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18 Though Tycho was during the greater part of the time he spent at Leipzig obliged to study astronomy in secret, he did not long content himself with the use of the Ephemerides of Stadius, but procured the Alphonsine tables and the Prutenic tables. We have already mentioned that the former were founded on the Ptolemean planetary system and the observations of the Arabs, as well as those made under the direction of Alphonso X. of Castile in the thirteenth century; while the latter, which got their name from being dedicated to Duke Albrecht of Prussia, were the work of Erasmus Reinhold, a disciple and follower of Copernicus. Tycho soon mastered the use of these tables, and perceived that the computed places of the planets differed from their actual places in the sky (even though he only inferred the latter from the relative positions of the planets and adjacent stars), the errors of the old Alphonsine tables being much more considerable than those of their new rivals. He even found out that Stadius had not computed his places correctly from Reinhold's tables. And already at that time, while Tycho was a youth only sixteen years of age, his eyes were opened to the great fact, which seems to us so simple to grasp, but which had escaped the attention of all European astronomers before him, that only through a steadily pursued course of observations would it be possible to obtain a better insight into the motions of the planets, and decide which system of the world was the true one. An astronomical phenomenon which took place in August 1563, a conjunction of Saturn and Jupiter, which in those days was looked on as a very important one, owing to the astrological significance it was supposed to have, induced him to begin at once to record his observations, even though they were taken with the crudest implements only. A pair of ordinary