1. CHAPTER 27 SECTION 2 EARTH AND SPACE AUSTIN HIGH SCHOOL Models of the Solar System

2. Early Models +2,000 years ago, Aristotle suggested an Earth-centered (geocentric) model of the solar system Sun, stars, and planets revolved around the sun Aristotle’s model could not explain whys some planets appeared to move backward in the sky relative to the stars (retrograde motion) Greek astronomer, Ptolemy, proposed changes to the model around 130 CE. He believed that planets moved in small circles as they revolved in larger circles around Earth called epicycles. Epicycles seemed to explain why planets sometimes appeared to move backward

3. Early Models 1543, Copernicus, proposed a sun-centered (heliocentric) model of the solar system In this model, 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 (that why some planets appeared to move backward from Earth) Later, Galileo observed that four moons traveled around Jupiter Showing him that objects can revolve around objects other than Earth

4. Early Models

5. Kepler’s Laws Danish astronomer named T. Brahe made detailed observations of the solar system After his death, on of his assistants J. Kepler discovered patterns in Brahe’s observation These patterns led Kepler to develop three laws that explained planetary motion

6. Law of Ellipse Kepler’s 1 st Law: states that each planet orbits the sun in a path called an ellipse, not a circle Ellipse closed curve whose shape is determined by two points called foci within the ellipse Ellipse can vary in shape The shape of an orbit can be described in a numerical form called, eccentricity Eccentricity if the degree of elongation of an elliptical orbit (symbol, e) Determined by dividing the distance between the foci of the ellipse by the length of the major axis Therefore, the eccentricity of circular orbit e= 0 and the eccentricity of an extreme elongated orbit is e=1

7. Parts of an Ellipse Eccentricity= distance between foci length of major axis

8. Increasing Eccentricity