Why does the Moon go through a Cycle of Phases?


After completing this activity, you should be able to:

MATERIALS (supplied by classroom teacher)


  1. Following your instructor's directions, complete the Moon Phase Diagram (supplied by your instructor).
  2. Answer the following activity questions.
  3. Using materials provided by the planetarium teacher, construct a "Moon Phaser." Visit the planetarium (students should bring the "Moon Phaser" to the planetarium).


  1. Why does the Moon go through a cycle of phases? (Before answering this question, you may need to refer to a demonstration of Moon phases.)

  2. Waxing and waning phases.

    1. During which phases does the shape of the Moon appear to grow larger (wax)?

    2. During which phases does the shape of the Moon appear to grow smaller (wane)?

    3. How can an observer on Earth recognize waxing from waning phases?

  3. Solar and lunar eclipses. (Note: Eclipses are named for the object which cannot be seen, e.g., during a solar eclipse, you cannot see the Sun.)

    1. During which phase(s) could a solar eclipse occur?

    2. During which phase(s) could a lunar eclipse occur?

    3. Why are solar and lunar eclipses so rare?

    4. You know that the Moon is much smaller than the Sun. Explain how the disk of the Moon can block most of the light from the Sun during a total solar eclipse.

    5. Is it safe to look directly at a solar eclipse? At a lunar eclipse?

  4. Using the following table, fill in the approximate time of moonrise and moonset. If necessary, refer to a "Moon Phaser."

    Phase Rise Highest Point Set
    New Moon 6 a.m. Noon 6 p.m.
    Waxing Crescent 9 a.m. 3 p.m.
    First Quarter
    6 p.m. Midnight
    Waxing Gibbous 3 p.m.
    3 a.m.
    Full Moon

    6 a.m.
    Waning Gibbous 9 p.m.

    Last Quarter

    Waning Crescent

  5. Which commonly used time unit (e.g., day, week, month, year, etc.) is derived from the revolution of the Moon around the Earth?

For Further Thought

When the Moon is in the waning gibbous phase, to an observer on the Moon, in which phase is the Earth?