The Big Blue Moon

What is a blue moon

A blue moon refers to the occurrence of an additional full moon within a calendar month. Typically, a calendar month has one full moon, but occasionally, due to the misalignment between the lunar month (which lasts about 29.5 days) and the calendar month (which typically lasts 30 or 31 days), there can be two full moons in a single month.

The term “blue moon” has been used for centuries, though its modern definition as the second full moon in a calendar month gained popularity in the latter half of the 20th century. This definition was popularized by an article in the March 1946 issue of Sky & Telescope magazine, and it has since become widely known.

The phrase “once in a blue moon” is also commonly used in English to describe something that happens very rarely or infrequently, perhaps because blue moons themselves are relatively rare occurrences, happening about once every 2 to 3 years on average.

It’s important to note that despite the name, a blue moon does not actually appear blue in color. The term “blue” here is used in its older sense, meaning “rare” or “unusual,” rather than referring to the moon’s color. However, there are rare atmospheric conditions, such as volcanic eruptions or wildfires, that can cause the moon to appear blue in color. This phenomenon is unrelated to the astronomical definition of a blue moon.

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Why is it called a blue moon?

The term “blue moon” traditionally refers to the phenomenon of having two full moons occur within the same calendar month. This occurrence is relatively rare, happening roughly once every 2.7 years. The phrase “blue moon” has its origins in folklore, where it was used to describe something that was rare or unusual, much like the idea of “once in a blue moon.”

Interestingly, the term “blue moon” doesn’t actually refer to the color of the moon. While the moon can sometimes appear to have a bluish hue due to atmospheric conditions such as smoke or dust particles, a “blue moon” is not inherently blue in color. It’s simply a name given to the second full moon in a calendar month.