Eudoxus of Cnidus proposed the first mathematical model of the cosmos around 365 BC. How many nested rotating spheres did the original model use to explain all observed celestial motion?
Eudoxus's homocentric-sphere model was the first attempt to derive observed planetary motion from pure uniform circular motion. The pairs of coupled retrograde-loop spheres for each planet used the geometric figure-eight construction called the *hippopede* ('horse-fetter'). Aristotle later expanded the system to 55 spheres to account for the physical interaction between adjacent planetary systems. The model could not explain the observed apparent-brightness variations (Mars at opposition vs conjunction), and was eventually replaced by Hipparchian and Ptolemaic epicycles.
Read the full story →Eudoxus of Cnidus proposed in the 360s BC that the apparent motions of the planets could be explained by a system of 27 concentric rotating spheres centred on the Earth. Aristotle expanded the model to 55 spheres. The framework substantively defined European cosmology until Tycho Brahe.
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