 In the third part of AN, chapters 22–40, Kepler deals with the path of the Earth and intends to offer a physical account of the Copernican theory. By so doing he includes the idea that a certain notion of power should be made responsible for the regulation of the differences in velocities of the planets, which in turn have to be established in relation to the planets’ distances. Now, the Copernican planetary theory departs from the general principle that the Earth moves regularly on an eccentric circle. For Kepler, on the contrary, the planets are moved irregularly, and the slower they are moved, the greater their distance is from the center of power, the Sun. Addressing the physical aspects of his new astronomy, he deals in chapters 32–40, perhaps the most idiosyncratic of the work, with his notion of motive power. Here, he combines different approaches and sources, sometimes producing—for the purpose of simplifying the whole geometrical construction of geometrical astronomy by introducing a power causing motions—a new confusion at the dynamical level. To begin with, it is not always absolutely clear what kind of power Kepler has in mind. He inclines, above all, to the idea of a magnetic power residing in the Sun, but he also mentions light and, at least indirectly, gravity (which he does not bring into operation in the central chapters of the AN but which is to a certain extent implied in his explanations using the model of the balance and which he surely accepts as true for the Sun-Moon system, as he explains in the general introduction). Secondly, it is not always clear what this power is and how it acts, especially when he is speaking merely analogically, “as if” (particularly in the case of light). Essentially, Kepler breaks down the motions of the planets into two components. On the one hand, the planets move around the Sun—at this state of the discussion—circularly. On the other hand, they exhibit a libration on the Sun-planet vector. The rotation of the Sun is responsible for the motion of the planets. Irradiating from the rotating Sun is a power which spreads at the ecliptic plain. This power diminishes with distance to the source of the power, that is, to the Sun. A decisive work for Kepler’s development in his physical astronomy is William Gilbert’s (1544–1603) De magnete (London, 1600), a work which also intends to offer a new physics for the new Copernican cosmology and which surely influenced Kepler’s thoughts about this power. One of the main problems was, of course, how to apply the general principles of magnetism to planetary motion, first to explain the difference in velocity on a circular path, and later to give an account of the motion on an ellipse. Kepler conceives of a model with parallel magnetic fibers which links the Sun with the planets in such a way that the rotation of the Sun causes the motion of the planets around it. The fibers are born in the planets parallel and perpendicular to the lines of apsides by a kind of “animal power”. The planets themselves are polarized, that is, with one pole they are attracted to the Sun, with the other pole they are pushed away from it. This explains very well the direction of planetary motion: the planets all move in one direction because the Sun rotates in that direction. Nevertheless, a further problem still seems to remain unresolved: according to Kepler’s explication, the planets should move around the Sun as fast as the Sun itself rotates, which is not the case. This phenomenon can be explained by referring to a property of matter, which for Kepler has an axiomatic character: the inclinatio ad quietem, that is, the tendency to rest (see especially AN, chap. 39; KGW 3, p. 256). As a consequence, the planets are moved around the Sun slower than they would be if the power of the Sun were at work alone.