This activity will help a
student of any age develop a clear comprehension of the complex
explanation of why the apparent shape of Moon (phase) changes from
night to night. It also illustrates that Moon is in the sky just
as often in day time as it is at night, though in daylight it is not as
noticeable. I have found that many students confuse phases with
a concept is rarely mastered with the first exposure. Repeated
in different forms is important.
Needed: Clear sunlight outside or a bright light in a
darkened room, outside at night. If the light is too close to a
reflective wall such as glass cabinet doors, reflected light may fill
in the shadows making the demonstration less clear. If a naked
bulb can be used, the student can revolve around it as the satellite
A ball of convenient size, preferable white, to serve as a model of
The student's head represents Earth.
A student work sheet for each student. The student
work sheet and answer key
can be printed by using the print command on your
browser. For best results, set the printer for "black only".
referring to models of objects of the Solar System and their orbits,
avoid using the terms; up, down, above, below. Their use tends to
reinforce some misconceptions of astronomical relationships.
While "up" and "down" describe valid relationships when
applied to the model, they are meaningless in relation to the objects
being modeled. See the discussion
( Redirecting thought processes ).
Note: The direction of the light represents the plane of Earth's
orbit around Sun. The direction of sunlight is in fact parallel
to the plane of Earth's orbit. The plane of Moon's orbit around
is at an angle about 5 degrees to the plane of Earth's orbit. How
ever, to make our models big enough to see and small enough to manage,
have to exaggerate the angle.
1. Show that one half of Moon is always lighted by Sun, not
always the same half (Moon rotates on its axis once each time it
around Earth). And not always the half we can see from Earth, but
half. We'll worry about eclipses later.
2. Have the student stand with back to the Sun (light
3. Holding the ball at arm's length completely in the light
(north of Earth's shadow). Full moon. Six months later full moon
south of Earth's shadow.
4. Rotateing slowly to the left, moving the ball so that when the
student faces the light, the ball's shadow is south of Earth.
In sunlight, a horizontal plane might work.
5. Make note of the apparent change in shape of the part of
the ball which is directly lighted.
6. Question: The book says that we see a full moon when Moon is
in the opposite direction from us as Sun, so why don't we see a lunar
eclipse every month?