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This eBook from the Gutenberg Project consists of approximately 53 pages of information about Kepler.
and for the sun.  Of the seventy odd circles or epicycles required by the latest form of the Ptolemaic system, Copernicus succeeded in dispensing with rather more than half, but he still required thirty-four, which was the exact number assumed before the time of Aristotle.  His considerations were almost entirely mathematical, his only invasion into physics being in defence of the “moving earth” against the stock objection that if the earth moved, loose objects would fly off, and towers fall.  He did not break sufficiently away from the old tradition of uniform circular motion.  Ptolemy’s efforts at exactness were baulked, as we have seen, by the supposed necessity of all the orbit planes passing through the earth, and if Copernicus had simply transferred this responsibility to the sun he would have done better.  But he would not sacrifice the old fetish, and so, the orbit of the earth being clearly not circular with respect to the sun, he made all his planetary planes pass through the centre of the earth’s orbit, instead of through the sun, thus handicapping himself in the same way though not in the same degree as Ptolemy.  His thirty-four circles or epicycles comprised four for the earth, three for the moon, seven for Mercury (on account of his highly eccentric orbit) and five each for the other planets.

It is rather an exaggeration to call the present accepted system the Copernican system, as it is really due to Kepler, half a century after the death of Copernicus, but much credit is due to the latter for his successful attempt to provide a real alternative for the Ptolemaic system, instead of tinkering with it.  The old geocentric system once shaken, the way was gradually smoothed for the heliocentric system, which Copernicus, still hampered by tradition, did not quite reach.  He was hardly a practical astronomer in the observational sense.  His first recorded observation, of an occultation of Aldebaran, was made in 1497, and he is not known to have made as many as fifty astronomical observations, while, of the few he did make and use, at least one was more than half a degree in error, which would have been intolerable to such an observer as Hipparchus.  Copernicus in fact seems to have considered accurate observations unattainable with the instruments at hand.  He refused to give any opinion on the projected reform of the calendar, on the ground that the motions of the sun and moon were not known with sufficient accuracy.  It is possible that with better data he might have made much more progress.  He was in no hurry to publish anything, perhaps on account of possible opposition.  Certainly Luther, with his obstinate conviction of the verbal accuracy of the Scriptures, rejected as mere folly the idea of a moving earth, and Melanchthon thought such opinions should be prohibited, but Rheticus, a professor at the Protestant University of Wittenberg and an enthusiastic pupil of Copernicus, urged publication, and undertook to see the work through the press.  This, however, he was unable to complete and another Lutheran, Osiander, to whom he entrusted it, wrote a preface, with the apparent intention of disarming opposition, in which he stated that the principles laid down were only abstract hypotheses convenient for purposes of calculation.  This unauthorised interpolation may have had its share in postponing the prohibition of the book by the Church of Rome.

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