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Hevelius's book on comets. Title page of Cometographia (1668), by the Polish astronomer Johannes Hevelius. Shown are three astronomers representing three theories about comets. From left, the theories are: Aristotle's sublunar theory (that comets orbit between the Moon and the Earth); Hevelius's theory that comets originate from Jupiter or Saturn and follow parabolic paths around the Sun; and Kepler's theory that comets move in a straight line. On the roof of the building (right), telescopes and sextants are being used to observe a comet (top left).
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