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Sometimes, a rare event called a blue moon occurs and this phenomenon has inspired the popular idiom ”once in a blue moon”. In this lesson, learn more about what a blue moon is and how often it occurs.

Blue Moons

You may have heard the expression once in a blue moon.

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People use this phrase to describe something that doesn’t happen very often. Have you ever heard this and thought, what exactly is a blue moon? If you have, this is your lucky day because you are about to learn all about blue moons and how often they occur! To understand blue moons, let’s first learn a little about the phases of the moon.

Why the Moon Appears to Change Shape

If you look at the moon every night for a month, you will see it slowly change shape. It will go from being completely round (a full moon) to a half moon, a crescent, and then will disappear altogether (a new moon) before beginning to grow again until you see another full moon.

This cycle repeats itself over and over every twenty eight days. Why exactly does this happen?

A full moon occurs every 28 days
full moon

First, let’s think about why we can see the moon at all. The moon does not produce any light, so the light we see when we look at the moon is actually just reflected sunlight. The moon circles the Earth every 28 days, and as it moves, light from the sun illuminates a changing area, causing the moon to appear to change shape. These changes in shape are called the phases of the moon. When the moon is fully illuminated by the sun, it produces a bright, round full moon.

When the sun is behind the moon, none of the surface facing Earth is illuminated, producing a new moon that appears dark in the sky.

As the moon moves around the Earth, different parts of it are illuminated, causing the moon to appear to change shape. These different shapes are the phases of the moon.

phases of the moon

When Does a Blue Moon Occur?

There are two popular uses of the term blue moon. But in both cases, the moon does not actually appear blue (that only happens if there is a lot of pollution in the air). The original definition of a blue moon was first recorded in the Old Maine Farmer’s Almanac in 1937, which said that a blue moon was the third full moon in a season with four full moons. Since each season (summer, fall, winter, or spring) is made up of three months, most of the time, there are only three full moons in a season.

Occasionally, however, there are four, and when this happens, the third one is called a blue moon. No one is really sure why this is called a blue moon, but some people speculate that when there were four full moons in a season, it was written down in blue ink in the almanac.

Farmers almanacs, like this one, provided detailed lists of the phases of the moon, including the times when a blue moon would occur
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