Astronomy final exam question: Comet C/2012 S1 ("ISON") just before perihelion

Astronomy 210 Final Exam, fall semester 2013
Cuesta College, San Luis Obispo, CA

An observing guide to spotting Comet ISON was published[*] before it fragmented and vaporized during its close pass to the sun. Explain how this diagram is consistent with the fact that Comet ISON was closer to the sun than Mercury[**] for late November 2013. Support your answer using a diagram showing the positions of the sun, Comet ISON, Mercury, and an observer on Earth.

[*] Joe Rao, "Comet ISON's Thanksgiving Sun Encounter: An Observer's Guide," November 23, 2013, http://www.space.com/23713-comet-ison-observers-guide-thanksgiving.html.
[**] "0.084 AU—Comet ISON’s forecasted distance from the sun on November 28, 2013, Mercury is 0.39 ± 0.09 AU from the sun," http://www.cometisonnews.com/distance/.

Solution and grading rubric:

• p = 20/20:
  Correct. Correct and complete diagram, with the sun, Mercury, Comet ISON, and an observer on Earth); shows and discusses how Comet ISON would be closer to the sun than Mercury on this diagram, consistent with the sunrise view given.
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Diagram and/or explanation has minor errors.
• t = 12/20: Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problems with either diagram or discussion. At least understands that Mercury and Comet ISON must be just above the sun when the sun is at (or just below) a horizon.
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Diagram and discussion problematic.
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Misconceptions or non-relevant concepts.
• y = 2/20:
  Irrelevant discussion/effectively blank.
• z = 0/20:
  Blank.

Grading distribution:
Section 70158
Exam code: finals0oB
p: 15 students
r: 3 students
t: 10 students
v: 4 students
x: 4 students
y: 2 students
z: 1 student

A sample "p" response (from student 0527):