 From a philosophical point of view, Kepler considered the HM to be his main work and the one he most cherished. Containing his third planetary law, this work represents definitively a seminal contribution to the history of astronomy. But he did not reduce his long prepared project to an astronomical investigation—his first thoughts on the notion of “harmony” arose already in 1599, although he did not publish his work until 1619—but instead extensively discussed its mathematical foundations and its philosophical implications, including astrology, natural philosophy and psychology. Thus, Kepler’s third planetary law appears in a context which goes far beyond astronomy and to a great extent takes up again the perspective of his youthful MC. When talking about “harmony” in Kepler, one needs to keep in mind that the background giving its full significance to this notion goes far beyond music theory. Of course, music is involved and plays a determining role – and along with it the corresponding mathematical concepts stemming from the Pythagorean tradition. However, in Kepler “harmony” represents the most general idea, the very core of the world as an ordered whole and, therefore, the ultimate goal to be pursued in his philosophical investigations. Social and political aspects are also included. It is surely not a minor detail that Kepler, on the threshold of the Thirty Years’ War, dedicated his HM to King James I of England “the man whom Kepler believed was best suited to apply its lessons to the most pressing harmony of all: the harmony of church and state” (Rothman 2017, p. 6).