20200316

Astronomy midterm question: "sliced" moon rising with coming darkness

Astronomy 210 Midterm 1, spring semester 2020
Cuesta College, San Luis Obispo, CA

The following excerpt describes the rising of a crescent ("sliced") moon around sunset:
Darkness was coming slowly down, a cool cloak draped over them. Soon, a sliced moon began to rise as a wind rose.
Discuss whether or not this description is plausible 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, moon, Earth, and an observer on Earth.

[*] Mitchell Smith, Moonrise: Book Three of the Snowfall Trilogy, Tom Dougherty Associates (2004), p. 195.

Solution and grading rubric:
  • p:
    Correct. Complete diagram (with the sun, moon, and observer on Earth), and discusses one of the following reasons why this description would be implausible:
    1. waxing crescent moon is overhead at 3 PM, and rose six hours earlier at 9 AM; while waning crescent moon is overhead at 9 AM, and rose six hours earlier at 3 AM, such that neither of these crescent phase moons could be rising just after sunset; or
    2. the moon phase that would be rising around/at/after sunset (6 PM) would be the waxing gibbous moon, full moon or waning gibbous moon, such that no crescent phase moon could be rising after sunset.
  • 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.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problems with either diagram or discussion.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to draw a moon phase diagram and apply rise/overhead/set times.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion not clearly based on a moon phase diagram.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 30674
Exam code: midterm01n7in
p: 15 students
r: 3 students
t: 4 students
v: 5 students
x: 0 students
y: 0 students
z: 0 students

Section 30676
Exam code: midterm01S7wY
p: 24 students
r: 2 students
t: 5 students
v: 9 students
x: 5 students
y: 0 students
z: 0 students

A sample "p" response (from student 2310):

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