Astronomy quiz question: zenith star set

Astronomy 210 Quiz 1, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[4.0 points.] An observer in San Luis Obispo, CA observes a constellation located at the zenith. This constellation will:
(A) never set below the horizon, as it is a circumpolar constellation.
(B) eventually set somewhere near the southwest horizon.
(C) eventually set somewhere near the northwest horizon.
(D) (None of the above choices, as it is not possible for a constellation to be at the zenith in San Luis Obispo, CA.)

Correct answer: (C)

Using a starwheel, selecting any constellation (e.g., Lyra) that is at the zenith at its highest position, will eventually set on the northwest horizon. (For Lyra, it is at the zenith as it passes the meridian at 8:00 PM on August 25, and will set at 5:00 AM the next day.) Or from discussion on paths of the sun, objects that rise northeast will pass the meridian closer to the zenith, and set northwest, than objects that rise in the southeast, which will pass the meridian closer to the horizon, and eventually set southwest.

Some students may be confusing the hub of the starwheel (corresponding to the celestial north pole, near which Polaris lies) with the zenith (corresponding to the geometric center of the blue oval representing the visible portion of the celestial sphere above the horizon).

Student responses
Section 70158
(A) : 38 students
(B) : 10 students
(C) : 21 students
(D) : 6 students

"Difficulty level": 34%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.43