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"...Kepler adopted the original five Ptolemy had espoused, in additional to eight more. The aspect known as conjunction occurred when one planet obscured another along the circumference (0°), while that of opposition occurred when two planets were situated opposite one another (180°). The next aspect was that of quartile, produced when two planets marked off 90° along the arc of the circle. The final two aspects were known as trine and sextile, respectively formed by the displacement of two planets by 120° and 60°.
Compelling evidence in the form of extensive weather observations had led Kepler to consider the eight additional aspects ranked in order of descending influence: Octile (45°), trioctile (135°), semisextile (30°), quincunx (150°), quintile (72°), tridecile (108°), biquintile(144°) and decile (36°). Each of these angular displacements corresponded to the measurement of a side of a constructible polygon inscribed in a circle. The potential influence of each configuration was a consequence of its geometrical properties as deduced by Kepler..."
Boner, P. J., Soul-searching with Kepler: an analysis of anima in his astrology, Journal for the History of Astronomy, 36(1), 7-20, 2005. Available: You are not allowed to view links.
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