Astronomy 10, Summer Session 2007
Cuesta College, San Luis Obispo, CA
Astronomy 10 learning goal Q3.2
Students were asked the following clicker question (Classroom Performance System, einstruction.com) near the end of their learning cycle:
[0.3 points.] As seen from San Luis Obispo, CA, when a planet is in retrograde motion, how will it move in a single night?
(A) It will rise from the east horizon, and set in the west horizon.
(B) It will rise from the east horizon, and shortly afterwards set in the east horizon.
(C) It will rise from the west horizon, and set in the east horizon.
(D) It will rise from the west horizon, and shortly afterwards set in the west horizon.
Correct answer: (A).
Most students are probably thinking that retrograde motion (planet moves a very small distance east-to-west relative to the background stars) is the reason why it will rise in the east horizon, and set in the west horizon. The next clicker question will test for this misconception.
Student responses
Section 8027
(A) : 11 students
(B) : 1 student
(C) : 2 students
(D) : 0 students
[0.3 points.] As seen from San Luis Obispo, CA, when a planet is in prograde motion, how will it move in a single night?
(A) It will rise from the east horizon, and set in the west horizon.
(B) It will rise from the east horizon, and shortly afterwards set in the east horizon.
(C) It will rise from the west horizon, and set in the east horizon.
(D) It will rise from the west horizon, and shortly afterwards set in the west horizon.
Correct answer: (A).
This follow-up question causes a lot of consternation for students, who must realize that in prograde motion, a planet moves a very small distance west-to-east relative to the background stars, which themselves over a single night rise from the east horizon, and set in the west horizon. Thus a planet will be seen to rise from the east horizon, and set in the west horizon in a single night, regardless if it is undergoing retrograde or prograde motion!
Student responses
Section 8027
(A) : 3 students
(B) : 1 student
(C) : 10 students
(D) : 0 students
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