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
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
They should now move their hand towards the left, about
(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
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
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.