Text: http://www.cco.caltech.edu/~winter/unilab/module1650/kepler/3d.html Kepler's first book was entitled Mysterium Cosmographicum, or "The Secret of the Universe." While teaching astronomy and mathematics in Graz, Kepler remained receptive to questions that would usually be ranked as philosophical. In particular, he wanted to know why there were six planets, why they were spaced as they were, and why they moved as they did. These were common questions for a natural philosopher to ask, but an astronomer was not usually reckoned responsible for these kinds of inquiries. Yet Kepler attempted not only to use Copernicus's heliocentric system, but to justify it as physically real. The theory based on the perfect solids, and published in the Mysterium, came to him, he claimed, in something approaching a revelation. Kepler believed that his model using the five Platonic solids was perfect because it answered two of his three questions so elegantly. He proposed that the six planets moved in spaces defined by the spheres fitted around the five solids. These solids were the simplest, and therefore the most desirable, three-dimensional figures, and Kepler reasoned that God had based the solar system on them. This would account for that fact that there were exactly six planets, and also for the spaces between their orbs. The two images above are his own portrayals of this model, that on the right being a close-up view of the central planets. Kepler spent considerable time and money trying to get a real device made to these specifications, in hopes of presenting it to a courtly patron. But he never succeeded. Even so, and although he modified his theory slightly when he wrote the Harmonices Mundi, he continued to believe in its fundamental principles until he died.

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