20160315

Astronomy midterm question: Mercury rising while Venus is setting?

Astronomy 210 Midterm 1, spring semester 2016
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:
??: At the same time you can see Venus setting, can you see Mercury rising?
DcM: Regardless of where they are in their orbits about the sun, while Venus can set in the west, it isn't possible to see Mercury rising in the east at the same time.
Discuss why this answer is correct 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, Mercury, Venus, Earth, and an observer on Earth.

[*] answers.yahoo.com/question/index?qid=20160114114820AAEJYPB.

Solution and grading rubric:
  • p:
    Correct. Complete diagram and reasoning includes the following explanations:
    1. for an observer on Earth at sunset (6 PM), Venus appears to be setting in the west (or some other late afternoon/early evening time) when placed in an orbit inside Earth's orbit;
    2. this sunset observer cannot see Mercury rising in the east, as that would mean Mercury would have to be in an orbit outside of Earth's.
  • 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. 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. Diagram and discussion problematic.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 30674
Exam code: midterm01n0rD
p: 6 students
r: 0 students
t: 4 students
v: 7 students
x: 4 students
y: 0 students
z: 1 student

Section 30676
Exam code: midterm01sN0w
p: 17 students
r: 15 students
t: 10 students
v: 8 students
x: 2 students
y: 1 student
z: 1 student

A sample "p" response (from student 2117):

A sample "p" response (from student 2919):

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