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Summer 1997

Physics Quiz #1

Michael Fowler

Physics Department, UVa

1. About 500 BC, the Pythagoreans had concluded that the earth was round. What evidence did they have for this belief?

2. Explain in terms of the relative positioning of the earth and the sun why it is warmer in summer than in winter.

3. Before 200 BC, Eratosthenes figured out the size of the earth, and he was right within about 10%. How did he do that?

4. The hands of a clock go around clockwise--when they're at the top of the dial, they're moving to the right. What can you conclude about where clocks were most likely invented?

5. Also before 200 BC, Aristarchus figured out the distance of the moon from the earth quite accurately. How?

This "quiz" was given to the class divided into groups of four, and really intended to stimulate discussion as to how the Greeks could possibly have figured out so much with so little technology (such as telescopes) to help them. The questions on Erastothenes and Aristarchus are fully answered in my Galileo and Einstein course, HERE.

The reasons the Greeks concluded the earth was round are summarized at the end of this lecture.

To solve the clockwise problem, think about sundials.

On the summer/winter problem, I recently spent ten or fifteen minutes working through it with a student by moving a globe on its stand around a light representing the sun. I illustrated how the length of day varied from season to season by slowly spinning the globe at different places in the orbit. At the end of this demonstration, I asked her if the origin of the seasons was now clear. "Of course I can follow your explanation", she said, rather condescendingly, "but it only works because your globe is tilted." When I then suggested to her that the earth's axis of rotation was itself tilted relative to the plane of the earth's orbit around the sun, she was dumbstruck. Why would the solar system be arranged in such an unappealing way?