as it would show the world the merits of the Landgrave's scientific work. In the preface Tycho refers to the length of time necessary to form a complete series of observations by which the restoration of astronomy might be accomplished. Though the solar orbit may be sufficiently investigated in four years, the intricate lunar course requires the study of many years, while it takes twelve years to follow the oppositions of Mars and Jupiter round the zodiac, and even thirty years to see Saturn move round the heavens. He had commenced his own observations at the age of sixteen, though the results of the first ten years' work were less accurate than the later ones. Ptolemy and Copernicus had not observed for such a length of time, and consequently the numerical values of astronomical constants had not been well determined by them. As already remarked, most of the letters are in Latin, only those of the Landgrave and some of Tycho's replies to him being in German, with a liberal sprinkling of Latin words and sentences, which almost render unnecessary the Latin translation which always follows. As also mentioned