Why is it dark at night?

Many years ago someone made a short film, asking Harvard graduates at commencement to explain why we have seasons. It's Harvard, so each answer is eloquent and delivered with perfect confidence -- and every one is badly wrong. I wish I could find a copy of that film on YouTube!

Motivated by the film, I've been asking the following question on exams given in courses on "conceptual physics" or physics for non-scientists. I've found that at both Berkeley and U Oregon, about 25% of students answer the question incorrectly (note this is at the end of the term, not the beginning). Remember, in our political system the votes of the people who get this question wrong count as much as yours or mine!


Question: It is dark at night in Eugene/Berkeley because

a) the night sky absorbs UV light
b) the moon blocks out the sun
c) the earth blocks out the sun
d) none of the above

Hint: draw a picture!

Further comments: Surprisingly the success rate for Berkeley students was about the same as for Oregon students, despite the difference in admissions selectivity. Nearly all of the students in these classes are humanities or social science majors. Many are upperclassmen, even seniors, trying to fulfill a minimal science requirement in order to graduate. These days the main claim of humanities programs is that they "teach critical thinking" :-)

See here for related discussion.