“Once in a blue moon” is an expression for something that rarely happens, and of course it leads people to ask, what is a blue moon?

A literal blue moon occurs when enough smoke or dust is present in the atmosphere to make the moon appear blue . This occurred back in 1883, when the Indonesian volcano Krakatoa exploded, and in 1950, when masses of smoke from Canadian bush fires filled the air .

The first use of the expression was in a 1528 book, where a blue moon referred to something that would never happen, similar to the saying “if the moon was made of green cheese ”. In the 1700s, meanwhile, saying “I’ll marry you when the moon is blue” was not a marriage proposal but a statement denying that there was any chance of marriage .

The expression “once in a blue moon” came to mean something that happened only occasionally . An edition of The Maine Farmers’ Almanac from the 1800s defined a blue moon as the third of four full moons that occurred in a season . The first full moon was known as Lenten Moon, the second was known as Paschal Moon and the third—if it did appear, which was a rarity—was known as Blue Moon, and it’s from the almanac that the modern meaning of the expression is derived .

In March 1946, the author of a Sky & Telescope magazine article misinterpreted the reference to the blue moon in the 1937 edition of the almanac, interpreting the blue moon to be the second full moon in a single calendar month, and this interpretation of the term became popular and still exists today . In fact, the average interval between full moons—i .e ., the lunar month—is twenty-nine and a half days, which is shorter than the average calendar month of thirty and a half days . This makes two full moons in the one month possible but rare . A blue moon generally happens forty-one times every century, which equates to once every two and a half years . It can occur in any month except Feb-ruary (which is too short) and is most likely to happen in either January or March, because they each contain thirty-one days and are separated by February .

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