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Rh of the tail never passed exactly through the sun, but seemed to pass much nearer to the planet Venus; he adds, that though the statement of Apianus was only approximately true, the opinion of Aristotle was far more erroneous, for according to him, the tails, as lighter than the head, should be turned straight away from the centre of the earth. The curvature he considers merely an illusion, caused by the head and the end of the tail being at different distances from the earth.

The eighth chapter is the most important in the whole book, as the consideration of the comet's orbit in space leads Tycho to explain his ideas about the construction of the universe. The "æthereal world," he says, is of wonderfully large extent; the greatest distance of the farthest planet, Saturn, is two hundred and thirty-five times as great as the semi-diameter of the "elementary world" as bordered by the orbit of the moon. The moon's distance he assumes equal to fifty-two times the semi-diameter of the earth, which latter he takes to be 860 German miles. The distance of the sun he believes to be about twenty times that of the moon. In this vast space the comet has moved, and it therefore becomes necessary to explain shortly the system of the world, which he had worked out "four years ago," i.e., in 1583. The Ptolemean system was too complicated, and the new one which that great man Copernicus had proposed, following in the footsteps of Aristarchus of Samos, though there was nothing in it contrary to mathematical principles, was in opposition to those of physics, as the heavy and sluggish earth is unfit to move,