Among the disciples and assistants who gathered round him here was Johann Kepler, whom we mentioned before. He was then twenty-eight years old, and lived to become an even greater astronomer than Tycho Brahe himself. He owed a great deal to the profound and extensive observations of his master on the subject of fixed stars, and with the aid of all the careful information which Tycho had gathered together and bequeathed to his favorite pupil on his death, he made important discoveries with regard to the movements of the planets, and elaborated a much more advanced idea of the universe. Curiously enough, Tycho Brahe, with all his astonishing industry, never completely accepted the system of Copernicus. His idea was that the earth was the center of the universe and the sun formed the center for the orbits of the planets, but the sun itself, together with the planets, moved round the earth.
By his diligent observation of a thousand fixed stars, he gave to the world a catalogue of accurate positions of these bodies which took the place of the old catalogue of Ptolemy of Alexandria, who lived in the second century. This catalogue of observations held its own for more than a hundred years, until telescopes and clocks of precision came into use. It was the mighty impulse that Tycho Brahe gave to practical astronomy that caused that science to be taken up at universities, among which those of Copenhagen and Leyden were the first to found observatories.
