Guest Post by Albert:
A well-known but little understood expression in English. ‘Once in a blue moon’ is used to describe something that happens very rarely, but which is not impossible. But what does it mean? Does it have an origin in astronomy?
What is a ‘blue moon’ or ‘blue sun’?
From experience, we know that the sky is blue, the sun yellow, and the moon dark grey. Seen through cloud, the Sun or Moon change brightness but their colour does not change, although it becomes harder to discern the colour. Only at sunset (or for early risers, sunrise) can we see a red Sun, caused by absorption and scattering of blue light in the atmosphere. The air molecules, water droplets and dust particles affect blue light more than they do red. With a long path through the atmosphere, red light gets through but blue light is removed. The result is a red Sun. The scattered light is what makes the sky appear blue.
There is however an exception. Particles of size around 1 micron affect red light more than blue. This unusual situation makes the Moon or Sun appear bluer. It would not be notable against a blue sky, but in such conditions the sky will become greyish (and perhaps even red): the contrast between the sky and Sun makes the Sun appear blue. It is magnified by the tendency of the human eye to assign colours in contrast. Photographs (this one from Brisbane, 2009) of the effect tend not to be show it as well, but the effect is marked to the eye. It only works if the particles are of quite uniform size: mix grains of sizes which are different by more than 50% and the effect goes away.
After the Krakatoa eruption in 1883, blue Suns and Moons were seen across the tropics for a month, following the large cloud of volcanic dust and ash moving around the globe. The cloud circled the Earth twice, each circle taking 13 days. The blue Sun was first seen around the Indian ocean. One report describes how the Sun appeared green at sunrise, becoming blue as it rose higher. Krakatoa was unusual in this respect, though: in general reports of blue Suns or Moons after volcanic eruptions are rare. In September 1950, large peat fires in Alberta led to blue Suns and Moons being visible from Canada and a few days later Europe. The event is still remembered across Britain. Patrick Moore wrote a few years later:
The moon was in a slightly misty sky and had a kind of lovely blue color comparable to the electric glow discharge. I never saw something similar before.
Many people have suggested that the expression ‘Once in a blue moon’ referred to rare events like this. I became curious about this, and tried to find out more.
How old is the expression ‘blue moon’?
It is a uniquely English expression, and does not occur in other languages. But there is surprisingly little mention of it in the older English literature. Where it is mentioned in 20th century writing, it is often as a question or speculation about the origin of the expression. Clearly it was part of the oral language but rarely used in written form. The earliest writing which uses it in something close to the modern form comes from Pierce Egan, Real Life in London, 1821 (the first year of this periodical). Egan was a journalist who described life in London, in the language used on the streets. The particular paragraph goes:
Their attention was at this moment attracted by the appearance of two persons dressed in the extreme of fashion, who, upon meeting just by them, caught eagerly hold of each other’s hand, and they overheard the following – Why Bill, how am you, my hearty? – where have you been trotting your galloper? – what is you after? – how’s Harry and Ben?- Haven’t seen you this blue moon
This depiction of London slang is very different from anything written before. Still, this particular phrase required a footnote provided by Egan, stating that ‘blue moon’ meant ‘a long time’. The expression was not commonly known, apparently. I found no earlier mention, and it is therefore likely that the expression arose in London slang shortly before 1820.
The suggestion has been made that it developed from an earlier expression ‘once in a moon’, with ‘blue added to describe something much rarer than monthly.
There is a much earlier mention in a document from 1528(!) but the meaning is so different that there does not appear to be any relation. It appears in a tract called Rede me and be nott wrotke, For I faye no thinge but trothe. written by William Roy and Jerome Barlow (but attributed to William Barlow who admitted being the author a few years later, when he tried to retract this rather unpleasant document.) The mention of ‘blue moon’ (in an inaccurate spelling for ‘blue’ even for those days) is in the following lines:
Holdynge the worlde vniverfall.
