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PHY121 Ch 4 Notes

# PHY121 Ch 4 Notes - Astronomy Chapter 4 The Origin of...

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Astronomy Chapter 4: The Origin of Modern Astronomy Ancient philosophers accepted as a first principle that the heavens were perfect, so philosophers such as Plato argued that, because the sphere was the only perfect geometrical form and carried a point on its surface around in a circle, the heavens must move in uniform circular motion . Aristotle – major authority on Astronomy until the late middle ages: he asserted that the universe could be divided into the imperfect changeable Earth and the divine heavens. Thus the universe had the following properties: Geocentric Universe : Earth should be at the center of the universe. Earth is imperfect and therefore and the lowest position. Heavenly bodies: they are close to the divine and so are expected to be perfect. The motions of heavenly bodies on the celestial sphere should follow ideal geometric paths (i.e. circles centered on Earth) The lack of any parallax in the positions of the stars gave astronomers confidence that Earth could not move. Really there was a parallax; it was just too far away to see. About AD 140, Ptolemy gave mathematical form to Aristotle’s model in the Almagest . Ptolemy preserved the principles of geocentrism and uniform circular motion, but he added epicycles , deferents , and equants to better predict the motions of the planets. To account for retrograde motion , his epicycles had to be quite large. Even so, his model was not very accurate in predicting the positions of the planets. Epicycles: the small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (the deferent) around the Earth Deferent: in the Ptolemaic theory, the large circle around Earth along which the center of the epicycle was thought to move Equant: in the Ptolemaic theory, the point off-center in the deferent from which the center of the epicycle appears to move uniformly Retrograde motion: the apparent backward (westward) motion of planets as seen against the background of stars. The problem of the place of Earth was solved when

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PHY121 Ch 4 Notes - Astronomy Chapter 4 The Origin of...

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