1. Early Astronomy 22.1

2. Astronomy The scientific study of the universe; it includes the observation and interpretation of celestial bodies and phenomena.

3. Geocentric Describes the concept of an Earth—centered universe.

4. Heliocentric Describes the view that the sun is at the center of the solar system.

5. Retrograde Motion The apparent westward motion of the planets with respect to the stars.

6. Ellipse An oval.

7. Astronomical Unit (AU) Average distance from Earth to the sun; 1.5x10⁸ Kilometers.

8. Key Concept In the geocentric model, the moon, sun, and the known planets—Mercury, Venus, Mars, and Jupiter– orbit Earth.

9. Key Concept In the heliocentric model, Earth and the other planets orbit the sun.

10. Key Concept Copernicus concluded that Earth is a planet. He proposed a model of the solar system with the sun at the center.

11. Key Concept Brahe’s observations, especially of Mars, were far more precise than any made previously.

12. Key Concept Kepler discovered three laws of planetary motion.

13. Key Concept Galileo’s most important contributions were his descriptions of the behavior of moving objects.

14. Key Concept Although others had theorized the existence of such a force, Newton was the first to formulate and test the law of universal gravitation.