This book explains Kepler's cosmological theory, based on the Copernican system, in which the five Platonic solids dictate the structure of the universe and reflect God's plan through geometry.

Kepler claimed to have had an epiphany on July 19, 1595, while teaching in Graz, demonstrating the periodic conjunction of Saturn and Jupiter in the zodiac: he realized that regular polygons bound one inscribed and one circumscribed circle at definite ratios, which, he reasoned, might be the geometrical basis of the universe.

By ordering the solids correctly—octahedron, icosahedron, dodecahedron, tetrahedron, and cube—Kepler found that the spheres correspond to the relative sizes of each planet's path around the Sun, generally varying from astronomical observations by less than 10%.

Much of Kepler's enthusiasm for the Copernican system stemmed from his theological convictions about the connection between the physical and the spiritual; the universe itself was an image of the Trinity, with the Sun corresponding to the Father, the stellar sphere to the Son, and the intervening space between to the Holy Spirit.

[6] With the support of his mentor Michael Maestlin, Kepler received permission from the Tübingen university senate to publish his manuscript, pending removal of the Bible exegesis and the addition of a simpler, more understandable description of the Copernican system (the Narratio prima by Rheticus) as an appendix.

His subsequent main astronomical works were in some sense only further developments of it, concerned with finding more precise inner and outer dimensions for the spheres by calculating the eccentricities of the planetary orbits within it.

Original to Kepler, however, and typical of his approach is the resoluteness with which he was convinced that the problem of equipollence of the astronomical hypotheses can be resolved and the consequent introduction of the concept of causality into astronomy—traditionally a mathematical science.

[10] Kepler corresponded with and provided courtesy book copies to a number of astronomers around the time of publication, including Galileo Galilei, Tycho Brahe, Reimarus Ursus, and Georg Limnaeus.