Astronomy midterm question: why no full moon in afternoon?

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

An astronomy question on an online discussion board(*) was asked and answered:

rraghunathan_in: In some afternoons we see both the sun and the moon in the sky... It is clear that the moon is directly exposed to the sun in the daytime sky, but only three-fourths or less...of the moon is illuminated. Why is it not a full moon at that time?
TicToc...: The moon is not full because it's not in opposition, which means the sun will be setting and the moon is rising at the same time.

Discuss why this answer is correct 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: http://answers.yahoo.com/question/index?qid=20120511191408AARz0Af.

Solution and grading rubric:

• p = 20/20:
  Correct. The moon cannot be full in the afternoon because the full moon rises at sunset, thus the opposition argument is correct. 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 a non-full moon visible during the afternoon, but does not directly address why the moon cannot be full in the afternoon.
• 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 70160
Exam code: midterm01n4rN
p: 6 students
r: 10 students
t: 4 students
v: 3 students
x: 6 students
y: 0 students
z: 1 student

A sample "p" response (from student 2513):
A sample "p" response (from student 1105):
A sample "x" response (from student 1964):