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