Labels:text | screenshot | font OCR: each planet inhabited its own sphere, which was concentric with the earth at the center of the world-the cosmos itself was spherical in shape. In his 2nd-century treatise Hypotheses on the Planets, Ptolemy proposed that the planets revolve in eccentric orbits within these concentric spheres, thus allowing for the possibility that each planet has its own center of gravity, surrounded by its own concentric spheres of water, air, and fire.
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