Astronomy clicker question: transit of Mercury

Astronomy 210, fall semester 2008
Cuesta College, San Luis Obispo, CA

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the middle of their learning cycle:

In a rare event, the planet Mercury was observed in November 2006 as a tiny dark dot in the middle of the disk of the sun, for observers in San Luis Obispo, CA. Which shadow zone were these observers in when that happened?
(A) The umbra of Mercury.
(B) The penumbra of Mercury.
(C) The negative shadow of Mercury.
(D) The umbra of Earth.
(E) The penumbra of Earth.
(F) (I'm lost, and don't know how to answer this.)

Section 70160
(A) : 7 students
(B) : 0 students
(C) : 16 students
(D) : 0 students
(E) : 2 students
(F) : 0 students

This question was asked again after displaying the tallied results with the lack of consensus, with the following results. No comments were made by the instructor, in order to see if students were going to be able to discuss and determine the correct answer among themselves.

Section 70160
(A) : 1 student
(B) : 0 students
(C) : 25 students
(D) : 0 students
(E) : 0 students
(F) : 0 students

Correct answer: (C)

The negative shadow zone is directly behind the umbra of Mercury, where from that location its angular size is too small to completely cover the sun.

Pre- to post- peer-interaction gains:
pre-interaction correct = 64%
post-interaction correct = 96%
Hake (normalized) gain <g> = 89%