The phases of the Moon run through a constant cycle of about 29.5 days. The period of time is a bit shorter than a month (29.5 x 12 = 354 days, while the sidereal year is 365.24 days). The extra 11 days per year mean that every 2-3 years there will be a thirteenth full moon in a calendar year. This extra full moon is called a blue moon. Indeed, four or five times in a century, a year will have two blue moons! Recently, popular usage has identified the second full moon occurring in a single calendar month as that year’s blue moon.
The origin of the term ‘blue moon’ appears to be in the late Medieval period in Europe. At the time, ‘blue moon’ was used as a symbol for the unfettered authority of the church. A quote from the time, translated into Modern English, reads:
“If they say the moon is blue, we must believe that it is true.”
The term blue moon thus began as a denigrating symbol for absurdities and impossibilities – something that does not exist became a ‘blue moon’.
Oddly, there are occasionally blue moons, which are probably where the notion of a blue moon arose. Following a volcanic eruption, huge forest fire, or a large meteoroid impact, the atmosphere is filled by smoke and dust particles. These extra particulates scatter light in such a way as to make the Moon appear blue in the sky.
In the folklore about blue moons, the original usage of ‘once in a blue moon’ seems to have been roughly equivalent to saying ‘when pigs fly’. However, the fact that there are occasional truly blue moons makes one think of Gilbert and Sullivan’s line from H.M.S. Pinafore – “What, never? Well, hardly ever.” The meaning has evolved from identifying something truly impossible to something which happens only rarely.
It actually appears that the modern meaning of a blue moon is the result of an American astronomer’s error. The article that promulgated this dastardly deed appeared in a 1946 issue of Sky and Telescope, then (and now) perhaps the primary magazine aimed at amateur astronomers. He sited as a reference a 1937 Maine Farmer’s Almanac, which he thought referred to a blue moon as the second full moon in a calendar month. However, when his sources were checked, it turned out that none of the full moons identified as blue moons in that almanac were the second full moon of the month. The legend of this meaning of the term ‘blue moon’ has survived intact to the present day, despite its flawed origins.