Astronomy midterm question: orbital eccentricity as reason for the seasons?

Astronomy 210 Midterm 1, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

[20 points.] A Harvard undergraduate student was interviewed just after his commencement ceremony to explain the causes of the seasons:
I think the seasons happen because as Earth travels around the sun, it gets nearer to the sun, which produces warmer weather, and [Earth] gets farther away [from the sun] which produces colder weather.
--A Private Universe, Science Media Group, Harvard University/Smithsonian Institution, Cambridge, MA (1989), http://www.youtube.com/watch?v=p0wk4qG2mIg.
With the assumption that this explanation is correct, discuss the type of seasons that an observer in San Luis Obispo, CA (above the equator) will experience compared to an observer in Sydney, Australia (below the equator). Support your answer using a diagram showing the positions of the sun, Earth, and observers above and below the equator.

Solution and grading rubric:
  • p = 20/20:
    Correct. Discusses that if the variation in sun-Earth distance is the cause of seasons (regardless of tilt), then SLO and Sydney will essentially experience the same seasons at the same time.
  • r = 16/20:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Some discussion of the student's assertion that distance is the cause of seasons, but emphasis on disproving student's assertion rather than on discussion of the seasons resulting from if the assertion were true. May have minor errors or inclusion of unrelated factors.
  • t = 12/20:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Explains (correctly with a diagram) how Earth's tilt causes SLO and Sydney to have opposite seasons, but does not discuss the type of seasons that SLO and Sydney will experience if the student's assertion that distance is the cause of seasons is true.
  • v = 8/20:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. As (t), but diagram or explanation of the effect of tilt on seasons is problematic.
  • x = 4/20:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y = 2/20:
    Irrelevant discussion/effectively blank.
  • z = 0/20:
Grading distribution:
Section 70158
p: 18 students
r: 5 students
t: 19 students
v: 5 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 4242):

A sample "t" response (from student 1134):

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