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Ptolemy was an astronomer and mathematician. He believed that the Earth was the center of the Universe. The word for earth in Greek is geo, so we call this idea a "geocentric" theory
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Halley is famous in his own right for analyzing the orbits of comets, which he showed to be elliptical and periodic. Halley's Comet returned, as he predicted, in 1758, 15 years after his death. (The comet last appeared in 1986, and next will appear in 2061.)
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Galileo di Vincenzo Bonaiuti de' Galilei was an Italian astronomer, physicist and engineer, sometimes described as a polymath, from Pisa. Galileo has been called the "father of observational astronomy", the "father of modern physics", the "father of the scientific method", and the "father of modern science"
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