Agaynfl god they are fo flobbourne/
That fcripture they tofle and tourne/ ‘
After their owne ymaginacion.
Yf they faye the mone is belewe
We muft beleve that it is true
(where the ‘f’ was used for the modern ‘s’). The text says that these are people who hold themselves above the world, god and scripture, and they think that if they declare the moon is blue, we should believe it. So in this context a blue moon is make-believe. It is just not true. This does not seem related to the later meaning of a ‘long time’.
It appears that this use of ‘blue moon’ never caught on. It did not become adopted into the English language until it was re-invented in London, 300 years later, now with the meaning of a ‘long time’.
What happened after 1820?
There are few mentions in British books. For instance, 1860. F. W. Robinson, Grandmother’s Money, I., 144, contains the line If he talked till a blue moon. In 1869, Edmund Yates published a book ‘Wrecked In Port’, a purported autobiographical account of a shipwreck survivor. It includes:
These gentry, who would have sat interested for that indefinite period known as â€a blue moon,â€ had the talk been of markets, and prices, and â€ quotations,â€ at length thought it time to vary the intellectual repast, and one of them suggested that somebody should sing a song.
Other occasions are in 1876, Mary Elizabeth Braddon, Joshua Haggard’s Daughter, x.\iv. Why should she stint as to one or two puddings a week . . . and a fruit pasty once in a blue moon (This is incorrectly listed as an unidentified book from 1871 in some articles on the blue moon), and in 1884, Robert. Francillon, Ropes of Sand, xxi. says: I’ve made bold to take the chance of your being at home for once in a blue moon, Mr. Carew,’ said she..
The expression survived, was in common use, but was not particularly common. All four authors lived in London, confirming an origin from London slang.
The expression resurfaced in North America. Blue moons had been seen there as wekll, following the Krakatoa eruption in 1883. A letter witten Jan 22, 1884 appeared in the Feb 1 issue of ‘Knowledge’ (an American periodical appearing between 1892-1911) stating:
There is a very old Norfolk saying, once in a blue moon. Can it have had its origin in the actual and yet very infrequent observation of that phenomenon? Or is it a mere random shot at an illustration of rare events? The moon here in November was of the intensest sapphire blue, the perfect clear sky looking rather slaty. This morning at 6:30 there was a fine sky-glow, and so last week. It certainly appeared to come from aqeous vapour. (taken from Wilson, 1946).
This is from Norfolk in Ontario (not the UK), an agricultural area attracting immigrants from poorer areas in the UK: Scotland, Devon, Cornwall, amongst others. The text shows that the expression was already old and well known, but also that the origin had been lost. It is also found in the Maine Farmers’ Almanac (a publication that existed from 1819 until 1967). In this almanac, and for the first time, it was given a specific definition. There is a long tradition in North America to give each full Moon in a year a name. Examples are hunter’s Moon (October) harvest Moon (September), lenten Moon (last full Moon of winter), and Easter Moon (first full moon of spring). Different names were used in different regions. Each season should have three full Moons. But some years have 13 rather than 12 full Moons, and in those years there is one season with four rather than three. This throws the naming out of kilter with the agriculural year. The almanac used the term ‘blue moon’ for the third (not the fourth!) full moon in a season of four. It is not clear when this was first used but it was in use by 1915 and may go a long way back. An article describing the detective work involved appeared in finding this obscure source is in Sky and Telescope in 2006.
The custom to give names to full Moons was used mainly in North America, and mainly in rural areas (where working outside at night becomes possible only with a full Moon). Therefore, this specific definition may well come from Maine or from a nearby area. The date is unclear, and neither is it known whether there is an oral tradition behind it or that this definition was invented by one of the Almanac editors.
