Questions

Review Questions

Questions preceded by an asterisk (*) are discussed in the Boxes.

1. Explain the difference between sunlight and moonlight.

2. (a) Explain why the Moon exhibits phases. (b) A common misconception about the Moon’s phases is that they are caused by Earth’s shadow. Use Figure 3-2 to explain why this is not correct.

3. How would the sequence and timing of lunar phases be affected if the Moon moved around its orbit (a) in the same direction, but at twice the speed; (b) at the same speed, but in the opposite direction? Explain your answers.

4. At approximately what time does the Moon rise when it is (a) a new moon; (b) a first quarter moon; (c) a full moon; (d) a third quarter moon?

5. Astronomers sometimes refer to lunar phases in terms of the age of the Moon. This is the time that has elapsed since new moon phase. Thus, the age of a full moon is half of a 29½-day synodic period, or approximately 15 days. Find the approximate age of (a) a waxing crescent moon; (b) a third quarter moon; (c) a waning gibbous moon.

6. If you lived on the Moon, would you see Earth go through phases? If so, would the sequence of phases be the same as those of the Moon as seen from Earth, or would the sequence be reversed? Explain using Figure 3-2.

7. Is the far side of the Moon (the side that can never be seen from Earth) the same as the dark side of the Moon? Explain.

8. (a) If you lived on the Moon, would you see the Sun rise and set, or would it always be in the same place in the sky? Explain. (b) Would you see Earth rise and set, or would it always be in the same place in the sky? Explain using Figure 3-4.

9. What is the difference between a sidereal month and a synodic month? Which is longer? Why?

10. On a certain date the Moon is in the direction of the constellation Gemini as seen from Earth. When will the Moon next be in the direction of Gemini: one sidereal month later, or one synodic month later? Explain your answer.

11. What is the difference between the umbra and the penumbra of a shadow?

12. Why doesn’t a lunar eclipse occur at every full moon and a solar eclipse at every new moon?

13. What is the line of nodes? Why is it important to the subject of eclipses?

14. What is a penumbral eclipse of the Moon? Why do you suppose that it is easy to overlook such an eclipse?

15. Why is the duration of totality different for different total lunar eclipses, as shown in Table 3-1?

16. Can one ever observe an annular eclipse of the Moon? Why or why not?

17. If you were looking at Earth from the side of the Moon that faces Earth, what would you see during (a) a total lunar eclipse; (b) a total solar eclipse? Explain your answers.

18. If there is a total eclipse of the Sun in April, can there be a lunar eclipse three months later in July? Why or why not?

19. Which type of eclipse—lunar or solar—do you think most people on Earth have seen? Why?

20. How is an annular eclipse of the Sun different from a total eclipse of the Sun? What causes this difference?

21. *What is the saros? How did ancient astronomers use it to predict eclipses?

22. How did Eratosthenes measure the size of Earth?

23. How did Aristarchus try to estimate the distance from Earth to the Sun and Moon?

24. How did Aristarchus try to estimate the diameters of the Sun and Moon?

Advanced Questions

Questions preceded by an asterisk (*) are discussed in the Boxes.

25. The dividing line between the illuminated and unilluminated halves of the Moon is called the terminator. The terminator appears curved when there is a crescent or gibbous moon, but appears straight when there is a first quarter or third quarter moon (see Figure 3-2). Describe how you could use these facts to explain to a friend why lunar phases cannot be caused by Earth’s shadow falling on the Moon.

26. What is the phase of the Moon if it rises at (a) midnight; (b) sunrise; (c) halfway between sunset and midnight; (d) halfway between noon and sunset? Explain your answers.

27. The Moon is highest in the sky when it crosses the meridian (see Figure 2-21), halfway between the time of moonrise and the time of moonset. At approximately what time does the Moon cross the meridian if it is (a) a new moon; (b) a first quarter moon; (c) a full moon; (d) a third quarter moon? Explain your answers.

28. The Moon is highest in the sky when it crosses the meridian (see Figure 2-21), halfway between the time of moonrise and the time of moonset. What is the phase of the Moon if it is highest in the sky at (a) midnight; (b) sunrise; (c) noon; (d) sunset? Explain your answers.

29. Suppose it is the first day of autumn in the northern hemisphere. What is the phase of the Moon if the Moon is located at (a) the vernal equinox; (b) the summer solstice; (c) the autumnal equinox; (d) the winter solstice? Explain your answers. (Hint: Make a drawing showing the relative positions of the Sun, Earth, and Moon. Compare with Figure 3-2.)

30. This photograph of Earth was taken by the crew of the Apollo 8 spacecraft as they orbited the Moon. A portion of the lunar surface is visible at the right-hand side of the photo. In this photo, Earth is oriented with its north pole approximately at the top. When this photo was taken, was the Moon waxing or waning as seen from Earth? Explain your answer with a diagram.

    

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    (NASA/JSC)

31. (a) The Moon moves noticeably on the celestial sphere over the space of a single night. To show this, calculate how long it takes the Moon to move through an angle equal to its own angular diameter (½°) against the background of stars. Give your answer in hours. (b) Through what angle (in degrees) does the Moon move during a 12-hour night? Can you notice an angle of this size? (Hint: See Figure 1-11.).

32. During an occultation, or “covering up,” of Jupiter by the Moon, an astronomer notices that it takes the Moon’s edge 90 seconds to cover Jupiter’s disk completely. If the Moon’s motion is assumed to be uniform and the occultation was “central” (that is, center over center), find the angular diameter of Jupiter. (Hint: Assume that Jupiter does not appear to move against the background of stars during this brief 90-second interval. You will need to convert the Moon’s angular speed from degrees per day to arcseconds per second.)

