In the early part of 1999 many newspapers articles appeared in North America to tell readers that they would be seeing two blue moons in that year, one in January and one in March.
This stopped me dead the first time I saw the phrase, as blue moon to me only has the meaning of some event that happens extremely rarely, if ever (it corresponds to a favourite expression of my father’s: “never in a month of Sundays”). I can find nothing in any of my dictionaries or books on phrase origins about two full moons in one month. But the expression has become common in that sense in recent years in north America, to the extent of almost usurping the older meaning.
The idea of a blue moon has been traced back to 1528, to a sceptical little item entitled Rede Me and Be Not Wrothe: “Yf they say the mone is belewe, we must believe that it is true”. This implies the expression had a meaning of something that was absurd, very like another moon-related proverb first recorded in the following year “They woulde make men beleue ... that ye Moone is made of grene chese”. Because it was absurd, saying that something happened only once in a blue moon was the same as saying it never happened. And this was what the phrase meant for several hundred years.
Charles Earle Funk suggested in 1948 that the two expressions are connected, the green cheese being the freshly pressed round cheese that looks white like the full moon, and the blue moon the one just before the new moon begins to show, when rarely the Moon’s surface, bathed only in faint Earthlight, may look blue. This is eminently plausible for green cheese, indeed it’s the usual explanation of how the saying came about, but his explanation doesn’t fit the fact that blue moon originally meant “never”.
The version that most of have grown up with has a sense that has shifted, like the opinion of the captain of the Pinafore, from “never” to “hardly ever”. This sense was first recorded only in 1821, but is probably eighteenth-century. Various writers have guessed that the change in meaning came about because people realised that the moon can indeed look blue, because of dust in the upper atmosphere, say from forest fires or volcanic eruptions (there was a huge one in 1816 that was supposed to have helped trigger this shift in meaning). As with Mr Funk’s thesis, that hardly sounds convincing, but as so often there’s no evidence either way.
So how did blue moon get this new meaning? I’m indebted to Philip Hiscock of the Folklore and Language Archive at the Memorial University of Newfoundland, who has done a lot of research on this phrase, the results of which were presented in the March 1999 issue of Sky & Telescope Magazine. He managed to trace it back to an edition of Trivial Pursuit published in 1986; its compilers got it from The Kids’ World Almanac of Records and Facts of the year before, which almost certainly got it from a radio programme in 1980 that got its information from an article in Sky and Telescope in March 1946, which was based on a mention in the same magazine in July 1943, which attributed it to the 1937 Maine Farmers’ Almanac.
In the May 1999 issue of Sky & Telescope the story has been taken further by Donald W Olson, Richard Tresch Fienberg and Roger W Sinnott. Their research into issues of the Maine Farmers’ Almanac over some decades has shown that its meaning of blue moon was neither the old or the new version, but an idiosyncratic one. It seems the Almanac commonly gave each of the 12 common full moons of the year a name (of which one was the well-known Hunter’s Moon), divided into four sets of three that matched the seasons. In years when there were 13 full moons in a year, the editor gave the third full moon in the season which had the extra one the name of blue moon.
This was mentioned in the 1943 piece in Sky & Telescope. In the 1946 article, James Hugh Pruett, an amateur astronomer from Oregon who was a frequent contributor to the magazine, misunderstanding the Almanac’s system, gave it to mean the second full moon in a month. As the years passed, this was taken up as I’ve already described, though this “two full moons in one month” meaning of blue moon only started to achieve much circulation from about 1988, no doubt as a result of the Trivial Pursuit reference.
So what we have is a truly modern piece of language folklore, and a fine example of the way that a supposed fact can become widely reported and accepted within a short space of time. It shows also how an expression can lurk in the language until something causes it to bursts upon the public stage. What is quite astonishing about all this is that the researchers have managed to trace the whole history of the shift in meaning and lay it out for our inspection, something that must surely be unique in the annals of lexicography.
Astronomers say that “two full moons in one month” type of blue moon is actually quite frequent. There is a month with two full moons in it rather more than once every three years. That’s because, though moon and month are intimately related words, all our months apart from February are a little longer than the interval between two full moons. Much rarer is to have two blue-moon months in one year. This happened this year, as both January and March had two, whilst poor old February had no full moon at all. The next years in which this happens are 2018 and 2037. Now that’s what I really call once in a blue moon.