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By discovering the first two laws of planetary motion, Kepler definitively demolished the millenarian axiom regarding the circularity and uniformity of the heavenly motion. These laws became the starting-point for reconsidering the physical causes of planetary behaviour. Kepler supposed the existence of a force emanating from the Sun. This force ? not attractive like Newtonian gravitation, but pushing or pulling the planets ? would have operated only along the plane of the orbits, generating a sort of vortex. But such a vortex should have moved the planets on circular orbits, concentric to the Sun. Therefore, why are planetary orbits elliptical? To answer this question, Kepler assumed that planets were "enormous spherical magnets". The magnetic axis of each planet was always orientated in the same direction, with one pole attracted by the Sun, and the other repelled. In the half of the orbit where the attractive pole pointed towards the Sun, the planet approached to the latter until the perihelion (the point of minimum distance from the Sun). Thereafter, the repulsing pole pointed towards the Sun, and for the remainder of the orbit the planet was repelled until the aphelion (the point of maximum distance from the Sun).
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William Gilbert, De magnete magneticisque corporibus, Londini, excudebat Petrus Short, 1600
[IMSS: Med 1919/1]