The cosmological model of concentric or homocentric spheres, developed by Eudoxus, Callippus, and Aristotle, employed celestial spheres all of which had the same center, the Earth. In this respect it differed from the epicyclic and eccentric models with multiple centers, which were used by Ptolemy and other mathematical astronomers until the time of Copernicus.
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“Perhaps a modern society can remain stable only by eliminating adolescence, by giving its young, from the age of ten, the skills, responsibilities, and rewards of grownups, and opportunities for action in all spheres of life. Adolescence should be a time of useful action, while book learning and scholarship should be a preoccupation of adults.”
—Eric Hoffer (1902–1983)
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