Only Once in a Blue Moon

Happening "only once in a blue moon" is the stereotype of rarity. Yet, if you ask people about the significance of the blue moon, the most common answer (if there's any answer) involves a definition that we attribute to astronomers. This is a definition of the blue moon as the second full moon in any calendar month. How often does that happen? If you do a simple calculation you find that, with this definition, you have a blue moon every two or three years—not of the rarity suggested by the common use. Furthermore, this definition gives no insight into the color blue. Our claim is that this is a trivial definition, hijacked from a much rarer optical effect that gives the moon (or sun) a blue appearance.

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