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

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

3. 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. 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. 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. 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. 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. 8 Heliocentric Planets-each revolves eastward around sun; earth third; natural motion, no forces! Greater distance from sun, longer period of revolution

9. 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. 10 Heliocentric-Pluses Simpler explanation of retrograde motion Natural order of planets from sun from periods Relative distances from observations and geometry

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

12. 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)