Once and For All, What’s a Blue Moon?

We’ve been hearing more and more about blue moons during the past few years, and I, like many people came to believe that, if there are two full moon during any given month, the second is a blue moon. Well, come to find out, that’s not true. Yesterday I came across an article on the Massachusetts Audobon Society blog, that explains the entire phenomenon. Why am I posting this science topic on a history blog? Because the author of the article in question, John Galluzzo,  has included key historical information about how the term “blue moon” came into such common usage.

But Galluzzo did not indicate why this special moon is designated as blue. According to Wikipedia, the earliest recorded English usage of the term  was in a 1524 pamphlet: “If they say the moon is belewe / We must believe that it is true”. Belewe? The word in Middle English can mean “blue”, but also means “betray”. Because the church arranged its theological calendar around the phases of the moon, when an extra full moon occurred near Easter, the clergy needed to warn the faithful that it was a “false” full moon. Alas, the people needed to continue their Lenten fast for another month until the next “true” full moon.

Does the moon ever appear to be the color blue? Occasionally. When the atmosphere is saturated with particles, such as dust or ash, that are wider than the wavelength of red light, the moon, and sometimes the sun (!), can appear blue or lavender. But now we’re leaving the realm of history for science, so I’ll end this little tale.


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