On the night of Jan 31th, 2018, do go out and take a look at the moon. It will be full and will seem either to be a bit brighter than usual, or very faint and red. In either case, it will be a blue moon. Confused? Let me help you.
This is an updated and extended repost of an article which appeared on volcanocafe in 2015
Let’s first do the easy bit: if it is faint and red, you will be looking at the lunar eclipse. There will be a total eclipse of the moon that night. This happens when the moon passes through the shadow of the Earth. Wherever the moon is above the horizon, its light seems to go out. But it does not become entirely invisible, because the Earth’s atmosphere bends light that passes through it a bit, and that bended light still reaches the moon. Imagine the moon being illuminated by all the sunsets and sunrises all around the world, simultaneously: it will not be a surprise that the moon becomes reddish to the eye. And the more volcanic dust there is in the air, the darker the moon becomes. After Krakatoa, the moon almost became invisible during total eclipses. The redness of the eclipsed moon can vary and is not easy to predict in advance. A red moon during total eclipse is sometimes called (at least in the US) a ‘blood moon’.
Lunar eclipses are among the few (regular) astronomical events that can be seen from within our cities. To see most stars, the occasional comet, auroral light – it really requires getting away from artificial lights (and from alcohol – it is amazing how much alcohol affects night vision). Your eyes will take about 15 minutes to adjust to the dark and reach maximum sensitivity. At astronomical observatories, cars drive around at night without their lights on, which can be scary when you know that the road follows the top of a cliff but you are not quite sure where the road is. In emergencies, you are allowed to use the indicators to see by. But the full moon is bright, and wipes out the vision of the dark sky as effectively as any city.
The total eclipse of Jan 31 will not be visible everywhere on Earth. Europe, Africa and South America will miss out. Alaska, Hawaii and the western Pacific are perfect places to see it.
If there is no eclipse, the moon will instead appear to be unusually bright. This is because it will be a bit closer to Earth than usual. The orbit of the moon is elliptical and therefore it is not always the same distance from us. It is not easily notable: it makes the moon appear a bit larger and smaller over the month but this is hard to see. But if closest approach coincides with a full moon, the closeness makes the moon appear quite a bit brighter. The moon will reach closest approach on Jan 30 when it is 358,900 km from Earth. The average distance is 382,900 km. Therefore, it will be 7% closer to us and 14% brighter than an average full moon. (In fact closest approach will be one day before full moon but it will still be notable the next day.) This is not extreme: in December 2052 there will be a full moon as close as 356,424 km – the ellipticity of the orbit varies a little and not all closest approaches are the same (the average closest approach is at 362,508 km). In recent years, the closest approach has been called a ‘super moon’. The definition of a ‘super moon’ is a bit fuzzy (actually, very fuzzy: it is not a term used in astronomy but was invented for astrology), but the term may be used if the moon comes closer than 360,000 km. No worries, it doesn’t affect us: the tides may be a few centimeters higher (added to the spring tide) but that is about it. (If you are more worried about this than about sea level rise from global warming, then you may want to reconsider.) This makes the Jan 31 full moon a ‘blood super moon’ – just about.
(If we are a bit more precise, during the exact moment of full moon, Jan 31 at 13:27 GMT, the moon will have moved away to 360,199 km, so the super moon ends before the full moon occurs. We’ll let this pass.)
But there is more. The full moon of Jan 31 will be the second full moon this January, after the one on Jan 1st. To have two full moons in one calendar month is not all that common. In recent decades, since about 1980, the second full moon within one month has been called a ‘blue moon’. So if you weren’t confused enough, we now have a ‘blue blood super moon’: it will be red, blue and big all at the same time!
Again, to be more precise, this complexity will not be true everywhere. If you have the misfortune to live in eastern Australia, or the fortune to live in New Zealand, Feb 1 will arrive before 13:27 GMT and so for you the full moon will happen in February – and it won’t be blue. As compensation, you will have a superb view of the total eclipse.
Moons of colour
From experience, we know that the sky is blue, the sun yellow, and the moon 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 scattered. The result is a red sun, whilst the scattered light makes the sky 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 often show a blue haze around the sun as well. To the eye the colour is marked. 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 (and green!) 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 forest 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 English expression ‘Once in a blue moon’ referred to rare events like this. It describes something that happens very rarely, but which is not impossible. But its history is more complex – and more obscure.
How old is the expression ‘blue moon’?
