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.
Kepler's construction looked nice, and it fit the approximate orbits of the planets, but this wasn't good enough for the mathematical perfectionist in Kepler. He spent a large portion of the book comparing actual astronomical observations to his predictions and trying to work out the differences.
Kepler was convinced that his idea was correct, and believed that if the facts didn't fit his theory, then there must be something wrong with the facts. So he played around with the numbers, trying to find ways the observations could be reinterpreted. One of his ideas for manipulating the data was to take a closer look at the center of the planetary orbits. In Copernicus's system, as in Ptolemy's, the planets orbited an imaginary point in space. Kepler wondered: What would happen if he put the sun in the physical center of the universe? So he did.
Unfortunately for Kepler, the shift didn't help fit his perfect solids system to the observations. But Kepler had accomplished something far more important. He had moved the sun to the center of the universe, replacing an imaginary point with a physical object, and making a gigantic leap toward achieving an accurate physical depiction of the universe. For all the wrong reasons, Kepler had done exactly the right thing, and it was something no one had thought to do for thousands of years.
Once it had occurred to Kepler to place the sun in the center, he realized that this was the only position that made sense. The sun was the most powerful and important object in the universe, he argued, so it followed that the sun should be physically responsible for the motion of the planets. Astronomers had long known that the farther away the planets were from the center of the universe, the slower they moved around their orbits – but no one had ever stopped to wonder why. Once again, Kepler asked the question that no one else thought to ask, and concluded that a solar force must be responsible for the movement. He imagined a force emanating from the sun that pushed the planets around in their orbits, and grew weaker with increased distance.
Kepler got it wrong. The force he imagined at this point is nothing like the gravity that actually guides the planets around in their orbits. But his theorizing is revolutionary nonetheless, because he was for the first time offering a physical explanation for celestial motion. With one small shift of the sun, Kepler made physical sense of the universe. He rejoined astronomy and physics, fields that had been separate since the fall of ancient Greece.