1. Origins of Modern Astronomy? OR
Earth-Centered (Geocentric) vs.
Sun-Centered (Heliocentric)

2. Why a Geocentric Universe?
The Earth does not feel like it is moving.
All objects in the sky appear to revolve around us.
It’s comforting to think we are so important.

3. A Strong Point in Favor of the Geocentric Model… there is no observed parallax.
June
Dec.

4. The Path of Mars across the night sky…
A Major Problem for the Geocentric Model… Retrograde Motion of the Planets
Mars Retrograde

5. This was explained by Ptolemy, using epicycles, orbits on top of orbits:
Animation

6. Copernicus: The Sun-Centered Universe
The Polish astronomer, Copernicus, believed that Ptolemy’s model was too complicated, so he proposed the heliocentric model of the universe….

7. By having planets, including the Earth, revolving around the Sun, the epicycles are eliminated.

8. In the heliocentric model, retrograde motion is explained by the Earth passing a slower planet.
Animation

9. Jupiter’s Retrograde Motion
May 2008
August 2007
Sept
Nov. 2007
Nov. 2008
Sept. 2008
It takes Jupiter 12 years to revolve around the sun.
Every Earth-year it goes through one Zodiac constellation.
When the Earth passes it, Jupiter “falls backward,” and then resumes its pro-grade motion.
June 2007
February 2008
July 2007
Oct. 2007
Dec. 2007

10. Jupiter
Each year Jupiter moves slowly to a new constellation in the zodiac.
During this time is goes through a retrograde stage when the Earth passes it.
2006
2007
2008
2009

11. Enter Galileo with the first physical evidence of a heliocentric universe!
THE TELESCOPE

12. The telescope greatly improved our ability to observe the universe

13. There are too many stars to be “pasted on” one sphere.

14. Galileo discovered that Jupiter had moons of its own - a mini-Solar System, independent of the Earth

15. More significantly, he discovered that Venus goes through phases like the Moon.

16. This is difficult to explain with an earth-centered universe.
Sketch made by Galileo:
The crescent Venus is much larger, since it is closer to Earth

17. Tycho’s Compromise
Danish astronomer Tycho Brahe was a superb observer of the heavens.
He didn’t observe any parallax, so he kept the Earth in the center.
Other planets revolved around the Sun, as it revolved around us.

18. Mathematical Evidence: Johannes Kepler

19. Kepler studied the records of planetary positions and came up with three laws…

20. Law #1: All the planets travel around the Sun in elliptical orbits.

21. Law #2: Planets move faster as they get near the Sun, slower when farther away
A1 = A2
Equal Area Ani. A2 A1
Animation

22. Law #3: The distance of the planet to the sun cubed is equal to the period of revolution squared: D3 = P2
For Mars: D = AU and
P = years
(1.5237)3 =
3.538
(1.881)2 =
3.538

23. Finally, there was Sir Isaac Newton, whose Laws of Gravitation proved the heliocentric model.

24. Orbits in a cone
Newton was able to show how the forces of gravity and the inertia of moving planets produces a Solar System.

25. Quiz! Why did the geocentric view hold on so long?
Who plotted out the epicycles for the known planets?
How did Copernicus, Galileo, and Kepler help promote the heliocentric model of the universe?
What was Newton’s contribution?