Astronomy 210 Quiz 2, Fall Semester 2008
Cuesta College, San Luis Obispo, CA
[4.0 points.] How does Ptolemy's 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 has a different orbital distance and speed around the sun from the Earth's orbital distance and speed around the sun.
(C) A planet moves around a circle that itself moves along an orbit around the Earth.
(D) 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.
Correct answer: (C)
Ptolemy's model of planetary motion used epicycles moving along deferents to explain the retrograde loops of planets. Response (A) is based on his (and Aristotle's) first principles, but in and of itself is not a model of retrograde motion; response (B) is Copernicus' heliocentric explanation of retrograde motion; and response (D) is Kepler's second law.
Student responses
Section 70160
(A) : 1 student
(B) : 2 students
(C) : 22 students
(D) : 8 students
"Difficulty level": 69%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.89
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