It's at the heart of a popular saying, it's featured in no end of songs and it provided the name of the detective agency in Bruce Willis finest (several) hour(s) Moonlighting. But what, oh what, is a blue moon?
In every year, there are 12 full moons. However since the lunar year is 11 days shorter than the solar year that informs our calendars, every two to three years, we are faced with an additional full moon.
Exactly which of the year's moons is considered a 'blue moon' (the name has its origins in English folklore) depends entirely upon the region's traditions. In most parts of the Christian world, it's the so-called Lent Moon (the full moon that occurs during the 40 days leading up to Easter) that's identified as a blue moon. However, in the US, the Farmers' Almanac - an agricultural magazine based in New England - claims that a blue moon is the third full moon in any season which contains four of the things.
Incidentally, there is a such a thing as a visible blue moon. Following catastrophic events such as volcanic eruptions, the ash that's thrown up into the atmosphere can leave the moon looking as if it has a blue-ish hue. Examples of this rare phenomena occurred in the wake of the Krakatoa and Mount St Helens disasters.