he German mathematician and astronomer Johannes Kepler (1571
1630) was an avowed Platonist, and set out early in his
professional career to demonstrate that the motion of the
planets was circular, in accordance with the established
Aristotelian doctrine, and that they could be described in terms
of the Platonic solids. However, he was also a friend and
assistant to the great Danish astronomer Tycho Brahe, who was
making precise and detailed observations of the planets and
stars. When Tycho Brahe died, in 1601, Kepler inherited this
enormous mountain of raw data. After studying this data for 20
years, Kepler came to understand that his earlier assumptions
about planetary motion had been naive, and that if an earth-
centered (Ptolemaic) understanding of the universe were
abandoned for a sun-centered (Copernican) model, then the motion
of the planets was clearly elliptical. From this basis, Kepler
generated his three famous laws of planetary motion: These laws
are illustrated in the following diagram: - The orbit