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Copernicus: Platonist Astronomer-Philosopher

Cosmic Order, the Movement of the Earth, and the Scientific Revolution

Matjaz Vesel

In 1543, Copernicus publicly defended geokinetic and heliocentric universe. This book examines why and how he became a Copernican and what his affirmation of heliocentrism means in the context of the Scientific Revolution. Close reading of Copernicus’ texts and examination of his sociocultural context reveals his commitment to the Platonist program of True Astronomy, which is to discover the well-proportioned, harmonious universe, hidden beyond visible appearances, but accessible through mathematical reasoning. The principal goal of the work is to show that the hypothesis of Copernicus’ Platonism brings unity and internal coherence to his project and provides historical background of his contributions to the Scientific Revolution.
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Chapter II. Celestial Spheres and the Problem of the Equant

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← 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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