University of Michigan - Department of Astronomy

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The Cause of Moon phases

O swear not by the moon, the inconstant moon,
that monthly changes in her circled orb,
lest that thy love prove likewise variable

-- Romeo and Juliet, Act 2 scene 2

Overview

Introduction

There are many misconceptions about what causes the phases of the Moon and the unpredictability of the phases. In fact, the phases are very predictable and have a fairly straightforward explanation. Through careful observation, you will determine what the cause is and learn to make predictions yourself.

Some of the questions below will have a set of words in parentheses: e.g. (before, after, at the same time as). Circle the best word from the set to complete the statement.


Part 1: Planetarium Observations

The show starts with the moon in 3rd (or last) quarter, a waning phase.This means the moon appears to get smaller night after night. The position of the 3rd quarter moon is shown in the following sketch. Note that the angle between the Sun, Earth, and Moon is 90º and the Moon is 3/4 of the way around its orbit.

last quarter

Observe the moon in the planetarium

  1. The Moon is lit on the (right, left) which corresponds to the (east, west) side. Fill in the east/west in table 1.

Locate the Sun. 

  1. What is the date?



  2. The Sun is (east, west) of the Moon. Enter this value in table 1. Does this make sense given your answer to question 1? Explain.







  3. How many degrees apart are the Sun and Moon (note one "day" on the ecliptic is roughly one degree)? Enter this value in table 1. Does this agree with the picture at the beginning of this section (Note the planetarium gears are only accurate to about 5 degrees)?





  4. The Moon rose (before the sun, after the sun, with the sun, as the sun set). Enter this answer in the last empty column in row 1 of table 1. NOTE: the moon rises before the Sun if it rises while it is dark out and after the Sun if it rises while it is light out. For example, if it rises 10 minutes before sunset it rises after the Sun, but if it rises 10 minutes after sunset, it rises before the sun.
  5. Label this position on Figure 2 below table 1.

The planetarium operator will move the projector ahead to the next phase.

  1. What is this phase? How is it different from 3rd quarter: is it bigger or smaller, lit on the same side or opposite side, closer or farther from the Sun, rises before or after the Sun...?






  2. Fill in the second row of table 1 and label this phase on figure 2.

The planetarium operator will move the projector ahead to the next phase

  1. Fill in the third row in table 1 and label figure 2.

The planetarium operator will move the projector ahead to the next phase

  1. Fill in the 4th row in table 1 and label figure 2.

The planetarium operator will move the projector ahead to the next phase

  1. Fill in the 5th row in table 1 and label figure 2.

The planetarium operator will move the projector ahead to the next phase

  1. Fill in the 6th row in table 1 and label figure 2.

The planetarium operator will move the projector ahead to the next phase

  1. Fill in the 7th row in table 1 and label figure 2.

The planetarium operator will move the projector ahead to the next phase

  1. Fill in the 8th row in table 1 and label figure 2.

The planetarium operator will move the projector ahead to the next phase

  1. Fill in the 9th row in table 1.

Table 1: Phases of the Moon
Phase Wax, wane, full or new Lit on... (E, W, full, new) E, W, Aligned with, or Opposite the Sun Degrees from the Sun Rises: before Sun, after Sun, with Sun, around sunset Days since 3rd qtr
3rd Quarter waning          0
             
             
             
             
             
             
             
             

Figure 2: Lunar position for each phase

phases

Concluding Questions

  1. How can you tell at a glance if a moon is waxing or waning?


  2. How long does it take the moon to go through a full cycle of phases?


  3. How long (on average) does the moon take to go from one position to the next in the sketch above?


  4. Compare the "degrees from the Sun" column of table 1 to figure 2. Do the numbers in the table generally agree somewhat well or very poorly with the figure? Is the planetarium very accurate at portraying the positions and phases of the moon?







  5. Why wasn't there an eclipse at the new or full moon positions?




  6. Which quarter moon will you see in the evening (near sunset)? Explain how you figured this out.



  7. How long will it be from that quarter moon until full moon?



  8. What phases can you see at noon (assuming a clear horizon)?  Explain how you figured this out.




  9. What phases can you see at midnight (assuming a clear horizon)? Explain how you figured this out.


  10. A friend of yours comments that when he was driving home 4 evenings ago, he saw a beautiful crescent moon near the horizon in front of him.
    1. What direction was he driving?




    2. Which crescent was it?




    3. Draw a sketch of what it would have looked like. Be sure the crescent is lit on the correct side and tilted in the right direction.














    4. What phase is the moon in now?







  11. You mention to someone in a study group from another class that you're studying the phases of the moon in astronomy class. One person says that should be easy, it's just the shadow of the Earth. Explain why that can't be the cause (give at least 2 reasons), what really causes the phases, and how they can observe that you're right.
















The questions below require information beyond what is covered above. Ask your GSI which of them you should answer.

  1. What are some reasons a projector like this one would not be very accurate at displaying the correct position and phase of the Moon (hint, ask your GSI to point out the Moon projector and look at how it moves)








  2. Explain why the full Moon is low in summer but high in winter.














  3. Explain why spring and fall are the best time to observe a very young or very old moon.







  4. Explain why moon maps and posters are usually made by combining 1st and 3rd quarter moon images instead of a single full moon image.







  5. Why can't you measure the height of the mountains or depth of craters on a full moon?










  6. If a base were built on the near side of the moon, would the residents see sunrise? Would they see "earthrise"? Explain.





















Updated: 7/9/10 by SAM

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