"The next subject... dealt with the relation between the distances of the planets and their times of revolution round the sun. It was obvious that the period was not simply proportional to the distance, as the outer planets were all too slow for this, and he concluded “either that the moving intelligences of the planets are weakest in those that are farthest from the sun, or that there is one moving intelligence in the sun, the common centre, forcing them all round, but those most violently which are nearest, and that it languishes in some sort and grows weaker at the most distant, because of the remoteness and the attenuation of the virtue”. This is not so near a guess at the theory of gravitation as might be supposed, for Kepler imagined that a repulsive force was necessary to account for the planets being sometimes further from the sun, and so laid aside the idea of a constant attractive force."
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Kepler
Kepler is a short history of events leading to the astronomical discoveries of Johannes Kepler. It was written by Walter William Bryant, a staff member of the Royal Observatory, Greenwich. The book was published in 1920 by the Society for Promoting Christian Knowledge, London, and by The MacMillan Company, New York, within a series of books called "Pioneers of Progress."
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