Though most people use the phrase to mean "very rarely", astronomers have a simpler way of identifying a blue moon: it happens when there are two full moons in a single month.
January 1999 fits the bill - its first full moon was on 2 January, and the second will be next Sunday, just squeezing in before the end of the month.
More remarkably, we will have to wait only until March for the next blue moon to arrive. "It's very unusual for this to happen," explained Simon Mitton of the Royal Astronomical Society. "It can only occur when one of the two blue moons is in January." Such a double helping will not happen again until 2018.
The reason is that the moon's own month is 29.5 days long. Usually, that only allows for a single full moon in each calendar month. But if there is a blue moon on the last day in January, then February - being only 28 days long most years - will miss out. That in turn means that March will also have two full moons.
But the most perplexing thing about a "blue moon" is where the expression comes from. There are many competing possibilities. The "two moons in a month" explanation became accepted because the board game Trivial Pursuit began using it in 1986, according to Philip Hiscock, a folklore specialist hired by Sky & Telescope magazine to investigate. Trivial Pursuit got it from a 1985 children's almanac; but the almanac's authors couldn't recall where they got it.
Among other possible origins turned up by Mr Hiscock are the 16th-century phrase "he would argue that the moon is blue" - to mean arguing that black is white. Yet another is that it is a corruption of the French "la double lune". Or a derivation from songs addressed by the lovelorn (blue) to the moon. Or even the Blue Moon cocktail, made of curacao, gin and a twist of lemon.
A rival explanation is that very occasionally, volcanic eruptions or strange weather threw dust into the atmosphere - making the moon look blue.
There is a stronger case, however, for the little-known Maine Farmers' Almanac, the source of two possible derivations. One is that when there were two full moons in a calendar month, the Almanac printed the first in red, and the second in blue.
The more baroque explanation is that in the past the various full moons of the year were named according to the order in which they occurred - provided there was only one per month. The names were Moon after Yule, Wolf Moon, Lenten Moon, Egg Moon, Milk Moon, Flower Moon, Hay Moon, Grain Moon, Fruit Moon, Harvest Moon, Hunters' Moon and Moon Before Yule.
The trouble was that about once every three years, there would be a 13th moon - the Blue Moon - which would upset schedules for church services and celebrations.
For astronomers, though, blue moons are non-events. "It's a completely artificial phenomenon, caused by our months being split into 30, 31 and 28 days," said Dr Mitton. In fact, they occur only with the Gregorian calendar: the Islamic and Hebrew calendars, which run on alternating months of 30 and 29 days, rule out such repeats altogether.