Astronomy midterm question: crescent moon setting just before dawn?

Astronomy 210 Midterm 1, fall semester 2014
Cuesta College, San Luis Obispo, CA

The following excerpt[*] describes the setting of a crescent moon:
[She] looked up and out, over the flat land with no intervening towers or hills, and saw the crescent moon...disappearing beyond the edge of the world. It would be dawn soon...
Discuss whether or not this description is plausible, 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.

[*] Pamela Dean, "Cousins," from Firebirds Rising: An Anthology of Original Science Fiction and Fantasy, Sharon November (ed.), Firebird (2005), p. 473.

Solution and grading rubric:
  • p:
    Correct. Complete diagram (with the sun, moon, and observer on Earth), and discusses how this scenario is impossible using at least one of two arguments:
    1. waxing crescent moon sets at 9 PM, and waning crescent moon sets at 3 PM, and thus neither crescent moon sets "soon" before sunrise;
    2. none of the phases that would set "soon" before sunrise (first quarter, waxing gibbous, and full moon; at 12 AM, 3 AM, and 6 AM respectively) are crescent phases.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Diagram and/or explanation has minor errors. Typically demonstrates that one of the crescent phases cannot set "soon" before sunrise, but does not eliminate the other crescent phase.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problems with either diagram or discussion. Demonstrates understanding of at least one of the crescent phase set times, but discussion regarding implausibility is missing, or somehow interprets results as plausible.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Diagram and discussion problematic. May instead discuss a gibbous phase.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
Grading distribution:
Section 70158
Exam code: midterm01neVs
p: 23 students
r: 5 student
t: 3 students
v: 12 students
x: 2 students
y: 0 students
z: 0 students

Section 70160
Exam code: midterm01s4Tn
p: 20 students
r: 2 students
t: 4 students
v: 6 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 1776), eliminating both waxing crescent and waning crescent phases as possibilities:

A sample "p" response (from student 1123), instead starting with phases visible just before dawn, and eliminating each as possibilities:

A sample "x" response (from student 6392), concerned with the local topography:

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