Chapter 4

Chapter 4 - Chapter 4 Ancient astronomers invented...

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Chapter 4 Ancient astronomers invented geocentric models to explain planetary motions Geocentric model – model in which Earth is center of the universe direct motion – eastward progress of planets retrograde motion – when planets appear to stop & back up. It is westward movement Apollonius of Perga and Hipparchus and created model later expanded upon by Ptolemy Ptolemaic system – the definitive version of the geocentric cosmogony of ancient Greece Epicycle – a moving circle in the Ptolemaic system about which a planet revolves deferent – a stationary circle in the Ptolemaic system along which another circle (an epicycle) moves, carrying a planet, the Sun, or the Moon In a Ptolemaic system, each planet is assumed to move in a small circle (epicycle) whose center in turn moves in a larger circle (deferent) which is centered approximately on the Earth. Both rotate in the same direction. When planet is on part of its epicycle nearest Earth, motion of planet is opposite to motion of the epicycle along the deferent. The planet appears to slow down and halt and then go backwards (westward) Famous model but criticized for treating each planet independently (wanted simple set of principles for all planets) Occam’s razor – the notion that a straightforward explanation of a phenomenon is more likely to be correct than a convoluted one Nicolaus Copernicus devised the first comprehensive heliocentric model Think of a car & a pedestrian (both moving same way but since you are so much faster he appears to move backwards) Heliocentric model – all the planets revolve around the sun Different planets take different lengths of time to complete an orbit, so from time to time one planet will overtake another Model also gave more precision to order of the planets (ones closer to sun had
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smaller orbits) inferior planets – a planet that is closer to the Sun than the Earth is superior planets – a planet that is more distant from the Sun than the Earth is Greatest eastern elongation -
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This note was uploaded on 09/29/2009 for the course ASTR 101 taught by Professor Christiansen during the Spring '08 term at UNC.

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Chapter 4 - Chapter 4 Ancient astronomers invented...

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