The 1937 issue of the Almanac explains the procedure (reproduced here):
However, occasionally the moon comes full thirteen times in a year. This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar. It became necessary for them to make a calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number. Also, this extra moon had a way of coming in each of the seasons so that it could not be given a name appropriate to the time of year like the other moons. It was usually called the Blue Moon. There are seven Blue Moons in a cycle of nineteen years. This year (1937) has a Blue Moon in August the same as 1918. In 1934 and 1915 Blue Moons came in November. The next Blue Moon will occur in May 1940 as it did in 1921. There was a Blue Moon in February 1924. In olden times the almanac makers had much difficulty in calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression “Once in a Blue Moon.”
This 1937 article includes a fair amount of unproven speculation. It was indeed the task of the priests to keep track of the calendars, and already in Jewish time they would order the insertion of a 13th (lunar) month when the seasons and the lunar calendar had clearly gone out of sync. Under the Julian and Gregorian calendars, their role was limited to the ecclestical year. But there is no evidence that in medieval times the extra lunar cycles were called ‘blue moons’, or that full Moons were given names. Neither is there support for the claim that the Almanac was the origin of the term “Once in a blue moon”. What is clear is that this expression was well known in 1937, but that the origin had been lost.
The definition changed again in 1946, this time through a mistake. An article in Sky and Telescope tried to answer a reader’s question on the definition of a blue moon. The answer was based on the usage in the Maine almanac, as explained the 1937 article, but the author misunderstood the explanation. He believed it was used for the second full Moon in a calendar month. The magazine repeated this altered definition in a 1950 article. It resurfaced in a 1980 radio show, became incorporated in a Trivial Pursuit question, and from there made it into modern ‘knowledge’. The article in Sky and Telescope which explains some of this history is apologetic in tone, but does not retract the new definition. How could it, if it had become part of Trivial Pursuit?
The earlier definition was much more useful, though. the Almanac defined the seasons as starting at the solstices and equinoxes, using a mean motion of the Sun (correcting for our elliptical orbit with its varying speed). These are moments in time everyone can agree on. The new definition depends on location. A full Moon may occur on different dates, depending on the time zone. For instance, in 2018 there will a second full moon within a calendar month on January 31 and again on March 31 (unusually close together, thanks to February having no full Moon). But this is not true worldwide. For instance, in New Zealand, these full Moons wil occur on Feb 1st and April 1st, and according to this definition, neither will be ‘blue’.
How often does a blue moon occur?
True blue Moons (or Suns) are rare. Regions with frequent dust storms may see it more, but areas which depend on chance smoke from the right kind of fires or ash from just the right volcano may experience it perhaps once a century.
The other definitions give more frequent occurences. Full Moons are 29.53 days apart, and the average length of a month is 30.43 days. Some months are longer than others, and have a higher chance of a second full Moon in one month (February, on the other hand, can never have two and occasionally has none). The time between two full Moons varies through the year, from 29.27 to 29.83 days. Steve Holmes (2011) put this together and found that each month has a chance of 3.28% of ‘being blue’. A second full Moon in one calendar month can therefore happen on average once every 30.48 months. The next occasion will be Friday, July 31, 2015, and this one is worldwide.
(The second full Moon in December 2009 coincided with a lunar eclipse, so for a brief moment the Moon was both red and blue simultaneously!) (From Steve Holmes, 2011 ).
A lunar year of 12 full Moons is 11 days shorter than a calendar year. Each calendar year therefore has a 36.3% chance of having a 13th full moon. This will happen on average once every 33.0 months. The next one will be Saturday, May 21, 2016. This will be by definition worldwide.
Grounded in astronomy?
In conclusion, the expression ‘once in a blue moon’ does not come from astronomy, nor from volcanology. It was a London colloquialism, dating from before 1820, but there was no observation or definition behind it. The suggestion that id derived from a medieval tradition, related to the ecclestical lunar calendar, has no supporting evidence of which I am aware. Instead, the astronomy was added later, perhaps by an editor of an American almanac (which sometimes were astronomers), and redefined in a popular astronomy magazine. Astronomy adopted an orphaned expression and gave it a new home.