In 1597, Kepler published his first major work, Mysterium Cosmographicum, or the Cosmic Mystery. In this long and rambling book, Kepler lays out his entire philosophy of the structure of the universe. As his ideas relied on a heliocentric system, Kepler began by trying to convince the reader that Copernicus had been correct. The heliocentric theory was still a new, untested, and unpopular idea – in fact, Kepler's book was the first major work to support the Copernican system since Copernicus's death, fifty years before.
Although the Copernican system was not yet accepted by the scholarly authorities of the time, Kepler took few risks in propounding it. Copernican advocates were not persecuted – the worst they could expect was a bit of ridicule from their colleagues, which in itself was enough to scare off most potential supporters. Many people's attitude at the time was that the Copernican system was mathematically sound but physically implausible.
This attitude offered a convenient out for scholars who wanted to use the Copernican system but feared the wrath of the Church should they try claim that the earth was actually revolving around the sun. Such rationalizing may stem from the publication of Copernicus's own treatise, whose preface included the following caution not to blame the author for his suggestions: "For these hypotheses need not be true nor even probable; if they provide a calculus consistent with the observations, that alone is sufficient." It continues to assert that astronomy will never be able to offer physical truths about the universe – nor should this be expected of it. For decades after the publication of De Revolutionibus, this was the attitude most scholars took toward the heliocentric theory.
Kepler refused to go along with the crowd. As he set out to write the Mysterium, the head of the theological faculty at Tuebingen warned him to steer clear of discussing "whether these theories correspond to existing things or not." Kepler ignored the advice. For him, the whole point of astronomy was to fit theories to "existing things." Throughout his career, he insisted on finding physical explanations, and it was this determination that propelled him toward his greatest discoveries.
By the time he published the Mysterium, Kepler had refined his grand ideas about the layout of the universe, but they continue to rely on one fundamental assertion: that the orbits of the six planets could be fit around the five perfect solids. In Kepler's system, the orbit of Saturn circumscribes a cube. Inscribed in the cube is the orbit of Jupiter, which circumscribes a tetrahedron. Inscribed in the tetrahedron is the orbit of Mars, which circumscribes a dodecahedron. Inscribed in the dodecahedron is the orbit of Earth, which circumscribes an icosahedron. Inscribed in the icosahedron is the orbit of Venus, which circumscribes an octahedron. Finally, the orbit of Mercury is inscribed in the octahedron.
It was an intricately beautiful creation, and it worked.