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# Ch4 - Assignment Chapter 4 Class Astronomy I Date September...

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Class: Astronomy I Date: September 5 th , 2007 Review Questions 1. How did the ancient Greeks explain why the sun and moon slowly change their positions relative to the background stars? The ancient Greeks imagined that the planets followed an ornate combination of circular paths. They assumed that the Sun and the Moon slowly revolve about a stationary Earth with respect to the stars orbiting the opposite direction. 2. In what direction does a planet move relative to the stars when it is in direct motion? When it is in retrograde motion? How do these compare with the direction in which we se e the sun move relative to the stars? When a planet is in direct motion, the orbiting process is eastward. In retrograde motion, the planet is in westward movement. These motions however are much slower than the apparent daily rotation of the stars. 3. (a) In what direction does a planet move relative to the horizon over the course of one night? (b) The answer to (a) is the same whether the planet is in direct motion or retrograde motion. What does this tell you about the speed at which planets move on the celestial sphere? a) A planet like Mars will be in direct motion most of the time, but will be in retrograde motion from time to time. In these occasions, the planet travels farther across the sky, and the speed of Mars relative to the stars is not constant. 4. What is an epicycle? How is it important in Ptolemy’s explanation of the retrograde motions of the planets? An epicycle is the small circle that each planet is assumed to move in. In Ptolemy’s explanation of the retrograde motion, he concluded that the epicycle and deferent, larger circle in center of Earth, rotate in the same direction. 5. What is the significance of Occam’s razor as a tool for analyzing theories? The ‘razor’ refers to shaving extraneous details from an argument or explanation. The idea itself had no proof or verification, but it appealed to the scientist’s sense of beauty and elegance, and it has helped lead to the simple and powerful laws of nature. 6. How did the models of Aristarchus and Copernicus explain the retrograde motion of the planets? Models of Aristarchus and Copernicus were based on a heliocentric model in which all planets including Earth revolve about the Sun. Their models suggested that the retrograde motion of the planets is a result of our viewing the universe from a moving Earth. 7. How did Copernicus determine that the orbits of Mercury and Venus must be smaller than the Earth’s orbit? How did he determine that the orbits of Mars, Jupiter and Saturn must be larger than the earth’s orbit? Copernicus realized that because Mercury and Venus are always observed near the Sun, their orbits must be smaller than that of Earth. On the other hand, Copernicus believed that Mars, Jupiter and Saturn must be larger than the Earth’s orbit because they are sometimes seen on the side of the celestial sphere opposite the Sun. Last Name: Sung

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Ch4 - Assignment Chapter 4 Class Astronomy I Date September...

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