series of images showing the phases of the moon

Arms Around the Moon

An activity by Padi Boyd

related to the song:
'Lunar Love'

Anyone on Earth who has looked up into the clear sky a few times knows that the Moon goes through phases. But why? Here's an activity that can be done with any class to demonstrate how and why the phases of the Moon occur. First graders may get something different out of it than high school seniors, but everyone can gain a concrete, hands-on understanding of lunar phases.

Before you begin, collect the following:

  1. A model Sun - a fairly bright light source, on a stick, that can be placed in a central location in the classroom. A bare 100 W light bulb in a floor lamp without a shade should do fine.
  2. For each student, one wooden skewer, such as those commonly used for the Thai dish chicken sate. These can be found in the international foods section of most grocery stores. A metal shish-ka-bob skewer will also work.
  3. For each student, one ping pong ball, with a hole pre-drilled into it so that the ping pong ball can be placed on the wooden skewer.
Have each student construct their own Moon model by placing a ping pong ball on the skewer. (Optional: students may first decorate their Moon with craters using paint or ink. A Moon with discernable surface features can be used to demonstrate how the Moon rotates exactly one time around its axis for each orbit about the Earth.)

Draw the blinds in your classroom windows, so as little light as possible comes through. Ask the students to arrange themselves in a circle around the model Sun. Once students are in place, turn on the model Sun, and turn all other classroom lights off. The activity works best when your model Sun is the only source of illumination.

Have students all face the Sun. Then ask students to hold their Moon at arms length toward the Sun. Ask students to look at their Moon, as well as the Moons held by the other students in different places around the circle. Each student should notice that their Moon does not appear to be illuminated. The Moon halfway across the circle should appear fully illuminated. The Moons of their nearest neighbors have just a sliver (a crescent shape) illuminated.

Explain to the students that, just like the Moon, the ping pong balls aren't giving off their own light. We see the light they reflect from the Sun. And this depends on our position with respect to the Sun, and on the Moon's position as well.

Now your students will demonstrate all the phases of the Moon for their own Moon model, by recreating the geometry of the Sun-Earth-Moon system.

Ask the students to look at their Moon again. They should hold their Moons in their left hands, with their arm extended straight in front of them. The side of the Moon that is illuminated by the Sun is facing away from them. (Note, don't block the Sun with the Moon, unless, of course, you want to simulate a solar eclipse!) This represents the new Moon. When the Moon is new it rises in the sky at the same time as the Sun. But the Moon is orbiting the Earth. And the result of this is that it moves in the sky, to the East with respect to the Sun. The student's view represents the view from Earth.

One day later, the Moon is about 12 degrees East of the Sun. We represent this by moving our outstretched left arm slightly to the left (counter-clockwise). Notice that we can now begin to see some of the illuminated surface of our Moon. This is the crescent phase. Keep moving to the left (eastward in the sky). The crescent is growing in size. This is called a waxing crescent.

Have students extend their left arm 90 degrees from the Sun - in other words, straight out to the side. What phase does the Moon look like now? (Answer: first quarter). Notice we're one quarter around a full circle, hence the name of the phase. Now if the Sun rises at 6 AM, about what time does the first quarter Moon rise? Students might be able to deduce this from the relative positions of the Sun, the Moon and themselves (the Earth). The Sun will be directly overhead when the first quarter Moon is on the horizon. So the first quarter Moon rises at noon.

Ask the students to turn with the Moon now and watch the phase continue to change. They will notice it growing from first quarter toward full. This is the waxing gibbous phase. When students have the Sun directly behind them, they should see that the Moon is totally illuminated (full moon). Do some students not see a completely illuminated Moon? Make sure their heads aren't blocking the light from the Sun (unless, of course, you want to simulate a lunar eclipse!). What time does a full Moon rise? (Answer: at sunset, approximately 6pm.)

Have students complete the full circle, noticing the Moon's phases now going through the waning gibbous toward third quarter (which rises at midnight), back through the waning crescent phases toward the next new Moon. We go from new to full to new in 29.53 days.

If your students had a feature painted on their Moon, they probably naturally kept that feature pointed toward them while revolving through one orbit. This is, of course, what really happens: we see only one face of the Moon from the Earth, because the Moon's orbit is "phase-locked": it rotates with the same period as it revolves around the Earth.

Ask them to demonstrate how a Moon that doesn't rotate in this way would look through one orbit (i.e. the feature on the Moon would point in the same direction with respect to a fixed spot in the room throughout the orbit.) Then, at full Moon, we'd see the opposite side of the Moon than at new Moon, since features that were facing us at new Moon were also facing a point directly behind us (on a wall, say). A Moon with no rotation would keep those features pointed toward the same point on the wall.

Having done the full cycle of lunar phases, you may now ask your students to demonstrate why lunar eclipses always happen at full Moon and solar eclipses happen at new Moon.

Through the month that follows this lesson, you might ask your pupils what phase the Moon is currently in. What time does it rise? What time does it set?