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Models of the Solar System

Models of the Solar System. Centered on the earth: geocentric Centered on the sun: heliocentric. Geocentric Assumptions. Earth at center (aesthetics, physics) Earth motionless (physics) Uniform motion on circles for celestial objects (aesthetics, geometry). Geocentric.

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Models of the Solar System

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  1. Models of the Solar System Centered on the earth: geocentric Centered on the sun: heliocentric

  2. Geocentric Assumptions • Earth at center (aesthetics, physics) • Earth motionless (physics) • Uniform motion on circles for celestial objects (aesthetics, geometry)

  3. 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

  4. 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

  5. Geocentric-Planets • Each planet’s “sphere” moves eastward relative to stars • Order based on decreasing average angular speed: Mercury, Venus, Sun, Mars, Jupiter, Saturn

  6. Geocentric-Retrograde • Planet moves on small circle centered on the perimeter of larger one; both “spheres” move eastward • Small circle period = interval between retrogrades

  7. 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

  8. Heliocentric • Planets-each revolves eastward around sun; earth third; natural motion, no forces! • Greater distance from sun, longer period of revolution

  9. 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

  10. Heliocentric-Pluses • Simpler explanation of retrograde motion • Natural order of planets from sun from periods • Relative distances from observations and geometry

  11. Geocentric vs. Heliocentric • Angular relations the same • Predictions about the same • Complexity about the same • Explanations comparable • Require different physics of motions

  12. 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)

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