33. The plane of the Moon’s orbit is inclined at a 5° angle from the ecliptic, and the ecliptic is inclined at a 23½° angle from the celestial equator. Could the Moon ever appear at your zenith if you lived at (a) the equator; (b) the south pole? Explain your answers.

34. How many more sidereal months than synodic months are there in a year? Explain your answer.

35. Suppose Earth moved a little faster around the Sun so that it took a bit less than one year to make a complete orbit. If the speed of the Moon’s orbit around Earth were unchanged, would the length of the sidereal month be the same, longer, or shorter than it is now? What about the synodic month? Explain your answers.

36. If the Moon revolved about Earth in the same orbit but in the opposite direction, would the synodic month be longer or shorter than the sidereal month? Explain your reasoning.

37. One definition of a “blue moon” is the second full moon within the same calendar month. There is usually only one full moon within a calendar month, so the phrase “once in a blue moon” means “hardly ever.” Why are blue moons so rare? Are there any months of the year in which it would be impossible to have two full moons? Explain your answer.

38. You are watching a lunar eclipse from some place on Earth’s night side. Will you see the Moon enter Earth’s shadow from the east or from the west? Explain your reasoning.

39. The total lunar eclipse of October 28, 2004, was visible from South America. The duration of totality was 1 hour, 21 minutes. Was this total eclipse also visible from Australia, on the opposite side of Earth? Explain your reasoning.

40. During a total solar eclipse, the Moon’s umbra moves in a generally eastward direction across Earth’s surface. Use a drawing like Figure 3-11 to explain why the motion is eastward, not westward.

41. A total solar eclipse was visible from Africa on March 29, 2006 (see Figure 3-10b). Draw what the eclipse would have looked like as seen from France, to the north of the path of totality. Explain the reasoning behind your drawing.

42. Figure 3-11 and Figure 3-13 show that the path of a total eclipse is quite narrow. Use this to explain why a glow is visible all around the horizon when you are viewing a solar eclipse during totality (see Figure 3-10a).

43. (a) Suppose the diameter of the Moon were doubled, but the orbit of the Moon remained the same. Would total solar eclipses be more common, less common, or just as common as they are now? Explain. (b) Suppose the diameter of the Moon were halved, but the orbit of the Moon remained the same. Explain why there would be no total solar eclipses.

44. Just as the distance from Earth to the Moon varies somewhat as the Moon orbits Earth, the distance from the Sun to Earth changes as Earth orbits the Sun. Earth is closest to the Sun at its perihelion; it is farthest from the Sun at its aphelion. In order for a total solar eclipse to have the maximum duration of totality, should Earth be at perihelion or aphelion? Assume that the Earth-Moon distance is the same in both situations. As part of your explanation, draw two pictures like Figure 3-11, one with Earth relatively close to the Sun and one with Earth relatively far from the Sun.

45. *On March 29, 2006, residents of northern Africa were treated to a total solar eclipse. (a) On what date and over what part of the world will the next total eclipse of that series occur? Explain. (b) On what date might you next expect a total eclipse of that series to be visible from northern Africa? Explain.

Discussion Questions

46. Describe the cycle of lunar phases that would be observed if the Moon moved around Earth in an orbit perpendicular to the plane of Earth’s orbit. Would it be possible for both solar and lunar eclipses to occur under these circumstances? Explain your reasoning.

47. How would a lunar eclipse look if Earth had no atmosphere? Explain your reasoning.

48. In his 1885 novel King Solomon’s Mines, H. Rider Haggard described a total solar eclipse that was seen in both South Africa and in the British Isles. Is such an eclipse possible? Why or why not?

49. Why do you suppose that total solar eclipse paths fall more frequently on oceans than on land? (You may find it useful to look at Figure 3-13.)

50. Examine Figure 3-13, which shows all of the total solar eclipses from 1997 to 2020. What are the chances that you might be able to travel to one of the eclipse paths? Do you think you might go through your entire life without ever seeing a total eclipse of the Sun?

Web/eBook Questions

51. Access the animation “The Moon’s Phases” in Chapter 3 of the Universe Web site or eBook. This shows the Earth-Moon system as seen from a vantage point looking down onto the north pole. (a) Describe where you would be on the diagram if you are on the equator and the time is 6:00 p.m. (b) If it is 6:00 p.m. and you are standing on Earth’s equator, would a third quarter moon be visible? Why or why not? If it would be visible, describe its appearance.

52. Search the World Wide Web for information about the next total lunar eclipse. Will the total phase of the eclipse be visible from your location? If not, will the penumbral phase be visible? Draw a picture showing the Sun, Earth, and Moon when the totality is at its maximum duration, and indicate your location on the drawing of Earth.

53. Search the World Wide Web for information about the next total solar eclipse. Through which major cities, if any, does the path of totality pass? What is the maximum duration of totality? At what location is this maximum duration observed? Will this eclipse be visible (even as a partial eclipse) from your location? Draw a picture showing the Sun, Earth, and Moon when the totality is at its maximum duration, and indicate your location on the drawing of Earth.

54. Access the animation “A Solar Eclipse Viewed from the Moon” in Chapter 3 of the Universe Web site or eBook. This shows the solar eclipse of August 11, 1999, as viewed from the Moon. Using a diagram, explain why the stars and the Moon’s shadow move in the directions shown in this animation.