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chapter 5

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Natural Science
NATS 1745
Robin Metcalfe

LECTURE 5:ANCIENT GREEKASTRONOMY January 21, 2014 NATS 1745: History ofAstronomy Astronomy in Ancient Greece • The fragmented geography and decentralized rule ofAncient Greece allowed for an intellectual freedom that led to a revolution in scientific thought. • The popularity of Plato andAristotle led to nearly 2000 years of widespread belief in the geocentric model, even though the heliocentric model was also taught in their time. • Thales (c. 624-546 BC): o Considered the “Father of Science” for attempting to find explanations for natural phenomena that didn’t involve the gods. LECTURE 5:ANCIENT GREEKASTRONOMY January 21, 2014 o His philosophy gained support when he demonstrated that some acts of nature (i.e. eclipses) are predictable. • Anaximander (c. 610-546 BC): o Set the Earth “afloat” in air. o Developed the 1 moving model of the Universe, in which the motion of the celestial bodies is explained by placing them on spinning wheels around Earth. • Pythagoras (c. 570-495 BC): o According to legend, when he discovered that musical pitch is determined by the length of the instrument, he realized: the Universe is a cosmos (a harmonious system that obeys knowable laws). o He taught that all celestial motion is perfectly circular, and that Earth is a sphere. • Evidence that Earth is a sphere: o Ships gradually disappear on the horizon bottom-first. o Earth’s shadow on the eclipsed moon is always round. LECTURE 5:ANCIENT GREEKASTRONOMY January 21, 2014 o When you travel North or South, the constellations rise and set more rapidly than they would is Earth was flat. Animation: Motion of stars due to a change in latitude • Philolaus (c. 470-385 BC): Set the Earth in motion; by allowing Earth to rotate once per day around a “central fire”, the daily motion of the celestial bodies was explained. • Herakleides (c. 375-310 BC): o With no reports from travellers of the “central fire”, he removed it, and set Earth spinning daily around its own axis. o He placed Mercury and Venus in orbit around the Sun to explain the Sun-centred appearance of their motion and their brightness changes. LECTURE 5:ANCIENT GREEKASTRONOMY January 21, 2014 • Aristarchus (c. 310-230 BC): o Used Earth’s shadow on the eclipsed moon to measure the Moon’s size relative to Earth’s. o 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. o His measurements weren’t accurate, but he correctly deduced: the Sun is much larger than Earth and Earth is larger than the Moon. LECTURE 5:ANCIENT GREEKASTRONOMY January 21, 2014 o 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). • Features of apparent planetary motion (i.e. the motion of planets we see across the sky): o Planets display both direct (forward) motion (W to E) and retrograde (backward) motion (E to W). o Aplanet’s apparent speed is not constant (planets appear to slow down and speed up across the sky). LECTURE 5:ANCIENT GREEKASTRONOMY January 21, 2014 o Aplanet appears to brighten and dim as it moves across the sky. • Plato (c. 428-348 BC): o Began an intellectual tradition in which theories of nature were accepted on faith as opposed to proven by observation. o Taught that all celestial bodies are perfect spheres with constant circular motion. (CCM) o The goal of the philosophers after Plato was to explain how the planets’paths in the sky can be explained by CCM. • Eudoxus (c. 410-355 BC):Attempted to explain planetary motion by placing the planets on systems of invisible nested spheres (“crystal orbs”), each spinning with CCM around Earth. LECTURE 5:ANCIENT GREEKASTRONOMY January 21, 2014 Animation: Eudoxus' explanation for retrograde motion • Aristotle (384-322 BC): o Unlike Plato, he taught that theories must match observation, but his neglect of quantitative analysis led h
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