Theories of Planetary Motion

1. Theories of Planetary Motion Part 1: Geocentric Theory

2. Why should they study Astronomy One main reason… Survival • Prepare for climate changes • Know when to plant crops • Know when to harvest crops

3. Geocentric Model Plato: 400 BC • Created idea of a “Celestial Sphere” • Sun, Moon, & Planets on separate spheres (26 total) • Earth is center of all spheres (center of universe)

4. Geocentric Model Plato (cont.): (Geocentric Acceptance) • Earth wasn’t part of the heavens • Earth wasn’t bright • Little change is the sky night after night • Our senses say we aren’t moving (no wind, objects not left behind, nothing flies off)

5. Geocentric Model Aristotle: 300 BC • Supported Geocentric Model • Added 29 more “Celestial Spheres”

6. Geocentric Model Aristarchus: 300 BC • Student of Plato – proposed idea of the Sun as center of universe. • Not accepted because: • It violated common sense • Went against religious beliefs • He couldn’t explain lack of parallax (shift / movement of an object due to a position change)

7. Geocentric Model Ptolemy: 85 – 165 AD • Provided theories (mathematically based) to support Geo. Theory • Most importantly, he explained retrograde motion -retrograde motion = the apparent backward motion of a planet in the sky.

8. Geocentric Model Ptolemy (cont.) • 3 new theories to explain retrograde motion • Epicycle= rotational path of planet while circling around earth • Equant= center of deferent (not earth, earth slightly to the side), earth is eccentric (off center) • Deferent= circular path around earth

9. Ptolemy’s Theory

10. Ptolemy’s Theory (cont.) • Retrograde Motion explained by Ptolemy: Retrograde Motion Animation