It is a uniquely English expression, and does not occur in other languages. (The closest is an expression in German for ‘working a blue Monday’ which refers to someone who hardly did any work before leaving the job.) But there is surprisingly little mention of it in the older English literature. When it occurs 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 gives a unique sound bite of the street language, very different from normal written work of the day. 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 around or 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. attributed to William Roy and Jerome Barlow (but William Barlow admitted being the author a few years later, when he tried to retract this rather unpleasant document.) The mention of ‘blue moon’ is in the following lines:
Holdynge the worlde universall
Agaynstl god they are so slobbourne
That scripture they tosle and tourne ‘
After their owne ymaginacion.
Yf they saye the mone is belewe
We must beleve that it is true
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 a few mentions in British books. For instance, 1860. F. W. Robinson, Grandmother’s Money, 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 a blue moon, had the talk been of markets, and prices, and quotations, at length thought it time to vary the intellectual repast, and one of them suggested that somebody should sing a song.
Published in 1876, Mary Elizabeth Braddon’s book Joshua Haggard’s Daughter, contains the line Why should she stint as to one or two puddings a week . . . and a fruit pasty once in a blue moon. In 1884, Robert Francillon, Ropes of Sand, 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 thus survived and was in use, but it was not particularly common. All four authors lived in London, confirming the local origin of the phrase.
The expression resurfaced in North America. Following the Krakatoa eruption in 1883 blue moons had been seen there as well. A letter written 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, 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 well known, but by now the origin had been lost.
The Maine Farmers’ Almanac (a publication that existed from 1819 until 1967) was the first to give it 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 of the four seasons should have three full moons. But some years have 13 rather than 12 full Moons, and in those years there must be one season with four rather than three. This throws the naming out of kilter with the agricultural 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 introduced but it was in use by 1915. 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 found 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) but also throws some doubts on the reliability of this source:
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 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 ecclesiastical 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. The origin of 13 as ‘unlucky number’ is unknown: the claim that it refers to an extra lunar month does not have any other source t back it up. Neither is there support for the claim that it was the Almanac that 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 become obscure.
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 had defined the seasons as starting at the solstices and equinoxes, using a mean motion of the Sun (correcting for the Earth’s elliptical orbit around the sun, with its varying speed). These are moments in time everyone can agree on. Instead, the new definition depends on location. A full moon may occur on different dates, depending on the time zone. This, in 2018 there is twice 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, in New Zealand, these full moons will occur on Feb 1st and April 1st, and according to this definition, neither will be ‘blue’.
How often does a blue moon occur?
True coloured moons (or suns) are rare. Regions with frequent dust storms or extreme pollution 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 figurative blue moon, such as that of Jan 31, occurs more frequently than that. 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.
Using the older definition, of four full moons in a season, blue moons occur a little less often. Each calendar year has a 36.85% chance of having a 13th full moon. Such a blue moon will happen on average once every 32.56 months.
Type ‘once in a blue moon’ into the google search bar, and it will helpfully give you a numerical answer: 1.16699016 × 10-8 hertz, which is equal to 991.788 days or 32.59 months. It does not say which definition is used. If it is the modern one, of two full moons in a month, google would be wrong and this would be inconceivable. Clearly, it used the older (better) definition.
So how often is ‘once in a blue moon’? It is about once every 2.7 years.
There are roughly 35 volcanic eruption on Earth per year. So ‘once in a blue moon’ is approximately ‘once every 100 eruptions’. And that should be an easy one to remember.
The story that the moon’s reflection in a pond or well is a cheese under water also goes back a long time, first appearing in writing in the 11th century. On the oldest versions, it is a hoax which the gullible wolf falls for. In English literature this idea first surfaces in 1548, where the moon os presented as green (or young) cheese, but again in the meaning of a hoax. Talla Hopper on this blog recounted the following story:
I live in Wiltshire where locals (particularly round Devizes) are called Moonrakers. When I first heard this expression (back in the 1950’s) it meant that the locals were a bit dim in the head. The story then was that a man passing a village pond at the time of a full moon saw some farm labourers raking the water. He stopped and asked them what they were doing and they said they were trying to rake “that gurt [great] cheese” out of the water. This was a sign of the simple-minds of agricultural workers. Much of Britain’s history was revised in the 1960’s and now the explanation is of local people out-witting the local Excise Men. The story now goes that locals were smugglers and had hidden their stash (probably rum) in the pond. The local Excise Men found them but they pretended to be daft and raking for the “gurt cheese in the water” so they were let off.
Sometimes the stories of the ‘green cheese moon’ is said to be the origin of the ‘blue moon’ expression. This seems very unlikely: they have very different meanings and histories.
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 around 1820, but there was no observation or definition behind it. The suggestion that it derived from a medieval tradition, related to the ecclesiastical 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 (who sometimes were astronomers), and redefined erroneously in a popular astronomy magazine. Astronomy adopted an orphaned expression and gave it a new home.