First Quarter Moon: Definition & Summary Video

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  • 0:01 Definition of a First…
  • 0:27 Phases of the Moon and…
  • 2:05 How You See the First…
  • 2:31 First Quarter Moon and…
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Lesson Transcript
Instructor: Mary Ellen Ellis
Have you ever wondered why a moon that looks like a half circle is called a first quarter moon? It has to do with the moon's trip around the earth and the lunar cycle as explained in this lesson.

Definition of a First Quarter Moon

Have you ever noticed when you look up into the night sky that sometimes the moon seems to be a perfect half circle? Would you be surprised to hear that it is officially called a quarter moon? How can that be when you can clearly see half of it? The first quarter moon is just one of many phases of the moon that we see during each lunar cycle. To explain and define it, we need to understand the movement of the moon around the earth.

Phases of the Moon and Lunar Cycle

The first quarter moon is just one of eight phases the moon goes through in one lunar cycle. It gets its name from the fact that it occurs one quarter of the way through this cycle. Another reason for the name is that, although it looks like you can see half of the moon, you are really only viewing one quarter of it. Three-quarters of the moon are in darkness and hidden from you.

The lunar cycle occurs because the moon revolves around the earth and each revolution takes about one month. As it moves around the earth, the part of the moon that is illuminated by the sun becomes visible to us to different degrees. When the moon is in between the earth and the sun, its illuminated side faces away from us, and we can't see it at all, which is called the new moon. When the earth is between the moon and the sun, that illuminated face is turned toward the earth, and we see the perfect circle we call the full moon.

As the moon revolves around the earth, revealing itself to different degrees, we get various glimpses of the surface of the moon. Sometimes the moon is full, other times it is no more than a thin crescent. These different views are called the phases of the moon.

As the new moon waxes, or grows, towards the full moon, it gets bigger and bigger. At the halfway point between the new and the full moons, we see what looks like a half circle. This is the first quarter moon. The full cycle from one new moon to the next is the lunar cycle, and the first quarter moon is one quarter of the way through the cycle. On the other side of the lunar cycle, at the three-quarter point, is another half-moon. This one is called the third quarter moon, or sometimes the last quarter.

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