The comet of 1570 was carefully observed by Tycho, and gave rise to a new theory of these bodies. He found the horizontal parallax to be 20', so that the comet was nearly three times as far off as the moon. He considered comets, therefore, as bodies placed far beyond the reach of our atmosphere, and moving round the sun. This was a severe blow to the physics of Aristotle, which regarded comets as meteors generated in the atmosphere. His observation. of the new star in 157e, was no less hostile to the argument of the same philosopher, which maintained, that the heavens are a region in which there is neither gas. nerzeion nor corruption, and in which existence has neither a beginning nor an end.
Yet Tycho„ with this knowledge of astronomy, and after having made observations more numerous and. accurate than all the astronomers who went before him, continued to reject the system of Copernicus, and to deny the motion of the earth. He was, however, convinced that the earth is not the centre about which the planets revolve, for he had him= self observed Mars, when in opposition, to be nearer to the earth than the earth was to the sun, so that, if the planets were ranged as in the Ptolemaic system, the orbit of Mars must have been within the orbit of the sun. He therefore imagined the system still known by his name, according to which the sun moves round the earth, and is at the same time 3 the centre of the planetary motions, It cannot be denied, that the phenomena purely astronomical may be accounted for on this hypothesis, and that the objections to it are ra ther derived from physical and mechanical considerations than from the appearances them selves. It is simpler than the Ptolemaic system, and free from its inconsistencies ; but it is more complex than the Copernican, and, in no respect, affords a better explanation of the phenomena. The true place of the Tychonic system is between the two •former ; an advance beyond the one, and a step short of the other ; and such, if the progress of discovery were always perfectly regular, is the place which it would have clarified in the history of the science. If Tycho had lived before Copernicus, his system would have been a step in the advancement of knowledge ; coming after him, it was a step backward.
It is not to hie credit as a philosopher to have made this retrograde moves neat, yet he is not altogether without apology. The physical augments in favour of the Copernican. sys tem, founded en the incongruity of supposing the greater body to move.round the smaller,
might not be supposed to have much weight, in an age when the equality' of action. and reaction was unknown, and when it was not clearly understood that the sun and the planets act at all on one another. The arguments, which seem, in the judgmeat of Tycho, to have balanced the simplicity of the Copernican system; were founded an certain texts of Scripture, and on the difficulty of reconciling the motion of the earth with the sensations which we experience at its surface, or the phenomena which we observe, the same, in all as if the earth were at rest. The experiments and reasonings of Galileo had not yet inetructedd men in the inertia of matter, or in the composition of motion ; and the follow ers of Copernicus reasoned on principles which they held in common with their adverse ries. A bail, it was said by the latter, dvopt from the mast-head of a ship under sail, does not fall at the foot of the meet, but somewhat behind it ; and, in the name manner, a stone dropt from a high tower would not fall, on the supposition of the earth's motion, at the bottom of the tower, but to the west of it, the earth, during its fall, having gone eastward from under it. The followers of Copernicus were not yet provided with the. true answer to this objection, viz, that the ball does actually fall at the bottom of the mast. It was admitted that it must fall behind it, because the ball was no part of the ship, and that the motion forward was not natural, either to the ship or to the ball. The stone, on the other hand, let fall from the top of the tower, wee a part of the earth ; and, therefore, the diur nal and annual revolutions which were natural to the earth, were also natural to the stone ; the stone would, therefore, retain the same motion with the tower, and strike. the ground precisely at the bottom of it.
It must be confessed, that neither of these logicians had yet thoroughly awakened from the dreams of the Aristotelian metaphysics, but men were now in possession of the truth, which was finally to break the spell, and set the mind free from the fetters of prejudice and authority. Another charge, against which it is more difficult to defend. Tycho, is his belief in the predictions of astrology. He even wrote a treatise in defence of this imagi nary art,, and regulated his conduct continually by its precepts. Credulity, so unworthy of a man ,deeply versed in real science, is certainly to be set down less to hia (ma account than to that of the age in which he lived,