Astronomy midterm question: Venus as morning and evening star in same day?

Astronomy 210 Midterm 1, fall semester 2012
Cuesta College, San Luis Obispo, CA

[20 points.] An astronomy question on an online discussion board(*) was asked and answered:

P-d...: ...If Venus is above the east horizon at sunrise (so it is a morning star), can it be above the west horizon at sunset (so it can be an evening star) later on that day?
aladdinwa: No, it cannot be both the morning and evening star on the same day... When Venus rises before the sun, you can see it before the sun rises and it is the morning star and it disappears below the horizon while the sun is still in the sky...

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, Venus, Earth, and an observer on Earth.

*Source: http://answers.yahoo.com/question/index?qid=20120511191408AARz0Af.

Solution and grading rubric:

• p = 20/20:
  Correct. Correct and complete diagram, with observers at sunrise/sunset on Earth (outer heliocentric orbit) and Venus (inner heliocentric orbit); shows and discusses how Venus above east horizon at sunrise (as a morning star) would not be visible anywhere in sky at sunset later that day (as an evening star).
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Diagram and/or explanation has minor errors. May show that Venus visible as an evening star would not then be visible as a morning star.
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problems with either diagram or discussion.
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Diagram and discussion problematic. May have Venus orbiting Earth, or in a heliocentric orbit outside of Earth's, but still discusses how Venus could not be visible at both sunrise and sunset on the same day.
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Misconceptions or non-relevant concepts.
• y = 2/20:
  Irrelevant discussion/effectively blank.
• z = 0/20:
  Blank.

Grading distribution:
Section 70160
Exam code: midterm01n4rN
p: 12 students
r: 5 students
t: 3 students
v: 7 students
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 0716):