A blue-colored moon (hypothetical representation), 2014
1 June 2020
Once a year or so, news and social media sites fill with articles and posts about how that month there will be a blue moon and that encourage people to go out and see this “rare” event. This astronomical definition, or more accurately definitions as there are two competing ones, is relatively recent in origin. Use of the phrase blue moon is quite old and has meant different things over the centuries.
The oldest of these senses is the idea that a blue moon is an absurdity, something that can’t actually exist. From the 1528 anti-Protestant polemic Rede Me and Be Nott Wrothe by William Barlow:
Agaynst god they are so stobbourne /
That scripture they tosse and tourne /
After their owne ymaginacion.
Yf they saye the mone is belewe /
We must beleve that it is true /
Admittynge their interpretacion.
This use, however, seems to be a singular one, chosen for the rhyme. There is little evidence of blue moon being used elsewhere in the sixteenth century to mean an absurdity.
Blue moon is, however, starting around 1700, used to mean a moon, or a graphic representation of a moon, that is literally blue. One of the earliest of these uses was in 1702 when lepidopterist James Petiver noted blue markings resembling moons on a species of butterfly:
Papilo Sulphureus, lunulis cæruleis, nigris lituris insignitus. This exactly resembles our English Brimstone Butterfly Mus. nost. No. 1. were it not for those black Spots, and apparent blue Moons in the lower Wings. This is the only one I have yet seen.
In 1816, Percy Bysshe Shelley writes of an actual moon that appears blue in his poem Alastor; or The Spirit of Solitude, lines 193–99:
Roused by the shock he started from his trance—
The cold white light of morning, the blue moon
Low in the west, the clear and garish hills,
The distinct valley and the vacant woods,
Spread round him where he stood. Whither have fled
The hues of heaven that canopied his bower
Of yesternight?
And of course, there is the 1934 Rodgers and Hart song Blue Moon, which has been covered by countless artists over ensuing decades, in which the color represents the singer’s state of sadness and loneliness:
Blue Moon
You saw me standing alone
Without a dream in my heart
Without a love of my own
The moon can actually appear as blue due to smoke or dust in the atmosphere, such as after a large volcanic eruption, forest fires, or from industrial pollution, and sometimes blue moon is used in this literal sense.
I’ve been including longer quotations than commonly occur in dictionary citations because sometimes an expanded context is necessary to understand the usage in question. An example is this one that would appear to be of just such a literal blue moon that appears in the Oxford English Dictionary from an 1883 article in Science. The OED citation reads:
The red sun and the blue moon mean higher temperature and general rain.
One might legitimately interpret this citation as referring to the moon in the sky, while in actuality, the article is about a means of signaling weather reports via a visual system on railroad cars. In fuller context, it reads:
The red signals are confined to predictions as to temperature,—rise in temperature, stationary temperature, falling temperature. The other color is blue, and that is confined to predictions in regard to the general state of the weather. The question of form was a good deal considered, and three forms were adopted. We adopted the sun, moon, and star, because everybody was familiar with those words. We experimented with the triangle, and finally rejected it. The device for attaching to the car is due to Mr. Anderson, who has been in the service of the board of commissioners for the past year; and it is a really happy device. The signal is made as large as possible, and the disk can be seen a long distance. The red sun and blue moon mean higher temperature and general rain.
So, this instance has nothing to do with astronomy and or how the moon appears in the sky, and one can sometimes be misled if one relies on the dictionary citations alone.
The moon has also long been used as a measure of time, in particular the passage of a month, and the phrase once in a moon, meaning once a month (or every 29.5 days if you want to be precise), dates back several centuries. From Andrew Boorde’s 1547 A Breuiary of Healthe:
Also there is an other kinde of madnesse named Lunaticus the which is madnesse that doth infest a man ones in a mone the whiche doth cause one to be geryshe, & wauerynge wyttid, nat constant, but fantasticall.
