Astronomy quiz question: Mars visible at sunset?

Astronomy 210 Quiz 2, fall semester 2017
Cuesta College, San Luis Obispo, CA

The locations of Venus, Earth and Mars are shown in the diagram below (not to scale, and orbits have been simplified as circles instead of ellipses).

At sunset, Mars will be visible:
(A) low over the east horizon.
(B) somewhere high up in the sky.
(C) low over the west horizon.
(D) not visible in the sky.

Correct answer (highlight to unhide): (C)

If a line is drawn from Earth to the sun, the observer at sunset (6 PM) is located perpendicular to that line. Since Mars is above that horizon line, it will be visible at that time, and would be located low over the west horizon. (Since Venus is below that horizon line, it would not be visible at sunset.)

Section 70160
Exam code: quiz02NoL4
(A) : 7 students
(B) : 3 students
(C) : 15 students
(D) : 4 students

Success level: 54% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64

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