The planetary increase of brightness during retrograde motion: An explanandum constructed ad explanantem |
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Authors: | Christián Carlos Carman |
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Affiliation: | 1. Department of Life Sciences, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel;2. National Institute for Biotechnology in the Negev, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel;1. Department of Molecular Physiology and Biological Physics and Department of Biomedical Engineering, University of Virginia, Charlottesville, VA 22908, USA;2. Science for Life Laboratory, Department of Cell and Molecular Biology, Uppsala University, Uppsala, Sweden |
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Abstract: | In Ancient Greek two models were proposed for explaining the planetary motion: the homocentric spheres of Eudoxus and the Epicycle and Deferent System. At least in a qualitative way, both models could explain the retrograde motion, the most challenging phenomenon to be explained using circular motions. Nevertheless, there is another explanandum: during retrograde motion the planets increase their brightness. It is natural to interpret a change of brightness, i.e., of apparent size, as a change in distance. Now, while according to the Eudoxian model the planet is always equidistant from the earth, according to the epicycle and deferent system, the planet changes its distance from the earth, approaching to it during retrograde motion, just as observed. So, it is usually affirmed that the main reason for the rejection of Eudoxus' homocentric spheres in favor of the epicycle and deferent system was that the first cannot explain the manifest planetary increase of brightness during retrograde motion, while the second can. In this paper I will show that this historical hypothesis is not as firmly founded as it is usually believed to be. |
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Keywords: | Epicycle Deferent Homocentric spheres Brightness change |
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