A35 Chapter 4 - Chapter 4 1 Ancient astronomers invented geocentric models to explain planetary motions Greeks and Chinese believe everything orbits

A35 Chapter 4 - Chapter 4 1 Ancient astronomers invented...

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Chapter 4 1. Ancient astronomers invented geocentric models to explain planetary motions • Greeks and Chinese believe everything orbits around Earth, sun, moon, stars as part of celestial sphere that rotates around the Earth • Knew about five planets, move slowly eastward relative to the stars (like sun and moon) called direct motion • Occasionally planets stop and appear to move back westward called retrograde motion • Ptolemaic system: each planet moves in small circle called epicycle whose center is part of larger circle called deferent which is centered on the Earth, both circles rotate in same direction, explained different motions of planets • Fairly accurate to predict paths and positions, became accepted system 2. Nicolaus Copernicus devised the first comprehensive heliocentric model • Sun-centered model explains retrograde motion of planets as viewing the universe from a moving Earth • Planets closer to sun in sky are closer than Earth to sun: inferior planets versus superior planets that appear high in the horizon • Greatest eastern elongation: angle between sun and planet viewed from Earth furthest east, position in the sky as far east from sun as possible so appears in western sky as morning star • Greatest western elongation: opposite, inferior conjunction: when Mars or Venus is between us and sun, moving from the evening sky into the morning sky, superior conjunction: on the opposite side of the sun moving to evening sky • Copernicus: discovered correspondence between time to complete one orbit (period) and size of the orbit • Synodic period: time elapses between two successive identical configurations as seen from Earth (i.e. two conjunctions), sidereal period: true orbital period around the sun relative to the stars
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  • Spring '09
  • RobertKirshner
  • Kepler's laws of planetary motion

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