Astronomy 210 Quiz 2, Fall Semester 2008
Cuesta College, San Luis Obispo, CA
[4.0 points.] How does Copernicus' model explain the retrograde motion of a planet?
(A) The only perfect shape is a circle, and the only perfect motion is uniform motion.
(B) A planet slows down in its orbit when it is further from the sun, and speeds up in its orbit when it is closer to the sun.
(C) A planet moves around a circle that itself moves along an orbit around the Earth.
(D) A planet has a different orbital distance and speed around the sun from the Earth's orbital distance and speed around the sun.
Correct answer: (D)
Copernicus' model of planetary motion explains retrograde motion as a "lapping illusion" when planets in different orbits traveling at different speeds move past each other. Response (A) is Ptolemy's model of retrograde motion; response (B) is Kepler's second law; response (C) is based on Aristotle's first principles, but does not explain retrograde motion.
Student responses
Section 70158
(A) : 9 students
(B) : 11 students
(C) : 14 students
(D) : 18 students
"Difficulty level": 39%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.35
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