Why are blue moons so rare?

Have you ever looked down on the sidewalk and found a toonie? Finding money in unexpected places happens once in a blue moon. That is to say, it doesn't happen often! We use the expression "once in a blue moon" to describe an event that occurs infrequently. This is because blue moons usually occur every two to three years. 

Typically, there is only one full moon during a month. According to Britannica Library, a season usually lasts around three months, but it could have four full moons instead of three. The extra full moon is called a blue moon. The lunar phase cycle (when you count from full moon to full moon) is 29.5 days long; not quite a full calendar month. So it's possible for these extra full moons to crop up. Another definition is when a month has two full moons instead of one — the second one is also called a blue moon. 

Is a blue moon blue? Most of the time, no. In order for the moon to look blue in colour, large particles must be present in the atmosphere. Volcanic ash, smoke from wildfires or water droplets in the air can scatter or bend red light. These particles filter out the red light and make the moon appear blue. After the 1883 eruption of Krakatoa, the volcano released so much ash into the atmosphere that people reported seeing a bluish moon for a whole year! 

Whichever definition you use, blue moons are pretty rare. At the time of this post, the last blue moon was Sunday, August 22, 2021. Don't feel blue if you missed it! The Old Farmer's Almanac reports that the next one will occur on August 30, 2023.

More moon wonders:
Can we live on the moon?
What is the dark side of the moon?


Image source: “Once in a Blue Moon” by different2une is licensed under CC BY 2.0.

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