Sample first exam questions

1. Describe the motion in the sky, as seen from Austin over a 24 hour period (pretending the bright skylight during the daytime didn't keep you from seeing the stars) of a star near Polaris (the north star).
   
   Describe the motion in the sky of a star on the celestial equator.
   
   
2. The Moon was new a week ago. It is now in first quarter phase. At about what time does it rise today?
   
   Draw a sketch of the Moon as it is rising today, showing the orientation of its lit face.
   
   
3. State where Mars is in its orbit, relative to us, when it is moving retrograde, and explain why its motion is retrograde at this time.
   
   If you could change the speeds of the planets in their orbits (violating Kepler's laws), how could you change Mars's speed to cause it to always move in the prograde direction?
   
   
4. An Earth-crossing asteroid has a perihelion of 1 AU and an aphelion of 4 AU. At perihelion its speed is 40 km/s. What is its speed at aphelion?
   
   Is its speed at aphelion faster or slower than that of an asteroid on a circular orbit at a distance from the Sun of 4 AU? Explain your reasoning briefly.
   
   What is the semi-major axis of its orbit?
   
   
5. How does the speed of the space shuttle orbiting the Earth compare to the speed of a small satellite orbiting the same distance above the Earth? Explain your reasoning briefly, using Newton's laws.
   
   
6. Why is it warmer here in June than in December?
   
   Why is it warmer in August than in June?
   
   
7. At what phase of the Moon can a lunar eclipse occur?
   
   Why doesn't a lunar eclipse occur every month?
   
   
8. How do the wavelength, frequency, and speed of a light wave change as it enters a piece of glass?
   
   Explain why the change in speed causes the direction the light wave is traveling to change as it enters a piece of glass.
   
   
9. What is a photon?
   
   Describe how photons can have wavelengths associated with them.
   
   
10. Summarize the argument that the fact that electrons have wave properties means that they can be in only certain orbits, and then the fact that electrons can be in only certain orbits means that they emit only certain wavelengths of light (not a continuous spectrum).
    
    
11. The Sun has a surface temperature of about 6000 K and is brightest at a wavelength of about 500 nm. Betelgeuse has a surface temperature of about 3000 K. At about what wavelength is it brightest?
    
    
12. How does the light gathering power of the Keck 10 m telescope compare to that of the Mt. Palomar 5 m?
    
    There is another reason for astronomers to want big telescopes (in addition to light gathering power) that is especially important for long-wavelength astronomy. What is it, and why is it more important at long wavelengths?
    
    
13. The Moon was full last Sunday. It will be in the 3rd quarter phase this Sunday.
    
    a) At about what time (to within a few hours) will it pass closest to overhead this Sunday? Explain how you figured this out.
    b) In what direction will the lit edge of the Moon face when it is near overhead this Sunday (north, south, east, or west)? Explain your reasoning.
    
    
14. A week ago Venus and Jupiter were right next to each other in the western sky in the evening. Now Venus has moved to a position above Jupiter and is setting later than it was last week.
    
    a) Is Venus moving in a retrograde or prograde direction? Explain your reasoning.
    b) Draw a sketch of the solar system showing the orbits of Venus, Earth, and Jupiter, and showing the current location of these three planets in their orbits. (Where you show the Earth in its orbit is arbitrary, but their location in the sky should tell you the locations of Venus and Jupiter relative to the Earth.)
    
    
15. Here are three incorrect applications of Kepler's laws. What's wrong with each one?
    
    a) A planet orbits the Sun on a circular orbit with a perihelion distance of 0.5 AU and an aphelion distance of 1.5 AU.
    b) The Earth orbits the Sun at a distance of 1 AU from the Sun and has an orbital speed of 30 km/s. Another planet orbits at 2 AU from the Sun with a speed of 15 km/s, so a line from the Sun to that planet and a line from the Sun to Earth sweep out equal areas in equal times.
    c) A moon of Jupiter orbits 1 AU from Jupiter with an orbital period of 1 year.
    
    
16. Assume the mass of the Moon is 1/100 the mass of the Earth. The Earth and the Moon orbit around their center of mass. Ignore the fact that both the Earth and the Moon also orbit the Sun.
    
    a) How does the force of the Earth's gravity on the Moon compare to the force of the Moon's gravity on the Earth? (Which is larger, and how many times larger, or are they the same?)
    b) How does the acceleration of the Earth in its motion around the Earth-Moon center of mass compare to the acceleration of the Moon? (Which is larger, and how many times larger, or are they the same?)
    c) How does the distance of the Earth from the Earth-Moon center of mass compare to the distance of the Moon from the Earth-Moon center of mass?
    
    
17. Why do hot atomic gasses emit emission line spectra (rather than continuous spectra like hot solids do)?