1. Lesson 2 Models of the Universe

2. Geocentric Theory
Ancient Greece

3. Geocentric Model Earth Centered Universe
All objects move around Earth, a fixed point.

4. Problems with Geocentric Theory
This model does explain apparent motions of celestial (space) objects

5. Geocentric Model: Ptolemy (87-150 AD)
Used epicycles to explain planetary movements

6. But… there was a problem…
Ptolemy’s system provided the first framework for all discussion of the universe for nearly 1500 years!!
But… there was a problem…

7. Heliocentric Theory

8. Heliocentric Model Sun Centered Universe
All celestial objects in the Solar system move around the Sun

9. Copernicus ( )
Suggests a model that the Sun was the center of the universe.
Argued that the motion of the Sun & planets could be equally described by a Sun centered (heliocentric) system.

11. Tycho Brahe (1546-1601): the greatest of the pre-telescope observers in Europe.
20+ years measuring the positions of the Sun, Moon and planets with great accuracy.

12. Johannes Kepler (1609)
Johannes Kepler was an assistant to Tycho Brahe. He used Brahe’s observations to study the orbit of Mars
Discovered three laws of planetary motion: predict with accuracy the motions of the planets
Copernicus ( )

13. Galileo ( ) first scientist to use a telescope to observe the sky. He observed the phases of Venus and the moons of Jupiter. Both observations supported the heliocentric model.

15. Kepler’s Laws

16. Each planet’s orbit is an ellipse with the Sun at one focus.
1st Law
Each planet’s orbit is an ellipse with the Sun at one focus.

18. Eccentricity A number indicating the roundness of an ellipse.
Eccentricity = Distance Between Foci
Length of Major Axis
e=d/L

19. Perfect circle
Straight line
e = 1
e = 0

20. ESRT pg 15

21. Eccentricity Practice

22. 2nd Law
Planets change orbital speed as they revolve around the Sun.

23. Caused by the Sun’s gravitational pull

24. 3rd Law
As distance from the Sun increases, a planet’s period of revolution increases.

25. ESRT pg 15