But when it comes to a blue moon, the period of time is rarely defined with such specificity. In 1821, writer Pierce Egan in his Real Life in London records a conversation in which the phrase is used to refer to a long period of time:
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 hands, and they overheard the following—“Why, Bill, how am you, my hearty?—where have been trotting your galloper?—what is you arter?—how’s Harry and Ben?—haven’t seen you this blue moon.”
And Egan includes this note at the bottom of the page:
Blue moon—This is usually intended to imply a long time.
Hence the phrase once in a blue moon refers to something that occurs rarely or never. The phrase appears in the early nineteenth century, about a decade after Egan wrote his book. There is this from a review of James Planché’s 1833 production of Verdi’s opera Gustavus the Third in the pages of the Athenæum in which the phrase is used and unremarked upon:
We are no advocates for the eternal system of producing foreign operas to the exclusion of the works of English composers, but once in a blue moon such a thing may be allowed.
Finally, we get to the astronomical definitions, which are both from the first half of the twentieth century. The first is from the 1937 issue of the Maine Farmer’s Almanac, which defines a blue moon as the third full moon in a season that contains four full moons. (Most seasons have only three full moons.) Such a happening occurs about once every 2.5 years. The explanation in the almanac, which I give in full, is historically and linguistically inaccurate, so put no stock in its factual claims (reading it makes the medievalist in me shudder):
THE MOON usually comes full twelve times a year, three times in each season. These moons were named by our early English ancestors as follows:
Yule
Winter | 0 Moon after Yule
Moons | 1 Wolf Moon
| 2 Lenten Moon
First Day of Spring
Spring | 3 Egg Moon
Moons | 4 Milk Moon
| 5 Flower Moon
The Long Day
Summer | 6 Hay Moon
Moons | 7 Grain Moon
| 8 Fruit Moon
Summer’s End
Fall | 9 Harvest Moon
Moons | 10 Hunter’s Moon
| 11 Moon Before YuleHowever, 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 Lunar 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 calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression “Once in a Blue Moon.”
The second, and more common, astronomical definition appears in an article by James Hugh Pruett in the March 1946 issue of Sky and Telescope magazine and is a misinterpretation of the definition in the earlier Maine Farmer’s Almanac. According to Pruett’s definition, a blue moon is the second full moon in a calendar month.
Seven times in 19 years there were—and still are—13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.
This second definition was used again in a 1950 issue of Sky and Telescope and was repeated by several other popular sources, most notably the 1986 release of the game Trivial Pursuit. As a result, the idea that a blue moon is the second full moon of a month has become widespread, but it is not an old definition and has nothing to do with the phrase once in a blue moon.
Sources:
Barlow, William, Rede Me and Be Nott Wrothe, 1528. Early English Books Online (EEBO).
Boorde, Andrew. A Breuiary of Healthe. 1547, Part 2, fol. 15. Early English Books Online (EEBO).
Egan, Pierce. Real Life in London. London: Jones & Co., 1821, 249.
Hiscock, Philip. “Once in a Blue Moon.” Sky and Telescope, March 1999, 53–55.
Mendenhall, T.C. “A Method of Distributing Weather Forecasts by Means of Railways.” Science, 2.29, 24 August 1883, 252.
Olson, Donald W., Richard Fienberg, and Roger W. Sinnott. “What’s a Blue Moon?” Sky and Telescope, May 1999, 36–38.
Olson, Donald W. and Roger W. Simott. “Blue-Moon Myster Solved?” Sky and Telescope, March 1999, 55.
Oxford English Dictionary, third edition, March 2013, s.v. blue moon, n.; June 2004, s.v. once, adv., conj., adj., and n.; December 2002, s.v. moon, n.1.
Petiver, James. Gazophylacii Naturæ & Artis. London: 1702, 16. Early English Books Online (EEBO).
Shelley, Percy Bysshe, Alastor (1816). Reprinted New York: AMS Press, 1975. University of Toronto, Representative Poetry Online.
“Theatricals.” Athenæum. 316. London, 16 November 1833, 780.
Photo credit: Wikimedia Commons, used under CC BY-SA 3.0 US.