Astr 2100 Descriptive Astronomy
Samples Questions for Test 2


  1. Which planet has the highest orbital speed? Mercury (this follows from Kepler's 3rd Law)
  2. Why did Copernicus still use epicycles in his heliocentric model of the Solar System? His orbits were still circular, which meant he could not get the details correct.
  3. What provided the definitive proof that the heliocentric model of the Solar System had to be the correct one? The phases of Venus
  4. Name the four Galilean moons of Jupiter. Io, Europa, Ganymede and Callisto
  5. If we were to suddenly discover a new (tenth) planet which is 50 AU from the Sun, how long would it take for this planet to orbit the Sun? 354 AU
  6. What is Kepler's 1st Law? The planets circle the Sun in orbits which are elliptical, with the Sun at one of the foci.
  7. Why do the planets sometimes appear to move backwards across the sky? Because of the different speeds of the planets in their orbits, when we are are on the same side of the orbit as the other planet then either we overtake them (Mars, Jupiter, etc.) or they overtake us (Mercury and Venus)
  8. If your weight is 160 lbs on the surface of the Earth, what would it be if you are 12,000 miles above the Earth's surface? (Take the radius of the Earth to be 4,000 miles). 10 lbs (note that would make you 4 Earth radii from the centre, not 3)
  9. What name is given to the shape of the orbit of a planet? ellipse
  10. What was Tycho Brahe's main contribution to the field of astronomy? Collection of very accurate data
  11. Are you truly weightless in the Space Shuttle? No, your weight is reduced (because you are further from the centre of the Earth) but it is not zero
  12. Why do we think that there are planets around other star's? Observed motion (wobble) of the star  (see Newton's Third Law of Motion)
  13. We have evidence of at least three planets orbiting around a single distant star. They have different sizes, different masses, and are different distances from the star itself. Which one has the longest year? The one that is furthest from the star  (Kepler's Third Law of Planetary Motion)
  14. What 2 main observations did Galileo make? Any two of
    1. Moons of Jupiter
    2. Sunspots
    3. Phases of Venus
  15. If you lived on Jupiter, which planets could you see in the eastern sky at sunset? Saturn, Uranus, Neptune
  16. Is the Moon falling? Yes, it has a force of gravity acting on it.
  17. How far would you have to be from the Sun to have a year exactly equal to 100 Earth years? 21.5 AU
  18. A planet goes around a distance star. If the planet is 6 AU from its star, and makes one orbit in 12 Earth years, what is the mass of the star (in solar masses)? 1.5
  19. The planets Bert and Ernie orbit the same star. Bert's orbit defines the "Bertian year". If Ernie is 8 times further from the star than is Bert, how many Bertian years does it take for Ernie to make one orbit.22.64
  20. According to Kepler, the planets move around the Sun in what shape of an orbit? ellipse
  21. What is an epicycle? A circular path whose centre lies on another circle
  22. You are the passenger in a car when it stops suddenly. You hit your head on the dashboard. What force pushes you forward into the dashboard? There isn't one (Newton's First Law of Motion)
  23. How do you determine the mass of a planet? Measure the distance to, and the orbital period of, any one of its Moons. An artificial satellite will do just as well.
  24. How do you determine the mass of the Sun? Measure the distance to, and the orbital period of, any one of its planets. An artificial satellite will do just as well.
  25. How do you determine the mass of the Moon? Measure the distance to, and the orbital period of, an artificial satellite in orbit around the Moon
  26. What is meant by the term 'weight'? The force of gravity acting on the object
  27. If we were to suddenly turn off the gravitational force provided by the Sun, what would happen to the Earth? It would move in a straight line at constant speed, off into space   (see Newton's Third Law of Motion)
  28. A planet orbits a star whose mass is twice that of the Sun. If the planet takes 16 Earth years to make one orbit, how far (in AU) is the planet from the star? 8 AU
  29. A planet is 0.12 AU from its star and takes 12 days to orbit once. What is the mass of the star? 1.6 solar masses
  30. How do you determine the mass of a planet? Time the orbit of any one of its moons. (For Venus which has no natural moon, you can use an artificial satellite.)
  31. How do you determine the mass of the Sun? Time the orbit of any one of its planets.
  32. How do you determine the mass of the Moon? Time the orbit of an artificial satellite orbiting the Moon (for example the Apollo spacecraft from the Moon missions)
  33. What is meant by the term 'weight'? The force of gravity acting on you.
  34. Where would you expect to find
    1. Olympus Mons Mars
    2. The Great Red Spot Jupiter
    3. Valles Marineris Mars
    4. Shepherd moons in the ring systems of the Jovian planets
    5. Phobos and Deimos Orbiting Mars
    6. Most of the asteroids between Mars and Jupiter
    7. Titan orbiting Saturn
  35. List the features which distinguish the terrestrial planets from the Jovian planets
  36. When was Saturn discovered, and by whom? No one knows, it has been known since ancient times
  37. When was Uranus discovered, and by whom? 1781 by William Herschel
  38. On the Earth your weight is 125 lbs. What would be your weight on a planet with 6 times the mass and 9 times the radius (both compared to the Earth.) 93. lbs.
  39. Two moons orbit the same planet. Moon 1 is 6 x 108 m from the center of the planet, and moon 2 is 4 x 108 m from the center. If moon 1 orbits the planet in 36 days, how long does it take moon 2 to orbit once? 19.6 days
  40. A planet takes 8 years to orbit of star whose mass is ½ the mass of the Sun. What is the radius of the planet's orbit? 3.2 AU.
  41. What were the major contributions of
    1. Copernicus Explained retrograde motion by using the heliocentric model
    2. Brahe Made the most accurate measurements that had been made until his time, of the positions of the planets
    3. Aristarchus Championed the heliocentric model of the Solar System

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