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Kepler placed the Cube between the orbits of Saturn and Jupiter because the ratio of the spheres inscribed in and circumscribed around a cube closely matched the ratio of these two planets' distances from the sun.
The Tetrahedron (a four-sided pyramid) was placed between Jupiter and Mars.
The inner planets are separated by the remaining solids: the Octahedron (eight faces) between Mars and Earth, the Icosahedron (twenty faces) between Earth and Venus, and the Dodecahedron (twelve faces) between Venus and Mercury. While Kepler later realized orbits are elliptical rather than perfectly circular, this geometric model was a crucial step in his search for the mathematical laws of the heavens.