Why Does the Moon Change Shape?

Contributor: Meghan Vestal. Lesson ID: 11037

If there is a full moon, is there an empty moon? Why do we get a "new" moon every month? Do you have a ball and some Oreos? Learn about and observe the lunar phases and create your own moon phase log!


Earth Science

learning style
Kinesthetic, Visual
personality style
Grade Level
Middle School (6-8)
Lesson Type
Dig Deeper

Lesson Plan - Get It!


The moon appears in different shapes, and some nights it does not appear at all!

  • What causes these lunar changes to occur?

Every night, when you look up at the moon, you should see something a little different.

moon phases

The shape of the moon depends on its position in relationship to the Earth and the sun.

The moon is a satellite, a body that orbits, or revolves, around a larger body. When you are able to see all or part of the moon, it is because light from the sun is reflecting off its surface. The area that you are unable to see is not in a position to reflect the sun's light.

  • What about those nights when you cannot see the moon at all?
  • Where has it gone?

If you are unable to see the moon, it is because the moon is positioned between the Earth and the sun. In this position, the sun's light is shining on the side of the moon that is not facing Earth. Therefore, the sun's light is being reflected away from Earth, making the moon seem invisible.

You can perform a simple demonstration to see how the sun's light reflects off the moon, causing it to appear in different shapes:

  1. Hold a kick-ball-sized ball at arm's length in front of you. This ball represents the moon, and you represent the Earth.
  2. Have a teacher, parent, or friend shine a flashlight on the ball as they walk around you. They represent the sun. You should remain stationary while the other person moves.
  3. As the other person moves, notice how the light shines on some parts of the ball while other parts remain dark.
  4. Then, ask the person to stand facing you, on the opposite side of the ball. When they shine the light on the ball, does any light show on the side of the ball facing you? This is how the Earth, the moon, and the sun are positioned when you are unable to see the moon at night.

You are probably familiar with what a full moon looks like.

  • But did you know that each of the moon's shapes has a name?

These are the phases of the moon. Look at the image below to see each of the moon's phases:

moon phases


The new moon is the first phase of a lunar cycle. It takes about 29-30 days to complete one lunar cycle.

Learn more about each of the moon's phases by watching Crash Course's Moon Phases: Crash Course Astronomy #4:


  • Do you recognize each of the phases?

Pay attention to the moon during the next few days and see if you can identify the phases.

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