Astronomy midterm question: prograde Moon

Astronomy 10 Midterm 1, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q2.1

[15 points.] An observer in San Luis Obispo, CA notices that the Moon goes through prograde (proper) motion each night, with respect to the background stars. Explain why this is so, using a diagram showing the positions and motions of an observer, the Moon, and the Earth.

Solution and grading rubric:

• p = 15/15: Correct. Draws a diagram showing that the from night-to-night, the stars will rise at approximately the same time (with a few minutes), due to the Earth rotating on its axis. However, from night-to-night, the Moon will move along its ccw orbit around the Earth, and thus appear to be in a slightly different position with respect to the stars. Diagram and/or accompanying explanation is convincing and clear.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Diagram demonstrates how relative speeds of Earth rotating and Moon orbiting Earth cause the Moon to appear in different parts of the sky from night-to-night, but directions may be reversed, or discussion may have minor omissions.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Diagram/discussion points out differences between motions of Earth rotation and orbiting Moon, but does not fully demonstrate how the relative speeds of these motions causes night-to-night prograde motion of the Moon, relative to the background stars. May argue that the Moon undergoes prograde motion, because it is never experiences retrograde motion, due to the Moon never being "lapped" by the Earth.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Diagram is incomplete, or has wrong or no motion(s) indicated, with limited corresponding discussion.
  
• x = 3/15:
  Implementation/application of ideas, but credit given for effort rather than merit. Missing or inapplicable diagram.
  
• y = 1.5/15:
  Irrelevant discussion/effectively blank.
  
• z = 0/15:
  Blank.
  

Grading distribution:
Section 0135
p: 2 students
r: 5 students
t: 19 students
v: 13 students
x: 0 students
y: 1 students
z: 0 students

A sample of a "p" response (from J. G.):