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This eBook from the Gutenberg Project consists of approximately 53 pages of information about Kepler.
wrote, “What I prophesied two and twenty years ago, as soon as I discovered the five solids among the heavenly orbits,—­what I firmly believed long before I had seen Ptolemy’s ’Harmonics’—­what I had promised my friends in the title of this book, which I named before I was sure of my discovery,—­what sixteen years ago I urged as a thing to be sought,—­that for which I joined Tycho Brahe, for which I settled in Prague, for which I have devoted the best part of my life to astronomical computations, at length I have brought to light, and have recognised its truth beyond my most sanguine expectations.  Great as is the absolute nature of Harmonics, with all its details as set forth in my third book, it is all found among the celestial motions, not indeed in the manner which I imagined (that is not the least part of my delight), but in another very different, and yet most perfect and excellent.  It is now eighteen months since I got the first glimpse of light, three months since the dawn, very few days since the unveiled sun, most admirable to gaze on, burst out upon me.  Nothing holds me; I will indulge in my sacred fury; I will triumph over mankind by the honest confession that I have stolen the golden vases of the Egyptians to build up a tabernacle for my God far away from the confines of Egypt.  If you forgive me, I rejoice, if you are angry, I can bear it; the die is cast, the book is written; to be read either now or by posterity, I care not which; it may well wait a century for a reader, as God has waited six thousand years for an observer.”  He gives the date 15th May, 1618, for the completion of his discovery.  In his “Epitome of the Copernican Astronomy,” he gives his own idea as to the reason for this Third Law.  “Four causes concur for lengthening the periodic time.  First, the length of the path; secondly, the weight or quantity of matter to be carried; thirdly, the degree of strength of the moving virtue; fourthly, the bulk or space into which is spread out the matter to be moved.  The orbital paths of the planets are in the simple ratio of the distances; the weights or quantities of matter in different planets are in the subduplicate ratio of the same distances, as has been already proved; so that with every increase of distance a planet has more matter and therefore is moved more slowly, and accumulates more time in its revolution, requiring already, as it did, more time by reason of the length of the way.  The third and fourth causes compensate each other in a comparison of different planets; the simple and subduplicate proportion compound the sesquiplicate proportion, which therefore is the ratio of the periodic times.”  The only part of this “explanation” that is true is that the paths are in the simple ratio of the distances, the “proof” so confidently claimed being of the circular kind commonly known as “begging the question”.  It was reserved for Newton to establish the Laws of Motion, to find the law of force that would constrain a planet to obey Kepler’s first and second Laws, and to prove that it must therefore also obey the third.

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