Models of the Solar System

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 gt
  closest to earth
• Suns sphere moves eastward in a year relative
  to the stars gt farther from earth

5
Geocentric-Planets

• Each planets 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 earths 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)