31 July 2015 is a special day. There will be a full Moon, which in itself is not unusual. But it will be the second full Moon this July, and this is rare. The second full Moon in a single calendar month is traditionally called a ‘Blue Moon’.
A second full Moon in one calendar month happens on average once every 30.5 months, the last time it happened was in August 2012. February never has two full Moons. In 2018, both January and March will have two full Moons but February will have none – a very unusual occurrence.
But where does the expression ‘Blue Moon’ come from?
Very occasionally, the Moon or Sun really does appear to be blue. This is caused by dust, and it has been seen after some volcanic eruptions. The last time this was widely seen in the UK was 1950, when it was caused by peat fires in Alberta. But this is not the origin of the expression.
‘Once in a blue moon’ means something that happens, but only very rarely. The oldest writing which uses the expression is 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.
Egan provided a footnote stating that ‘blue moon’ meant ‘a long time’, which suggests that the expression was not commonly known. There are a few other uses of the expression in later books from the 19th century, all from writers who lived in London (Mary Braddon, Edmund Yates, and others). So the origin of the modern expression ‘Once in a blue moon’ probably lies in London, around 1820. The single older reference to a blue Moon is in a tract from 1528 by William Barlow:
After their owne ymaginacion.
Yf they faye the mone is belewe
We muft beleve that it is true
Here a blue moon (belewe mone) is an example of something that isn’t true. The meaning is completely different, and the date 300 years earlier. It does not seem to be related to Egan’s use.
There is an expression in German about a ‘blue Monday’. A possible origin is in the first Monday after Three Kings (Jan 6) when some medieval guilds had a day of celebration, and churches covered their altars with blue cloth. The modern usage is that of a day when little or no work is done. It is not related to the English expression.
When was a full Moon first called ‘blue’?
The modern definition of a ‘Blue Moon’ comes from America, specifically the Maine Farmers’ Almanac. In 1915, this used the term for a season that had four rather than three full Moons. All usual full Moons had been given names (e.g. Hunter’s Moon in October, Lenten Moon in February/March), but if a season had four full Moons, one of these was nameless. The Almanac called the extra full Moon the ‘blue Moon’.
The definition changed in 1946, when an article in Sky and Telescope misunderstood the usage in the Maine almanac, and said it was used for the second full Moon in a calendar month. The error was repeated in a 1950 article, resurfaced in a 1980 radio show, became incorporated in a Trivial Pursuit question, and from there made it into modern ‘knowledge’. The new definition has never been retracted. How could it, once it had become part of Trivial Pursuit?
Grounded in astronomy?
In conclusion, the expression ‘once in a blue Moon’ was London slang, dating from around or before 1820. The astronomy was added later, by an editor of an American almanac, and redefined in a popular astronomy magazine. Astronomy adopted an orphaned expression and gave it a new home.
More information can be found here
Blogger details: Albert Zijlstra is Professor of Astrophysics at the School of Physics and Astronomy, the University of Manchester, and is a past Director of the Jodrell Bank Centre for Astrophysics.