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Once in a Blue Moon

Nodoka M., Astronomy Club Editor
Superstitionists, beware! Halloween night of 2020, in a strangely prophetic fashion, will be lit up by a gorgeous full moon for the first time since 1944, 76 years ago. It also just happens to be a blue moon. But before you make plans to catch a glimpse of this rare event, there’s a question you probably wanted answered: What exactly is a blue moon, anyway?
I’m sure most readers have heard of the phrase, ‘once in a blue moon.’ It’s an idiom used to describe events that happen very rarely. This is because blue moons themselves are rare: they only happen once every two or three years. A blue moon refers to the third full moon in a season that has four full moons. Typically there are only twelve full moons in a year, and each of the four seasons has three; however, because the lunar cycle is shorter, by a day or two, than the typical month, every two to three years there are thirteen full moons instead of the typical twelve. This definition comes from the August, 1973 edition of the Maine’s Farmer’s Almanac.
More recently, however, another meaning for the blue moon has become popular. It’s used in reference to the occurrence of a second full moon in a calendar month. This second definition arose after a misinterpretation of the original definition in 1946. Technically, both are correct!
There are a lot of misconceptions surrounding blue moons, the most common being that the moon appears blue during these times. While the moon gaining a slight blue tint is a very possible scientific scenario, it would be caused by particles in the air (such as from after a volcanic eruption), and would have nothing to do with how many full moons there are in a season.
For more information on why blue moons are called “blue moons,” how the moon might look blue, the history behind blue moons, and other interesting articles, consider checking out the links below!

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