Physics 109 Homework #2

Due Thursday September 27

1. The time of day is of course determined by how the sunlight falls on the rotating Earth. Some sundials make this obvious by being really miniature Earths, like the one in the garden behind Pavilion IV:

Go take a look at it, and answer he following questions:

a. What angle does that arrow make with the horizontal? What does that relate to?

b. Are the hour markers equally spaced? Would you expect them to be? Why?

(Jefferson made a sundial: read all about it here.)

2. a. Draw a diagram of an appropriate sundial to use at the north pole (in summer, of course!)

b. How could you design a sundial to use at the equator? Suppose you just took a vertical stick, what path would its shadow take at different times of the year? Can you think of a better design?

3. Here’s another sundial, for sale on the web:

If you look closely, you will see the numbers go in the opposite direction to those on the Pavilion IV sundial!

Who’s going to buy this sundial? Can it be of any use to anybody?

4. The moon goes round the earth once a month. Explain briefly why we don’t see an eclipse of the sun every month.

5. Go to the solar system simulator at http://space.jpl.nasa.gov/ Click the option to have the image take up 30% of the width.

Look at: Mars as seen from Earth, Earth as seen from Mars, Jupiter as seen from Earth, Earth as seen from Jupiter. Draw three concentric circles approximately representing Earth, Mars and Jupiter. Figure out, from the shadowing you see on the views you looked at, where the three planets must be in their orbits relative to each other, and indicate the positions, approximately, on your orbit diagram.

6. As we went over in class, the chain of transmission of Greek knowledge ultimately to Europe has some surprising links. One was the Nestorians, declared heretic and kicked out of the Roman Empire, from their monastery at Edessa in present day Turkey (not the Edessa in Macedonia). They were welcomed by the Persians and set up shop at Gundishapur (or Jundishapur). Some were later given court appointments by the Arab Muslims in Baghdad. Use Google Earth to find out where these places are, put in place markers, and print off the image. Find the approximate distances from Edessa to Gundishapur to Baghdad, and state the approximate dates of these movements.

State briefly why (a) the Persians, then later (b) the Arabs, welcomed these heretic Christians.

7. Mars in its path through the sky sometimes loops backwards. Explain in your own words why this happens (a) using Ptolemy’s model, and (b) with our present day picture of the Solar System.