Johannes had been teaching at a seminary in Graz when he was struck by what seemed to him a divine revelation about the structure of the universe. Under the still-prevailing Ptolemaic system of astronomy, the Earth was understood to be immobile in the centre of the universe, surrounded by the fixed stars and seven planets (the sun and the moon were also considered to be planets). Johannes, however, was inclined to put his faith in the revolutionary cosmology proposed by Copernicus in 1543, which placed the sun in the centre, demoting Earth to planet status and the moon to a satellite of Earth. At that time—telescopes had not yet been invented—only the five naked-eye planets were known to exist: Mercury, Venus, Mars, Jupiter and Saturn. Adding Earth...
...to their company brought the total to six. But why, Johannes wondered, had God made only six planets? And why had He arranged their orbits in the particular proportions that He had? There must be an answer: God would not make an unintelligible Universe.
The revelation, which occurred on July 19, 1595, in the middle of teaching a class on planetary alignment, involved the curious geometric properties of Platonic solids. These are three-dimensional polygons, every face of which is identical: A cube is a Platonic solid. So is a tetrahedron (a pyramid with three faces and one base which are all equilateral triangles). It had been understood since Antiquity—and much discussed by Plato, hence the name—that there can only be five such polygons: The laws of geometry forbid any others.
Within a year, Johannes had published this theory in a book, Mysterium Cosmographicum, which he sent to every astronomer and mathematician he could think of, including Galileo and Tycho. The numbers did not exactly agree with the theory, but there was enough of a promising match to give him hope that if he got hold of the best observational data, everything would click into place.
But within a year, Tycho was dead, and Johannes had inherited the role of Imperial Mathematician, finally gaining complete access to all of Tycho’s books. The astronomical data within was so precise, and Johannes so respectful of its precision, that it enabled him to write four monumental treatises based on this data and in the process discover the three foundational laws of modern astronomy: the elliptical nature of planetary orbits; the precise variance of planetary speed; and the exponential relationship between planetary speed and distance from the sun. Three laws upon which Newton built his theory of universal gravitation, laws known to posterity by Johannes’s surname: Kepler. When Newton spoke about standing on the shoulders of giants, it was Johannes Kepler, along with Tycho, Galileo, and Descartes, he had in mind.
