Phases of the Moon

Explaining Phases of the Moon

[from the planetarium show Moons of the Solar System.]


1. Planetarium

Your planetarium must have the capabilities of diurnal motion and Moon phases with proper position relative to an image of the Sun.

2. Sun Simulator

In the center of the planetarium, mount a short unfrosted tubular light bulb (about 25 to 40 watts) for simulating the Sun. Such a clear, single filament bulb is necessary to create crisp shadows on the model Moons. Ideally, supply electrical power to the bulb through a dimmer switch. Place a top shade over the bulb to prevent reflection of white light from the dome. (See diagrams for ideas for how to make Sun Simulation Light.)

A wire stand can be fashioned to support the shade/cover over the tubular light.

3. Moon Models Make a class set of moon models that can be reused for each program. These can be made with white polystyrene balls, about 2”-3” in diameter, mounted on sticks or pencils. Such balls are made by Plasteel Corp. in Inkster, Michigan. Google “polystyrene balls.” Polystyrene balls are also available at craft and hobby stores. Styrofoam does not work as well, since it is translucent.

Presenter's Script:

Why does the Moon seem to change shape? [Accept any answers.]

Practically every culture throughout human history has come up with a different explanation in answer to that question. One rather interesting theory was invented by the Egyptians who believed that a new Moon was born each month (literally) and it grew and grew until it was full. At the moment the Moon reached the fullness of maturity, a giant pig attacked it and kept feasting on it for the rest of the month until there was no Moon left, at which time a new Moon was born.

Do you think that’s really how the Moon changes its shape?

Even though we don’t believe this explanation now, in its time this was a perfectly good explanation and accounted for the phases of the Moon quite well.

The theory most accepted in modern times has to do with relationships between the Earth, Sun and Moon. To see how this works let’s make a working model.

Will everyone please stand up?

Turn on the white light (tubular bulb) to about 1/2 brightness.

Let’s pretend that this light is the Sun. Pretend that your head is the Earth. All you need to complete the model is the Moon.

Give each student a ball on a stick.

Hold your Moon so that it is directly in front of the Sun.

Does your Moon look dark? [Yes.]

At this time of month, in reality, the Moon is so dark you could not see it at all. The Moon doesn’t stay in one place; it orbits (goes around) the Earth. Slowly move your Moon to your left, just beginning the orbit of the Moon around the Earth (your head). Move the Moon until you see a small part of it lit by the sunlight.

Go around and check to see each student understands and is observing the crescent Moon.

What shape would you call that lit part of the Moon? [Crescent.] Does it look like the shape of the Moon when we first saw it in the planetarium sky today? [Yes.]

Now continue the Moon in its orbit, moving it slowly to the left, until you see a half disc lit up, which you may recall is the “quarter Moon.” Notice that when the Sun is setting, from your viewpoint, the first quarter Moon is directly overhead.

Continue moving your Moon in its orbit until you see the gibbous phase (nearly full). Now try to hold your Moon in a place where it is fully lit and could be called a full Moon.

Let the students discover the shadows of their heads. If necessary, hint that they hold their Moons above those shadows.

We have now modeled the Sun-Moon-Earth system so that we have seen everything that we observed in the planetarium sky. The lit part of the Moon grew from nothing to full and in reality it takes about two weeks for it to do that. However, the Moon does not stop there in its orbit.

What do you think happens as the Moon continues in its orbit? [Appears to get smaller.]


If your head represents the Earth, the shadow of your moon ball should be only about 1/20 inch (1 mm) wide. (In that scale, the light bulb (sun) should be about a mile and a half away and your arm would have to be 20 feet long to hold your moon ball at the proper distance from your head!)

Try slowly moving your Moon the rest of the way in its orbit around its Earth. Try going slowly through a couple more orbits so you can observe the complete cycle of the phases of the Moon. In reality, it takes about one month for the Moon to complete such a cycle (29.5 days, to be exact; for young students it is fun to refer to this as a “Moonth.”)

While we have this model working, let’s see if we can explain a couple of other kinds of events that have startled and terrified people through the ages. Hold your Moon right in front of the Sun so that it blocks the Sun.

What is the name for the event in which the Moon blocks the Sun? [Solar eclipse.]

While you hold your Moon so that it blocks the Sun, look around the room at the other “Earths” which are other peoples’ heads.

Do you see the shadows of their Moons on them? What phase must the Moon be in for there to be a solar eclipse? [New.]

In the real Sun-Earth-Moon system, that shadow of the Moon on the Earth during a solar eclipse is only about fifty miles (80 km) wide. In comparison, the whole Earth is about 8000 miles (13,000 km) wide.* During a total eclipse of the Sun, only people located in that narrow shadow region can see the eclipse.

Now move your Moon around until it moves into the shadow of the Earth (your head).

What is this type of eclipse called? [Lunar eclipse.] What phase must the Moon be in for there to be a lunar eclipse? [Full.]

People on the whole night time half of the Earth, the half that points away from the Sun, can observe a total lunar eclipse.

Could people who live on the back of your head see the Moon move into the Earth’s shadow? [No, it’s daytime for them.]

Many more people have seen lunar eclipses than have seen solar eclipses. This is because whenever a lunar eclipse occurs, people on half of the Earth have the opportunity to see it, but to see a solar eclipse, you must be where the comparatively tiny shadow of the Moon sweeps across the Earth.


Show image of Lunar Eclipse.


Show image of Full Moon while telling Egyptian story of the giant pig eating the Moon.