University of Michigan - Department of Astronomy

Version: short

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



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 is the true cause is and learn to make predictions yourself.

Some of the questions below will have a set of words in brackets: 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 line called that the Sun is on? Is the Moon also on that line?
  2. Read the date off the line the Sun is on. Enter the date in table 1
  3. Did the Moon rise before the sun, after the sun, with the sun, or 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 the Sun is down (it is dark out) and after the Sun if it rises while the Sun is up (it is light out). For example, if it rises 10 minutes before sunset the Sun is still up, so it rises after the Sun. If instead it rises 10 minutes after sunset, the Sun is down and it is getting dark, so the moon rises before the sun.
  4. Label this position as 3rd quarter on Figure 2 below table 1 (see also fig. 1 above).

The planetarium operator will move the projector ahead to the next phase. Watch the Moon as it moves to this new position and be prepared to select the correct answers from the sentences below.

  1. The Moon traveled [eastward, westward] along the [ecliptic, equator]. When it started moving, the lit fraction got [larger, smaller] which means it was [waxing, waning]. It moved [closer to, farther from] the Sun. It passed through the [new, full] phase and when it stopped, the lit fraction was getting [larger, smaller] so it is [waxing, waning].
  2. Fill out the second row in table 1 and label this position in figure 2.

The planetarium operator will move the projector ahead to the next phase. Watch the Moon as it moves to this new position.

  1. Fill in the third row in table 1 and label figure 2.
Table 1: Phases of the Moon
Phase Waxing, waning, full or new Lit on... (E, W, full, new) Rises: before Sun, after Sun, with Sun, around sunset Date Days since 3rd qtr
3rd Quarter waning       0



Figure 2: Lunar position for each phase


Concluding Questions

  1. How long does it take the moon to go through a full cycle of phases?
  2. The Moon always travels [eastward, westward] compared to the stars. During the course of a single day however, it travels [eastward, westward] through the sky.
  3. A waxing moon will be lit on the [left, right] when viewed from Ann Arbor. 24 hours latter, it will be [larger, smaller] and [closer to, farther from] the Sun, and it rises [before, after] the Sun.
  4. The waxing crescent moon falls between new and first quarter. Mark this position on figure 2 and label it "waxing crescent".
  5. A friend of yours comments that when he was driving home 5 evenings ago just after sunset, he saw a beautiful crescent moon near the horizon in front of him.
    1. What general direction was he driving? [N, S, E, W]
    2. Which crescent was it? [waxing, waning]
    3. Label the position of this phase in the figure here:phases
    4. 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.
  6. Based on your observations in this activity, explain the cause of the Moon Phases.

Updated: 7/9/10 by SAM

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