2. Models of the Solar System Ch 27.2 Page 691

3. Early models  Around 2,000 years ago, Aristotle suggested the earth- centered or geocentric model of the solar system  In this system everything revolved around the Earth

4. Ptolemy made modifications  Around 130 CE, thought that planets moved in small circles called epicycles as they revolved in larger circles around the earth  They helped explain retrograde motion

5. RETROGRADE MOTION The apparent backward movement of planets

6. Heliocentric model  In 1543 CE, Nicolaus Copernicus proposed a sun-centered, or heliocentric model of the solar system  Planets revolved around the sun in the same direction but at different speeds and distances from the sun  Fast moving planets passed slow moving planets  This explained retrograde motion

7. Galileo  Galileo’s observation of four moons that traveled around Jupiter gave support to Copernicus's model

8. Kepler’s Laws of Planetary Motion

9. Law 1 Law 1 : Each planet moves in an elliptical orbit having the sun at one focus.

10. Law 2: An imaginary line from the sun to any planet sweeps out equal areas in equal time intervals.

11. Law 3: The square of any planet’s period is proportional to the cube of the planet’s average distance from the Sun. p 2 = d 3

12. Newton’s Explanation of Kepler’s Laws  Newton said a moving body will remain in motion and resist a change in speed or direction until an outside force acts on it  This concept is called inertia  It is Newton’s First Law

13. orbit  Since a planet does not follow a straight path, some outside force must be acting on it  Newton called this force gravity  He said this force exists between any two objects

14. Two forces combine to create orbit  Inertia makes things move in a straight line  Gravity pulls it downward  The result is an ellipse