Full Moon: Definition, Facts & Features

Full Moon: Definition, Facts & Features
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  • 0:00 Phases of the Moon: Full
  • 0:50 The Lunar Cycle
  • 2:05 Fact vs. Fiction About…
  • 4:04 Lesson Summary
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Lesson Transcript
Instructor: Mary Ellen Ellis
The full moon has long been a part of myths and legends in cultures around the world. We see a full moon approximately once a month because of how the earth, sun, and moon align. Learn more about it in this lesson.

Phases of the Moon: Full

As seen from our vantage point on Earth, the moon is always changing. You can imagine what people in ancient times must have thought about this. We know now that the moon seems to change because its position relative to the earth and the sun changes. The transition from no moon to full moon and back again inspired many myths and legends in older times, before scientists figured out what was really happening.

A full moon occurs when one side of the moon is fully illuminated from our perspective on Earth. The sun, earth, and moon are lined up, with the earth in the middle. This positioning allows full illumination because the rays of the sun are fully and directly hitting the moon from an Earth perspective. Even with the earth in between the moon and the sun, we can still see this full illumination.

The Lunar Cycle

The moon, as it appears to us on Earth, changes its shape as the positions of the earth, sun, and moon shift relative to each other. Of course, the moon isn't really changing shape, but how we see its illumination by the sun does. The changing illumination occurs in a cyclical manner, which means that you get to see a full moon about every 29 days, or roughly once a month. This is known as the lunar cycle. This lunar cycle is slightly less than the length of a calendar month, so every once in a while we get two full moons in the same month. We call that second monthly full moon a blue moon, and it happens once every two to three years. This is where the term 'once in a blue moon' to refer to something uncommon comes from.

The full moon is the halfway point in one lunar cycle, which begins with the new moon. The new moon occurs when we can't see it because it is between the earth and the sun. The side of the moon that is illuminated by the sun is facing away from us. As the moon revolves around the earth, the illuminated side appears to us a sliver at a time. From the new moon, it grows into the full moon and then shrinks back to the new moon at the end of one lunar cycle.

Fact vs. Fiction About Full Moons

The moon has played an important role in myths and legends throughout human history - especially the full moon. There are some interesting facts and fictions about the full moon. One fact is that a lunar eclipse can only occur with a full moon. A lunar eclipse occurs when the moon passes through the earth's shadow. This can only happen when the earth is between the moon and the sun - the same conditions for a full moon. We don't get an eclipse with every full moon, though. This is because the moon's orbital plane around the earth is tilted. When the moon is in the full position, it is often above or below the earth's shadow.

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