Astronomy quiz question: inferior planets at midnight

Astronomy 210 Quiz 2, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

Why are Mercury and Venus never visible in the San Luis Obispo, CA sky at midnight?
(A) Mercury and Venus never exhibit retrograde motion.
(B) Mercury and Venus are too small to reflect enough sunlight.
(C) The orbits of Mercury and Venus are smaller than the orbit of Earth.
(D) Mercury and Venus cannot exhibit gibbous phases.

Correct answer: (C)

Since the orbits of Mercury and Venus are smaller than the orbit of Earth (making them inferior planets), they can never appear behind Earth, with respect to the sun, and thus cannot be overhead (or anywhere in the sky) at midnight for typical temperate latitudes such as for San Luis Obispo, CA.

Student responses
Section 30674
(A) : 4 students
(B) : 5 students
(C) : 34 students
(D) : 2 students

Success level: 78% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.71

Student responses
Section 30676
(A) : 10 students
(B) : 8 students
(C) : 45 students
(D) : 11 students

Success level: 64% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.85

Previous posts:

• Astronomy quiz question: inferior planets at midnight, Fall Semester 2008.
• Astronomy midterm question: midnight Venus?, Fall Semester 2009.