Kepler's first major astronomical work, Mysterium Cosmographicum (The Cosmographic Mystery), is the first published defense of the Copernican system.

As he indicates in the title, Kepler thinks he has revealed God’s geometrical plan for the universe.

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

His first manuscript of Mysterium contains an extensive chapter reconciling heliocentrism with biblical passages that seem to him to support geocentrism.

With the support of his mentor Maestlin, Kepler had 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 as well as Kepler’s new ideas.

Mysterium was published late in 1596, and Kepler receives his copies and begins sending them to prominent astronomers and patrons early in 1597; it is not widely read, but it establishes Kepler’s reputation as a highly skilled astronomer.

The effusive dedication, to powerful patrons as well as to the men who control his position in Graz, also provides a crucial doorway into the patronage system.

Though the details will be modified in light of his later work, Kepler will never relinquish the Platonist polyhedral-spherist cosmology of Mysterium Cosmographicum.

His subsequent main astronomical works are 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.