Astronomy midterm question: moon rising at later time a few days later

Astronomy 210 Midterm 1, fall semester 2015
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked:

Pdg: If the waxing gibbous moon rises at a certain time today, why does it rise at a slightly later time a few days later when it is full?
TmS: It's due to the rotation of Earth and the moon's orbital motion.

Explain why this answer is correct for the waxing gibbous moon and full moon, and how you know this. Support your reasoning using a diagram showing the positions of the sun, the moon, Earth, and an observer on Earth.

[*] answers.yahoo.com/question/index?qid=20150924234103AA6ZbCz.

Solution and grading rubric:

• p:
  Correct. Complete diagram and reasoning includes the following explanations:
  1. the waxing gibbous moon is highest overhead at 9:00 PM, and would have risen six hours before at 3:00 PM;
  2. the full moon is highest overhead at 12:00 AM, and would have risen six hours before at 6:00 PM;
  3. thus because of the moon's revolution around Earth, the full moon occurs several days after the waxing gibbous moon, and due to Earth's rotation (which makes the moon appear to rise and set over a given day), the full moon rises several hours later a few days after the waxing gibbous moon rises.
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. As least has diagram indicating the times each moon phase is highest overhead.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Section 70158
Exam code: midterm01sF1A
p: 26 students
r: 5 student
t: 11 students
v: 0 students
x: 0 students
y: 0 students
z: 0 students


Section 70160
Exam code: midterm01nW6b
p: 10 students
r: 5 student
t: 4 students
v: 12 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1995):