Astronomy midterm question: correcting the National Geographic Society

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

Astronomy 10 learning goal Q3.2

[15 points.] Consider the following excerpt (Paul Raeburn, Uncovering the Secrets of the Red Planet: Mars, National Geographic Society, Washington, D.C., 1998, p. 30) regarding Copernicus' heliocentric model of planetary motion:

Copernicus had begun the job of dethroning Ptolemy, but he hadn't quite finished the work. And this is what brings us back to Mars. For all the strengths of the Copernican system, it failed to explain the motion of Mars, including its occasional, peculiar backwards march across the constellations of the zodiac.

Discuss why this statement is wrong. Support your answer using a diagram showing how Copernicus' model does explain the "occasional, peculiar backwards" motion of Mars.

Solution and grading rubric:

• p = 15/15: Draws a diagram with the Sun at the center (Copernicus' heliocentric model of planetary motion), showing how the Earth (moving faster in an inner orbit) "lapping" Mars (moving slower in an outer orbit) explains the retrograde motion of Mars.
  
• r = 12/15:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Understands how the "lapping" illusion explains retrograde motion, but does not explicitly draw a diagram, or explanation/diagram has minor errors/omissions.
  
• t = 9/15:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least demonstrates understanding of Copernicus' heliocentric model with a diagram or explanation.
  
• v = 6/15:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Explains retrograde motion using Ptolemy's geocentric model.
  
• x = 3/15:
  Implementation/application of ideas, but credit given for effort rather than merit. Missing or inapplicable diagram. Problematic explanation of Ptolemy's geocentric model.
  
• y = 1.5/15:
  Irrelevant discussion/effectively blank.
  
• z = 0/15:
  Blank.
  

Grading distribution:
Section 1073
p: 19 students
r: 2 students
t: 4 students
v: 19 students
x: 4 students
y: 0 students
z: 0 students

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