1. Copernicus was the first person to propose the details of a model of a heliocentric (Sun- centered) Cosmology in the 1500's. However, he was not the first person to propose the idea that the earth revolved around the sun. The ancient Greek astronomer Aristarchus proposed a heliocentric universe to explain the retrograde motion of the planets. Based on your understanding of the Solar System and the orbits of the planets, how would you defend a model of an earth-centered (geocentric) solar system? What difficulties would you run into in defending this model? How would you explain retrograde motion?

2. How would you prove or disprove the proposition that the Earth moves around the Sun in an orbit, using only the data that can be obtained with the naked eye?

3. If you discovered another planetary system in our galaxy, what properties of a planet would you need to observe in order to determine the mass of its "sun"? How would you then determine the mass of its "sun"?

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