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Lecture 6

Lecture 6 – Astronomy in Ancient Greece (continued), Middle Ages.docx

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Natural Science
NATS 1745
Robin Metcalfe

Lecture 6 –Astronomy in Ancient Greece (continued), MiddleAges Thursday, January 23 , 2014 1. Astronomy inAncient Greece (continued) 2. MiddleAges 1. Astronomy inAncient Greece (continued) -9) Aristarchus (c. 310-230BC): - i. Used Earth’s shadow on the eclipsed moon to measure the Moon’s size relative to Earth’s - ii. Used the angle in the sky between the Sun and quarter moon to measure the Sun’s distance and size relative to the Moon’s - iii. His measurements weren’t accurate, but he correctly deduced the Sun is much larger than Earth, and the Earth is larger than the Moon  Though Sun + Moon appear same (i.e. angular size),Aristarchus’discovery revealed the Sun to be close to 19 times bigger - iv. This led him to propose a heliocentric model of the Universe, with only the Moon in orbit around Earth (owing to the Moon’s straight night-to-night path around the sky) -10) Features of apparent planetary motion (i.e. the motion of planets we see across the sky: - i. Planets display both direct (forward) motion (West to East) and retrograde (backward) motion (East to West) - ii.Aplanet’s apparent speed is not constant (planets appear to slow down and speed up across the sky) - iii. Aplanet appears to brighten and dim as it moves across the sky -11) Plato (c. 428-348BC): - i. Began an intellectual tradition in which theories of nature were accepted on faith as opposed to proven by observation - ii. Taught that all celestial bodies are perfect spheres with constant circular motion (CCM) - iii. The goal of philosophers after Plato was to explain how the planets paths in the sky can be explained by CCM - Plato and Aristotle’s preservation and promotion of Pythagorean ideas most likely result of their hesitation towards change (they lived in a time of political chaos) -12) Eudoxes (c. 410-385BC): - Attempted to explain planetary motion by placing the planets on systems of invisible nested spheres (“crystal orbs”) each spinning with CCM around Earth -13) Aristotle (384-322BC): - Unlike Plato, he taught that theories must match observation, but his neglect of quantitative analysis led him to the wrong conclusions -14) Aristotelian model: - i. Earth is fixed at the centre of the Universe (geocentric) - ii. The Universe is divided into the terrestrial realm (Earth) and the celestial realm (the heavens), where different laws of nature apply - iii. All matter in the terrestrial realm is composed of four elements: Earth, water, air, and fire - iv.All bodies in the celestial realm are perfect spheres and composed of a pure and imperishable substances called “quintessence” (5 element) or ether (“pure air”) - v. Planetary motion
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