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Models of the Solar System

Centered on the earth: geocentricCentered on the sun: heliocentric

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Geocentric Assumptions•Earth at center (aesthetics, physics)

•Earth motionless (physics)

•Uniform motion on circles for celestial objects (aesthetics, geometry)

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Geocentric

•Sky-entire heavens (all “spheres”) moves westward daily relative to horizon

•Made of a special material (the “fifth essence)

•Division of earth/sky by matter and motions

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Geocentric

•Moon- “sphere” moves eastward relative to the stars in a month; fastest angular speed => closest to earth

•Sun’s “sphere” moves eastward in a year relative to the stars => farther from earth

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Geocentric-Planets

•Each planet’s “sphere” moves eastward relative to stars

•Order based on decreasing average angular speed: Mercury, Venus, Sun, Mars, Jupiter, Saturn

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Geocentric-Retrograde

•Planet moves on small circle centered on the perimeter of larger one; both “spheres” move eastward

•Small circle period = interval between retrogrades

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Heliocentric•Sky-daily rotation of earth eastward (west to east)

•Moon- eastward revolution of moon about earth monthly

•Sun-reflection of earth’s yearly eastward revolution around sun

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Heliocentric

•Planets-each revolves eastward around sun; earth third; natural motion, no forces!

•Greater distance from sun, longer period of revolution

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Heliocentric

•Planetary retrograde-illusion caused by one planet passing another (relative motion)

•Closer a planet is to the sun, the faster it revolves once

•Earth retrogrades seen from other planets

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Heliocentric-Pluses

•Simpler explanation of retrograde motion

•Natural order of planets from sun from periods

•Relative distances from observations and geometry

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Geocentric vs. Heliocentric •Angular relations the same

•Predictions about the same

•Complexity about the same

•Explanations comparable•Require different physics of motions

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Geocentric vs. Heliocentric•Very different place for

the earth (center of cosmos vs. another planet)

•Crucial observation: heliocentric stellar parallax (stars very far away; telescopes needed)