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Educator's Guide to Moon Phases

Courtesy of the Jet Propulsion Laboratory

Easier Done Than Said

Phases of the Moon is at the top of the list of things that students seriously misunderstand. Most teachers run into problems in trying to explain the Moon's phases to youngsters, and evidence suggests that many have a very difficult time with the concepts. The problem starts immediately when the teacher uses a light piece of chalk on a dark board. Is he or she making the drawing as a positive or a negative?

The supplies for this activity are rather modest. Each student will need a light colored sphere of some sort. Ideally it can be placed on the end of a pencil. Try 5-centimeter (2-inch) or greater white Styrofoam balls. Get a larger sphere (15 centimeters or so) for your use as leader. You need a light source to serve as the Sun. A lamp with a bright bulb (400 watts) and the shade removed works fine. A dark room is also required.

With the lamp in the center of the room, have each student place the ball at arm's length between the bulb and their eyes. They should hold the pencil in their left hand. The bulb is the Sun, the ball is the Moon and they are Earth. The view from their eyes is the same for both this exercise and for observations of the real sky.

At the start, the "Moon" is blocking the "Sun." (This is actually demonstrating a total solar eclipse which is very rare for any given location on Earth.) Usually the Moon passes above or below the Sun as viewed from Earth. Have the students move their moon up or down a bit so that they are looking into the Sun. As they look up (or down) at their moon they will see that all of the sunlight is shining on the far side, opposite the side that they are viewing. This phase is called "new moon" (like "no moon").

They should now move their hand towards the left, about 45° (1/8) of the way around counterclockwise. Have them observe the sunlight on their Moon now. They should see the right hand edge illuminated as a crescent. The crescent will start out very thin and fatten up as the Moon moves farther away from the Sun. (Note: although the Moon is closer to the Sun during new and crescent phases, it is still 400 times closer to Earth; i.e., the Sun is VERY far away in reality.)

When their Moon is at 90° to the left students will see the right half of the Moon illuminated. This phase is called "first quarter." Remember that fully one half of the sphere is illuminated at all times (except during lunar eclipses) but the illuminated portion that we observe changes as the Moon changes position.

As they continue to move counter-clockwise past first quarter, the Moon goes into its "gibbous" phase (more than half but less than fully illuminated) which grows as the Moon moves towards 180°.

When the Moon reaches the position directly opposite the Sun, as viewed from Earth, the half viewed from Earth is fully illuminated (unless the student's head is causing a lunar eclipse). Of course only half of the Moon is illuminated. It has taken the Moon about two weeks to move from new to full. This growth in illumination is known as "waxing." The Moon chases the Sun across the (day and night) sky.

Students should now switch the pencil to their right hand and face in the general direction of the Sun. Starting with the Moon at full, students should continue the Moon's counterclockwise motion. They will observe the reverse of the Moon's phases seen so far with the left portion of the Moon illuminated.

After the gibbous phase diminishes, the Moon will reach the 270° position, straight out to the right. This is "third" or "last quarter." It is followed by a thinning crescent and a return to new moon. From full to new the Moon has been "waning" and leading the Sun. The phase cycle takes 29.53 days. Be sure to observe the real Moon! Most newspapers give the Moon phases along with the weather data.