Kepler conceives in Graz, where he teaches mathematics, his first astronomical treatise: the first open, passionate defense of Copernican heliocentrism as physical reality (not merely a mathematical tool). His central theory — the planets nested within the five Platonic solids — turned out to be wrong, but the book was decisive along another path: Kepler asked not only how the planets move but why, proposing a magnetic-like physical force (anima motrix) emanating from the Sun. This turn toward celestial physics, and the need for better data to test his model, led him to seek out Tycho Brahe, whose data he would later use to discover that the orbits are elliptical. Published in Tübingen in 1596, he sends courtesy copies to several European astronomers, including Galileo.