On Friday, August 31, there will be a blue moon in the sky. Yet, quite often, there is no color change. A blue moon is actually when there are two full moons in one month. And it happens once every 2.7 years, the last one happened to fall on New Year's Eve 2009. So they're not incredibly rare, especially in universal terms. Halley's commet appears every 75-76 years. Comet Ikeya–Zhang appears every 366.51 years.
Occasionally, dust particles in the atmosphere can make the moon at times appear blueish, but that doesn't happen all too often.
Wikipedia explains the origins of the term "Blue Moon:"
The earliest recorded English usage of the term "blue moon" was in a 1524 pamphlet violently attacking the English clergy, entitled "Rede Me and Be Not Wrothe" ("Read me and be not angry"; or possibly "Counsel Me and Be Not Angry"): "If they say the moon is belewe / We must believe that it is true" [If they say the moon is blue, we must believe that it is true].
Another interpretation uses another Middle English meaning of belewe, which (besides "blue") can mean "betray". By the 18th century, before the Gregorian calendar reform, the medieval computus was out of sync with the actual seasons and the moon, and occasionally spring would have begun and a full moon passed a month before the computus put the first spring moon. Thus, the clergy needed to tell the people whether the full moon was the Easter moon or a false one, which they may have called a "betrayer moon" (belewe moon) after which people would have had to continue fasting for another month in accordance with the season of Lent.
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