What is a Waning Moon? - Definition, Facts & Phases

Instructor: Mary Ellen Ellis
As the moon shrinks in the sky from one night to the next, you observe what is called a waning moon. The moon appears to get smaller, but it's really all in how we see it.

The Moon Gets Smaller and Smaller

You have surely noticed during your time on Earth that the moon doesn't always look the same. Sometimes you can see it in the middle of the day in a cloudless blue sky. Some nights it is large, round, and bright and illuminates everything. Sometimes the moon is just a sliver of a crescent, and other times you can't see it at all.

These changes in the appearance of the moon can be explained by the lunar cycle. The way we see the moon changes throughout the cycle because its position relative to the earth and sun changes. When the moon is getting smaller every night, we call it is a waning moon.

The Lunar Cycle and the Phases of the Moon

The reason we can see the moon at all is because sunlight strikes it and reflects from its surface. When the reflected sunlight is visible to us on earth, we can see the moon. The percentage of the illuminated moon that we can see depends on how the moon, sun, and earth are all positioned relative to each other. For instance, when you see a full moon, the earth is between the sun and the moon. We can see the entire illuminated surface of the moon, and it looks full and round.

On the other hand, when the moon is between the earth and the sun, the illuminated side is facing away from us and we can't see it at all. We call this a new moon, and it marks the beginning of a new lunar cycle. A lunar cycle takes about 29 days, and the moon works through its different phases as it revolves around the earth.

When the moon is full, it is at its maximum illumination from our vantage on earth. After the full moon, the percentage of the illuminated surface that we can see gets smaller and smaller until it disappears and we have a new moon again. During that period of getting smaller, the moon is called a waning moon. To 'wane' means to get less and less, or to decline.

The waning moon occurs between the full moon and the next new moon as our view of it gets smaller and smaller.
moon phases

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