Key concepts
Sun and moon
Solar system
Orbits

From National Science Education Standards: Changes in Earth and sky

Introduction
Why does the moon seem to change shape each night?

The moon itself, of course, isn't changing. But because it is moving around Earth as we move around the sun, the amount of light that we see reflecting off the surface of the moon varies from day to day. About every two weeks, the moon goes from being nearly invisible—what is called a new moon—to being bright and full in the night sky—a full moon.

An easy way to understand these changes is by using a lamp and an orange to create a rough model of the moon's orbit around Earth.

Background
Here on Earth, we see the moon cycle through its phases about every 29.5 days (which is slightly different from the amount of time it takes the moon to orbit around Earth, which is 27.3 days). The different appearance of the moon in the sky—from a skinny sliver to a great, round full moon—is the result of the moon's position relative to Earth and the sun.

A lunar eclipse happens when the moon is lined up directly on the other side of Earth from the sun so that Earth essentially casts a big shadow over the moon. Because a lunar eclipse depends on Earth's and the moon's orbits being just right, one doesn't happen every time there is a full moon. The moon usually passes just under or above Earth's shadow.

On the other hand, a solar eclipse happens when the moon is lined up directly between Earth and the sun, so the moon casts a shadow on Earth's surface. Solar eclipses don't happen every month for the same reason there isn't a lunar eclipse every month. At new moon, Earth usually is slightly below or above the moon's shadow.

Materials
•    Lamp without a lamp shade (preferably, a bulb no brighter than 40 watts, which is easier on the eyes)
•    Dark room
•    Orange
•    Sharpened pencil

Preparation
•    Place the lamp (without its lamp shade) on a table or other surface. A good height is just above head level of the person doing the activity.
•    Carefully push the sharp end of the pencil through the center of the orange deep enough so that it is stable if you hold the pencil.
•    Turn on the lamp and darken the room. Get ready to observe your orange moon! (Be careful not to touch the lit bulb, which is very hot.)

Procedure
•    Stand at least arm's length from the lamp light source, and face it directly. The lamp is your sun, and your head represents Earth.
•    Hold up the pencil-topped orange moon out with a straight arm in front of you. Hold the orange slightly above or below the bulb, and don't look directly at the lamp. What does the surface of the orange moon that you can see look like? Is it relatively dark?
•    Now, with your arm still outstretched, start rotating your body slowly to the left. Can you see a crescent of light begin to creep onto the surface of your orange moon?
•    Once you have turned about 90 degrees from the lamp, your moon should be at the first quarter phase. Which side of the moon is the light is falling on (the left or the right)? Does the moon like a "D" or a mirror image of a "D?"
•    Keep slowly rotating in place with your arm out straight. When you arrive with your back to the lamp, what phase is the orange moon in? (Tip: if your head or body is blocking the lamp, try squatting down until the moon gets full light in this position—you should be looking at a completely illuminated full moon. When your head or body blocks the light so that the moon appears dark, you've created an eclipse. Is it a lunar or a solar eclipse?)
•    Continue turning slowly counterclockwise. Which direction is the light moving on the surface of your moon?
•    Once you are about 90 degrees from completing the full turn, you have reached the third quarter phase. How does this phase look different from the first quarter phase?
•    Finish rotating until you are back to the starting point: a new moon, when you (and people on Earth) only see the side of the moon the sun isn't hitting—the unlit nighttime side of the moon.
•    Extra: Try to create a lunar eclipse. These lunar eclipses happen when Earth blocks the sun's light from hitting the moon by lining up right between the two. If your head is Earth, can you create a lunar eclipse with your orange moon?

Read on for observations, results and more resources.

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