1. 22.1 Early Astronomy

2. Solar System Models
geocentric model: model of the solar system that states that the Earth is the center of the solar system
believed until 1600’s

4. heliocentric model: model of the solar system that states that the sun is the center of the solar system and the planets revolve around it

6. the stars don’t seem to move in our sky; planets do because they are much closer than stars
planets move in an eastward movement
sometimes planets appear to move in a backward movement, this is called retrograde motion
a loop occurs because each planet travels around the sun at different speeds

8. Watch retrograde motion!
BBC Video

9. The path a planet follows around the sun is called its orbit

10. Kepler’s Laws of Planetary Motion
1st law : planets orbit the sun in a path called an ellipse

11. perihelion
aphelion

12. astronomical unit: the average distance between the earth and the sun
150 million km
93 million miles

13. 2nd law: planets move fastest when they are closest to the sun in their orbits

14. 3rd law: the farther away a planet orbits, the longer the planet takes to revolve around the sun