PHYSICS 109N

Michael Fowler

Physics 307

mfowler@virginia.edu

Homework Assignment: Due Tuesday 19 September, 2:00p.m.

1. Finding the angular size of the moon: choose some suitable round object, such as a dime, quarter, a tennis ball, ping pong ball or whatever, and while you are looking at the moon, have your partner hold the ping pong ball (say) at just the right distance that it looks to you exactly the same size as the moon. You could try it directly in front of the moon, to just block it, or side by side with the moon, whatever works best for you. Now, measure the distance from your eye to the ball, and measure the diameter of the ball. Given that the moon is 230,000 miles away, figure out the moon's diameter. What is the angular size of the moon? That is, if you take two pencils, point one at the bottom of the moon, and one at the top, as seen from here, what is the angle between them? You can answer in degrees or radians. (One radian is the angle of a piece of pie having the curved side the same length as the straight sides.)

2. Find the North Star (Polaris). Now, find how high Polaris is above the horizon by pointing a pencil directly at Polaris and having your partner measure the angle between the pencil and a horizontal line running directly beneath it.

Just after dark, find the Big Dipper, and sketch the Big Dipper together with Polaris. Three hours later, check them again. Has Polaris moved? Has the Big Dipper moved? Draw on your sketch how they have moved, if at all, in the sky.

3. Explain in a sentence or two, together with a simple diagram of the earth and the sun, why it is warmer in summer than in winter.

4. Some years ago, a very romantic picture of the Rotunda as seen from the middle of the Lawn was published. It depicted the Rotunda at night, and there was a full moon visible in the sky just above the Rotunda. How can you prove this picture is a fake?