Demonstrating Moon Phases

from Lake Afton Public Observatory

Teacher's Activity Guide


This demonstration will give students a chance to experience first-hand the relative positions of the sun, Earth, and moon. By examining these positions, they will discover why the moon goes through phases.

Materials Needed:

Bright Light Source, Styrofoam ball (2" to 3" across), Pen or small stick


     1.   Depending on the level of your class, you may decide to do only a
          portion of this activity.
     2.   If you are only interested in demonstrating why the moon goes through
          phases, it is only necessary to do the Motion and Phase of the Moon
          section.  This part of the activity will take students through half of the
          moon's cycle -- new moon to full moon.
     3.   If you are interested in having your students also know the moon's
          position in the sky at each phase, then you should complete the next
          activity as well.


Anyone that has watched the moon in the sky for a period of two weeks has noticed that it seems to change its shape, from a thin crescent to a bright white ball. These changes in the moon's shape or phase are due to the moon's motion around Earth and how we view it.

The moon, like every object in the solar system, has half of its surface sunlit and the other half dark. As the moon orbits Earth, we see different amounts of the moon's sunlit half. At new moon the moon's sunlit half is facing away from us; at first quarter we see half of the sunlit half; while at full moon, we see the entire sunlit half. At no time does the shadow of Earth fall on the moon to cause its phases.

One of the difficulties with most models which demonstrate the phases of the moon is that the student must imagine himself or herself standing on the ball which represents Earth. In this activity the student's head is the Earth, a bright light source is the sun, and a styrofoam ball on a stick or pen held at arms length represents the moon.

Procedure / Discussion Questions

Motion and Phase of the Moon

1. Have your students place the styrofoam ball on the pen or small stick. This ball represents the moon. Have them hold the ball so the light shines on it. Light is shining on how much of the ball?

Half of the ball is lit up.

If the ball is held in a different place, is more, less, or the same amount of it covered with light?

Regardless of where the ball is held, half of it is always light.

2. Now have them face the sun and hold the ball in front of them. As they look at the ball do they see any of the light that is falling on it?

They should not see any of the light falling on the ball. Why don't they see light falling on it?

The half of the ball that is lit is facing away from them.

3. Have them move the ball slowly to their left (counterclockwise). What begins to happen?

They should begin to see part of the ball's lighted half.

4. How far must the ball be moved before half of the lighted side is visible?

The ball must be moved one-quarter of the way around their head.

Before all of the lighted side is visible?

half-way around

5. What happens if you keep moving the ball in the same direction?

They begin to see less and less of the lighted half of the ball.

6. The bright light represents the sun and the styrofoam ball represents the moon. As the moon revolves around Earth we see different amounts of its sun lit half. This causes the phases of the moon! Where is the moon in its orbit at:
a crescent? About one-eighth of the way around
first quarter? One-fourth of the way around
full moon? Half way around
third quarter? Three-quarters of the way around
new moon? Directly between the Sun and Earth at the beginning of the cycle

Procedure / Discussion Questions

Location of the Moon

The position of the moon in our sky at different phases can be demonstrated. The position of the sun and Earth will first be used to establish directions.

1. Have your students stand facing the sun (bright light). What time of day does this represent?

Since they are facing the sun, the sun is overhead in the sky. Thus it is noon.

What direction are they facing?


2. Which direction is to their left? East

to their right? West

3. Now have them turn so the sun sets. In which direction did they have to turn (left to right or right to left)?

In order to have the sun set in the western part of the sky, the students will have to turn to their left.

4. What time is it when the sun is:
directly to their right? Sunset
directly to their left? Sunrise
directly behind them? Midnight

5. To get some idea of where the moon is in the sky at different phases have the students turn so the sun is setting.

What direction are they facing? They are facing south.
Left and right are what directions? Left is east and right is west.

6. Now have them hold the moon so that it is in the same direction as the sun. This is the new moon phase since the sun lit half of the moon is facing away from us. Now have them move the moon in its orbit (to the left).

What happens?

They begin to see part of the lighted half of the ball -- a crescent moon.

7. In what direction should they look to see the crescent moon at sunset?

8. Have the students move the moon so it is in front of them.

What phase is this? First Quarter
In what direction is the moon? Due south

9. Have the students continue until they reach full moon.

In which direction should they look for a full moon?
In the east

What happens if they move the moon further in its orbit?
It can no longer be seen.

When should they look for it?
They must look for it later than sunset.

10. Repeat the above procedure, except have the students turn so the sun is rising.

What direction are they facing now? South
What directions are right and left? Left is east and right is west.
Where should they put the moon in its orbit so that it is full? On the opposite side of the sun, in the west.

Have them slowly move the moon in its orbit. How does the phase and position change?
As the moon moves toward the east, the students will see less of the sunlight half.

Why is this different from before?
Before they were seeing more and more of the sunlit half as the moon moved from the west to east and now they will see less and less.

Additional Activity

This same kind of activity can be used to illustrate why the constellations appear to move from east to west during the evening. Just have the students turn and notice that objects in the room that were once to their left move in front of them and finally to their right. The same thing happens with the constellations.