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Phases of the Moon

Rebecca Teed, Wright State University
This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
Initial Publication Date: January 17, 2006

Summary

This exercise has students use a simple physical model of the Earth, sun, and moon to understand why the moon changes phases from the perspective of Earthly observers. Students hold up balls representing the moon in a room with a single light source (representing the sun). As they turn relative to the sun, holding the "moon" out in front of them, they will see changes in how much of the side that they can see is lit, but they can also check and see that overall, half of the "moon" is lit (the side facing the sun), no matter what position it is in relative to them. It's all a matter of perspective.

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Learning Goals

This experience is intended to enable students to
  • Understand why the moon changes phases
  • Recognize the importance of an observer's perspective
  • Get personally involved in the lesson

Context for Use

This exercise will take about half an hour, depending on how long the students have to work out their initial and final explanations of why the moon changes phases.

Teaching Materials

Each student or group of student gets a sphere, tennis-ball size or larger, to represent the moon.

A grapefruit illuminated by a lightbulb shows phases too

Ask the students, in groups of four, to work out an explanation of the phases of the moon that they can write down (legibly) on an index card that they then hand in (for a participation grade, the information is not expected to be correct). Students then pick up the spheres, and the instructor turns out all of the overhead lights (make sure windows are covered by curtains or blinds) and turns on one light source, taking the role of the Sun (a powerful one if teaching in a lecture hall).

Have each student with a "moon" hold it up in front of them as they face the instructor. Ask them what phase the moon is in, assuming that they are the Earth ("new moon"; make sure they agree on this). Then ask them (or students without "moons", if there are any) "How much of the moon is lit?" (one-half, make sure everyone can see this, if need be by looking over their shoulders at the "moons" behind them).

Next, have the students with "moons" turn 90 degrees to their right. "What phase is the moon in now?" (one-quarter, crescent). "How much of the moon is lit?" (one-half). Then have the students turn until their back is to the "sun" and hold the moon up high so that no shadow falls on it. "What phase is it in now?" (full). "How much of the moon is lit?" (still one-half).

Turn the lights on and have the student groups once again work out an explanation of the phases of the moon that they can write down (legibly) on an index card that they then hand in.


Teaching Notes and Tips

Items for further discussion include: lunar eclipses, the "dark" side of the moon, whether it's possible to see a crescent moon directly overhead at midnight.


Assessment

The index cards allow the instructor to take roll (and assign participation grades) and to see how well students understood the lesson and how much it changed their previous ideas.

References and Resources

Related Web Sites:
  • A Private Universe Project (more info) : A very similar version of this exercise, intended for young kids, includes excellent questions that could be substituted for the index-card exercise that will help reveal student misconceptions about moon phases.
  • Phases of the Moon (more info) : This site contains a series of visualizations of the sun/moon/Earth system ans of the changing face of the moon in the form of Java applets, forms for field observation of the moon, and a collection of exercises and background material (as .pdf's).