Phases of the Moon: Lesson for Kids

Lesson Transcript
Instructor: Dominic Corsini

Dominic Corsini has an extensive educational background with a B.S. in Secondary Biology and General Science with a Minor in Environmental Education, an M.Ed. in Educational Leadership, an M.S. in Biology, and a K-12 Principal Certification Program. Corsini has experience as a high school Life, Earth, Biology, Ecology, and Physical Science teacher.

Why does the moon change throughout the month? What role does the sun play in all this? This lesson addresses these questions through a detailed investigation into phases of the moon.

What Is a Moon Phase?

When you look up at the night sky, you probably see the moon shining brightly overhead. Sometimes, the moon is perfectly round and full, and other times only a portion of it is illuminated. When something is lit up, it is illuminated, like a lamp you've just turned on. These different illuminated faces of the moon are what we call moon phase, which is the appearance of the moon in the night sky.

Moon phase is controlled by the positions of the earth, moon, and sun, and it slowly changes from night to night. Over the course of a month, the moon might appear full one night, then as a crescent shape only a few weeks later. To understand what causes this change, let's investigate the position of the earth, moon, and sun in outer space.

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  • 0:03 What Is a Moon Phase?
  • 0:51 What Causes Moon Phases?
  • 2:02 Various Moon Phases
  • 3:38 Lesson Summary
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What Causes Moon Phases?

Moon phase is caused by the position of the earth, moon, and sun. Once every 29 ½ days the earth finds itself positioned between the moon and sun. This means that the side of the moon facing the sun is completely illuminated by the sun's rays. From Earth, this is called a full moon because we see the entire face of the illuminated moon.

As the moon moves around the earth the view of its illuminated half appears to change. For example, during a waning crescent moon phase, half of the moon is still illuminated. However, from Earth we can only see a small sliver of that illuminated half. When this happens, the moon phase appears to be a crescent shape. That's because we're viewing the very edge of the illuminated half.

Getting it? At first it sounds confusing. People sometimes struggle to understand that half of the moon is always lit by the sun. It's just we can't always see that half. Consider a new moon for example. During a new moon we see nothing from Earth. That's because the side of the moon facing Earth also happens to be facing away from the sun. This means it isn't lit up for us to see. The back side of the moon is lit up, but it could only be viewed from outer space.

Various Moon Phases

If this makes sense, then we're ready to move ahead to the names for various phases. These names are fairly simple. To begin, let's start with the new moon, a phase in which the side of the moon facing Earth is dark. Therefore, during a new moon we cannot see the moon from Earth. Shortly after the new moon phase, we begin to see the right edge of the moon. This is called the waxing crescent moon phase.

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