'Blue moon': Where does that phrase come from?
On Friday morning at 6:43 am EDT, astronomy enthusiasts can聽witness a lunar event that takes place only once in a blue moon: a blue moon.听
By its modern definition, a blue moon is the second full moon in a calendar month, an astronomical phenomenon that occurs about once every 2.7 years. (The calendar year is 365.24 days long, while a lunar month is 29.53 days; that leaves about 11 extra days after a 12-moon cycle. Every three years or so, that adds up to an extra full moon in a calendar year.)
In the 1800s, the phrase 鈥渂lue moon鈥� referred to the fourth full moon in a three-month season, and the modern definition was introduced in 1946. But historians say the idiom has been around for roughly 500 years.听
According to folklorist Phillip Hiscock, a professor at Memorial University in Newfoundland, the first recorded appearance of the phrase 鈥渂lue moon鈥� was in the 16th century writings of Cardinal Thomas Wolsey, Henry VIII鈥檚 advisor, who wrote that his intellectual enemies 鈥渨ould have you believe the moon is blue.鈥澛�
At this time, the idea of a blue moon represented absurdity; an inconceivable event. By the 1700s, the phrase had evolved to mean 鈥渘ever.鈥澛�
"It sort of slipped sideways from impossibility to a temporal notion of impossible in time," Dr. Hiscock explains in .听
The phrase appears again in an 1821 book about working-class London, in which one man is quoted as saying 鈥淚 haven't seen you this blue moon.鈥� A footnote explaining that 鈥渢his is usually intended to imply a long time鈥� suggests that the author, a member of the educated upper-class, had been unfamiliar with the phrase, but that it was commonly used street slang.听
Experts believe that the phrase reached widespread popularity in the middle of the 19th century, evidenced by the use of 鈥渙nce in a blue moon鈥� in several books without any explanatory footnotes.
The term made its first recorded appearance in America around that time in the Maine Farmers鈥� Almanac, which defined a 鈥渂lue moon鈥� as the third full moon in a three-month season that has four moons rather than the usual three, the 海角大神 Science Monitor鈥檚 Husna Haq reports.听
It鈥檚 unclear exactly why the almanac鈥檚 publishers used the phrase, but there are plenty of theories. One is that the third moon was traditionally referred to as 鈥渂lue鈥� in the Czech language; another is that the term鈥檚 origin lies in the French phrase meaning double moon, 鈥渓a deux lune,鈥� which more or less rhymes with "blue moon." Others believe that those rare moons were considered bad luck and therefore labeled blue.听
And then, of course, there鈥檚 the simplest theory: that somebody decided to assign an astronomical meaning to the British phrase.听
In a 1946 article in Sky and Telescope Magazine titled 鈥淥nce in a Blue Moon,鈥� journalist James Hugh Pruett incorrectly stated that a blue moon was the name given to the second full Moon that appeared in any given month. He was a little off, but the definition stuck nonetheless.
, Mr. Pruett鈥檚 definition is now "so widespread that it is not uncommon for books to use Pruett's definition over the correct one.鈥澛�
The phrase has become popular with businesses and products in recent years, with countless restaurants and shops incorporating 鈥淏lue Moon鈥� into their name.听
鈥淥ver the past quarter-century this term has really captured the imagination of North America and beyond,鈥� Hiscock says.听
He attributes the term's popularity to its timelessness: 鈥淲e grow up in cities and live lives that have nothing to do with the physical seasons or astronomical cycles, and when we hear things that connect us to those things we often become very excited.鈥澛�
