Causes of the Phases of Moon

Kinesthetic lessons in astronomy

Kenneth Fuller
(copyright information 2004)

Moon's Phases

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 eclipses.  Remember, a concept is rarely mastered with the first exposure.  Repeated review in different forms is important.

 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 revolves.

A ball of convenient size, preferable white, to serve as a model of Moon.

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".

Caution:  When 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 Earth 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, we 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 revolves around Earth).  And not always the half we can see from Earth, but always half.  We'll worry about eclipses later.

2.  Have the student  stand with back to the Sun (light source).

3.  Holding the ball at arm's length completely in the light (north of Earth's shadow). Full moon.  Six months later full moon will be 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?

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