Astronomy midterm question: watching two moons rising?

Astronomy 210 Midterm 1, fall semester 2012
Cuesta College, San Luis Obispo, CA

[20 points.] Shown below is an excerpt from an online comic strip(*) posted on June 6, 2012. Assuming that Earth can have two moons, discuss why this view of these two moons is not possible for an observer in San Luis Obispo, CA, and how you know this. Support your answer using a diagram showing the positions of the sun, the moon, Earth, and an observer on Earth.


*Source: "Steampunk Summer," Incidental Comics, Grant Snider, http://www.incidentalcomics.com/2012/06/steampunk-summer.html.

Solution and grading rubric:

• p = 20/20:
  Correct. The full moon rises at 6 PM, while the smaller waning crescent moon would rise at 3 AM, so it would not be possible to watch both moons rising at the same time. May instead demonstrate it is impossible for an observer to see these moons with two different phases simultaneously in the same part of the sky, as two moons in the same part of the sky must have the same (or nearly identical) phases. Complete diagram and reasoning.
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Depicts how these moons with two different phases are in different locations in their orbits, but does not explicitly discuss how these moons cannot be seen rising at the same time, or seen together in the same part of the sky, as these moons could still be seen in (different parts of) the same sky between 12 AM to 3 AM.
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problems with either diagram or discussion.
• 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: moon phases created by Earth's shadow, diagrams with the moon orbiting the sun, etc.
• y = 2/20:
  Irrelevant discussion/effectively blank.
• z = 0/20:
  Blank.

Grading distribution:
Section 70158
Exam code: midterm01sLa6
p: 7 students
r: 10 students
t: 9 students
v: 7 students
x: 6 students
y: 0 students
z: 0 students

A sample "p" response (from student 2210):
Another sample "p" response (from student 8669):