2. Solar System Models

3. Geocentric Model Earth Centered Moon, Sun, Planets, and Stars revolve around the Earth Feels right No observed parallax of stars Later adopted as we are the center of the universe and everything is perfectly made for us.

4. Aristotle Model

5. Greeks knew that the Earth was round Ships sink as they sail away Shape of Earth’s shadow on the Moon during a lunar eclipse

6. Eratosthenes (200 BC) Used position of the sun from two locations to determine the circumference of the Earth α = 1/50 of a circle Figure it out

7. Problems with Geocentric Model Wanders: – Planets vary brightness and speed as they move along the ecliptic Apparent Retrograde Motion – backward motion of planets at opposition – Video Video

8. Ptolemy Proposal Epicycles – small cycle on orbit – allow more accurate predictions demo

9. Ptolemy Model Earth in the center – Moon, sun, planets and stars revolve around us – The stars are far away, therefore they must be going real fast

10. Heliocentric Model Sun-centered System

11. Copernicus Model Was able to use simple math to determine relative distances to planets. Not perfect but simpler; he added some small epicycles. Motion of sun and planets are caused by movement of the Earth Space is really big, stars are very far away

12. Tycho Very careful observations with calibrated sighting tube, no telescope Proposed variation – Earth-centered/Sun- centered system

13. Kepler The orbits of planets are ellipses with the sun at one focus Equal area in equal time – They go faster when they are closer to the sun The period 2 is proportional to the distance 3

14. Galileo Used small telescope to look at planets Evidence that things were not as they thought – Moon has features – Jupiter has moons – Saturn has appendages – Sun has spots – Milky Way is lots of stars – Venus has phases and changes size

15. Evidence for the Geocentric Model Feels right – Do you feel like the Earth is spinning? – Do you feel like we are traveling through space as we revolve around the sun?

16. What about Apparent Retrograde Motion? Do the planets back up? Video

17. Evidence for the Geocentric Model No parallax was observed – If we are traveling around the sun then we should be able to see it.

18. Evidence against the Perfect Universe model The moon was not a perfect object, it had craters, highlands and seas.

19. Evidence against the Heliocentric model Jupiter has moons, therefore not everything revolves around the earth.

20. Evidence against the Perfect Universe model Saturn had moons or arms on the side that sometimes disappeared.

21. Evidence against the Perfect Universe model Sun has spots – It was not perfect – It rotated

22. Evidence against the Perfect Universe model The Milky Way is really a lot of stars

23. Evidence for the Heliocentric model Venus has phases and it changes size.

24. Which is correct?

25. The End