lecture notes

# Age of aquarius when the sun on the spring equinox

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year. (“Age of Aquarius”: when the sun on the spring Equinox reaches the Aquarius constellation). 2. All star coordinates shift .014 degree East per year. (By 2200, Polaris will no longer mark the NCP; aka “precession of the Pole star”) * Animation: Precession of the Pole Star 8. Eratosthenes (~276-195 BC) - Estimated Earth’s circumference by measuring the length of the shadow cast by a stick.

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9. Apollonius (~262-190 BC) Devised the epicycle model of constant, circular planetary motion, which explains: - Retrograde motion - Changing angular speeds (planet’s tangential component can vary) - Changing brightness (planet’s distance can vary) This model was in use for nearly 300 years.
Animation: The epicycle model 1. Claudius Ptolemy (~85-161 AD) - Author of Mathematical Treatise (~150 AD), a.k.a The Almagest (“The Great Treatise”), the definitive astronomy textbook for 1400 years. - It presents the lost works of the ancient Greeks, a star catalogue, and the Ptolemaic model of planetary motion. 2. The Ptolemaic Model Ptolemy modified the epicycle model to improve its accuracy: - He displaced Earth from the center. - He added the equant point, which forces the epicycle to have non-constant motion. Animation… 3. Ptolemy further modified his model to explain the different paths of the inner vs outer

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planets Applet: Ptolemaic motion of an inner planet Applet: Ptolemaic motion of an outer planet 4. Ptolemy’s order of the planets a. Moon is closest (since it’s never eclipsed by a planet). b. 5 visible planets, from fastest to slowest (sun between Venus& Mars to separate inner vs outer types of motion). c. Sun is on the center , not the earth d. The fixed stars.
5. By assuming his order of the planets was correct with no space between adjacent epicycles, Ptolemy estimated the size of the Universe to be ~20,000 Earth radii (actual: 260,000). Despite the error, this proved the enormous distance to the stars Mercury’s min distance = Moon’s max distance Chapter 4 1. Astronomy in the Middle Ages (~5 th -15 th C AD) In the Dark Ages (5 th -7 th C), knowledge of Astronomy declined as the church became more involved in State affairs: - Study of the sky not emphasized in the Bible. - Rejection of Ancient Greek “pagan” ideas. - Loss of Ancient Greek texts (Latin became the official language of scholarship) 2. Between the 9-12 th C, Islamic scholars continued from where the Ancient Greeks left off: - The Qur’an encourages knowledge of the sky for time-keeping - Interactions with Greek refugee scholars led to translations of the ancient Greek texts into Arabic (e.g. Almagest). 3. - New star catalogues were created in which the stars were renamed with Arabic descriptions - When the Arabic catalogues later made their way into Europe, the Arabic star names persisted (still used today).

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Buridan dismantled the Aristotelian argument that an arrow’s motion proves that Earth is fixed. His theory of impetus (momentum) shows: - A Fixed Earth and a moving Earth re equivalent from our perspective on Earth.
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• Fall '11
• RobinMetcalfe

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