Key concepts
Sun and moon
Solar system

From National Science Education Standards: Changes in Earth and sky

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.

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.

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

•    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.)

•    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.

Observations and results
How did the moon change as you slowly rotated? Were you able to see the familiar crescent and full moon phases?

Want to get even more specific with the moon phases? Between the new moon and first quarter, the moon is said to be a "waxing crescent". ("Waxing" is another word for getting larger.) Between the first quarter and full moon, it is said to be a "waxing gibbous". After the full moon and up to the last quarter, it is a "waning gibbous" moon. ("Waning" means getting smaller.) Any guess as to what we call it between the last quarter and new moon? (answer: a "waning crescent") What phase is the moon now where you live?

Like the sun, the moon also rises and sets each day, moving across the sky. But it is also often out during the daylight hours. (Have you ever noticed a pale moon in the sky on a clear sunny day?) During a new moon, the moon is actually rising about the same time as the sun, although you can't see it. Each day the moon comes up later, so that by the time it is a full moon, it rises about the same time the sun sets.

The moon often looks bigger as it's rising and smaller when it's high in the sky. This is an optical illusion: It looks relatively large because we are seeing it in comparison with objects on the horizon, such as buildings or trees. But this apparent size change as it travels across the sky has nothing to do with its distance from Earth, which averages about 238,866 miles (384,417 kilometers) away!

A lot of cultures have used the moon as a way to keep track of time. These systems are known as lunar calendars.

Bonus: The time for the moon to orbit Earth (27.3 days) is different from the time for the moon to go through all its phases (29.5 days). Why? Because Earth is not stationary but itself is moving around the sun! Because Earth takes about 12 months to go once around the sun, during the 27.3 days (about one month) that the moon is going around Earth, the Earth has moved about one twelfth the way around the sun. As a result, the moon has to go a little farther in its orbit to have the same appearance to us on Earth. In fact, we can estimate that this would take 27.3 + (1/12) X (27.3) = 29.5 days!

Share your orange moon observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Remove the pencil from the orange (being careful with the sharp point); turn off the lamp and use caution—the lightbulb is hot.

More to explore
"Is It Just a Coincidence That the Moon's Period of Rotation and Revolution Are Identical?" from Scientific American
"Why Do the Moon and the Sun Look So Much Larger Near the Horizon?" from Scientific American
Complete Sun and Moon Data for One Day from the U.S. Naval Observatory
"The Phases of the Moon" overview from Woodlands Junior School
The Moon Book by Gail Gibbons, ages 4–8
The Moon by Seymour Simon, ages 9–12

Up next…
Washed Away: Rivers and Streams in an Instant

What you'll need
•    Large, shallow rectangular pan or tray with edges (the plastic lid of a storage container will work, too)
•    Disposable plastic water bottle
•    Scissors
•    Flour or another light powder
•    Different color powder, such as cocoa powder, to represent "topsoil"
•    Small round snacks, such as seeds, to represent "rocks"
•    Medium-sized round snacks, such as small strawberries or dried figs, to be "boulders"
•    Longer snacks, such as carrot sticks, to represent obstacles
•    Cereal boxes (or other thin objects that can be used to prop up one side of the tray)
•    Sink (or other larger container to catch any excess water runoff)