Earth centered universe
- If you stop pushing -- you stop moving
- Didn’t realize the force of friction was at work
- without friction, you continue moving
- demo with skateboard
- Galileo and Newton (Chap. 6) figured this out
Concluded the earth was still, and the heavens rotate
- Place Earth at center of universe, fixed
- place stars on celestial sphere, rotate 1/day
- place sun, planets on other spheres, rotating
- reproduces basic motions seen in sky
Problem: retrograde motion
- Planets usually move West -> East on cel. sphere
- At opposition, they move backward f. 4-11
- which planets will show retrograde motion?