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160 of this comet cannot therefore compare in accuracy with his later ones, but still they were immeasurably superior to those made by other observers, and they demonstrated most decisively that the comet had no perceptible parallax, and was consequently very far above the "elementary sphere" to which the Aristotelean philosophy had consigned all comets as mere atmospherical phenomena. By showing that the star of 1572 was situated among the stars, Tycho had already dealt the Aristoteleans a heavy blow, as it was now clear that new bodies could appear in the æthereal regions. But still that star was not a comet, and Tycho, who had formerly believed in the atmospherical origin of comets, now took the opportunity of testing this matter, and found that the comet had no appreciable daily parallax. Though he was not the only observer who placed the comet beyond the moon, his observations were known by his contemporaries to be of very superior accuracy, and his authority was so great that this question was decided once for all.

Before proceeding to pass in review the book which Tycho prepared on this comet, we shall shortly allude to the other comets observed at Hveen. On the 10th October 1580 Tycho found a comet in the constellation Pisces. It was observed at Hveen till the 25th November, and again after the perihelium passage on the morning of the 13th December. The observations are more numerous and better than those of the previous comet, and time determinations with a quadrant were made nearly every night, while there are very few quadrant observations of the comet. Moestlin had seen it already on the 2nd October, and both he and Hagecius observed it assiduously, but their