New research into the origins of the phrase "once in a blue moon" has discredited the popular notion that it stems from the relatively rare occasion when a month has two full moons. Historians have traced sayings that include references to a blue moon for more than 400 years but its more modern meaning - two full moons in a month - has become common only in the past 20 years.
Although many people assumed this must have referred to ancient observations concerning the rarity of two full moons in a month, the earliest record of this usage dates back only to 1946, when it appeared in an issue of Sky & Telescope magazine.
The author of the piece, James Hugh Pruett, an astronomer from Oregon, cited a 1937 edition of the Maine Farmers' Almanac. However, an investigation by Roger Sinnott, the current editor of Sky & Telescope, and Donald Olsen, a Texas physicist and authority on almanacs, revealed that there was no mention of a second full moon of the month in the Maine Farmers' Almanac.
They scoured more than 40 editions of the almanac and while they found plenty of blue moons, not one of them was the second full moon of the month.
"Several clues in the almanacs pointed to a strong connection between blue moons and the seasons of the year, which normally have three full moons each," Mr Sinnott said.
They determined that themeaning of "blue moon" was a reference to the third full moon in a season when there were four, but only if you mark the beginning of the seasons using an outmoded rule based on the dates of Easter and Lent.
Mr Sinnott said that until now nobody had realised that Mr Pruett had misinterpreted the Maine almanac and that his erroneous description had developed into a legend that had been repeated many times over the past 52 years.
Mr Pruett's definition of a blue moon became so popular, in fact, that it even surfaced in the game of Trivial Pursuit in the Eighties. Trivial Pursuit got it from a 1985 children's almanac; which had, in turn, got the information from Mr Pruett's erroneous research.
Although the phrase can no longer be applied to the event the occurrence of two full moons in a single month is a rare one. Rarer still is two such occurrences in the same year, but this year has been an exception, with both January and March each containing two full moons.
Such a double helping will not happen again until 2018.