Cosmic Order, the Movement of the Earth, and the Scientific Revolution
Chapter II. Celestial Spheres and the Problem of the Equant
← 50 | 51 → CHAPTER TWO
CELESTIAL SPHERES AND THE PROBLEM OF THE EQUANT
Let us once again recapitulate Copernicus’ presentation and criticism of astronomy in his Commentariolus and Preface to De revolutionibus.
Our predecessors, says Copernicus in the Commentariolus, assumed a large number of celestial orbs for the purpose of explaining the apparent motion of the planets by the principle of uniformity. It was absurd to them that perfectly spherical heavenly bodies would move any way but uniformly. Working from within this principle, however, they discovered that by connecting and combining uniform motions in various ways they could make any celestial body appear to move to any position. Eudoxus and Callippus met this requirement of astronomical science with the theory of concentric orbs, but their model failed to account for the varying distances of the planets from the earth. In Copernicus’ reconstruction – and in historical reality – this particular problem with the concentric model eventually leads to the Ptolemaic astronomy of eccentrics and epicycles. The theories of Ptolemy and many other astronomers readily predicted the apparent motions of the planets. But they also presented no small difficulty because, in order to provide more accurate predictions of celestial motions, Ptolemy was compelled to introduce the equant, a device which violated the principle of uniform circular motion. Hence, Copernicus concludes that Ptolemy’s “notion (speculatio) seemed ← 51 | 52 → neither absolute enough nor sufficiently in accordance with reason (non satis absoluta neque rationi satis concinna).”42
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