Once in a Blue Moon

To say something happens “once in a blue moon” is to say it is very rare. Your husband doing the dishes, for example. But like many expressions in the English language, this one’s meaning has got muddied along the way.

It dates back some 400 years, but back then, it would have been akin to a modern day “when pigs fly”: a nonsensical thing that will never happen. Like your husband cooking dinner.

The earliest recorded example comes from 1528, in an anti-clerical pamphlet by William Roy and Jerome Barlow (complete with Old English spellings):

O churche men are wyly foxes… Yf they say the mone is blewe / We must beleve that it is true / Admittynge their interpretacion.

So why did the meaning change?

What is a blue moon exactly?

This is the problem, there are two answers to that question because there are two phenomena that we could be referring to when we’re saying “blue moon”.

To start with the more literal meaning, there are instances in history where, owing to large amounts of dust particles and/or smoke in the atmosphere, the moon actually appears blue in the night sky.

Below is an illustration (well, a lithograph, but anyway) of one of the most famous examples of this: the 1883 eruption of Krakatoa, which shot smoke and dust and sulphur in to the sky and resulted in the moon literally appearing blue for two years (as well as making the sky look red, the sun look lavender and sunsets appear green).

So why, of all colours, did the moon appear blue? Well, the particles in the ash clouds that were spewed in to the sky were just thick enough to scatter red light while allowing other colours through. It speaks for the size of this eruption that it literally filtered the light for years after.

But to call it an eruption is an understatement. It is often referred to as the time Krakatoa “exploded” which gives a truer indication of the force behind this thing. Some 36,000 people died (from the initial explosion and the resulting tsunamis), the reflective nature of the sulfur dioxide and other particles that had been thrown in to the atmosphere cooled the average temperature of the Earth by 0.4 °C, and it is said to have been the inspiration for Edvard Munch’s The Scream.

Just to be clear, this sort of thing doesn’t happen with every volcanic eruption: this was literally a “once in a blue moon” event, resulting in an actual blue moon.

Other examples of this phenomena since are appropriately sparse: in 1927, India experienced a particularly long dry season, resulting in more dust being blown in to the sky as usual, and a blue moon in their sky. In 1951 the smoke from gigantic forest fires in Canada gave those in northeastern North America a blue moon to look at for a short period of time. And most recently in 1980 as a result of another volcanic eruption, this time Mount St. Helens in the USA: another doozy.

Perhaps these recorded events are why the meaning of the expression “once in a blue moon” changed from never to rare. Suddenly it wasn’t an absurdity anymore, and saying something happened “once in a blue moon” meaning never was simply wrong. Pig’s never fly, but the moon does go blue sometimes.

The other Blue Moon

But because we can’t have nice things and because as humans we love to change meanings and overcomplicate everything, there is of course a second definition of a “Blue Moon”, which is less literal and less rare.

To explain: the average lunar cycle (full moon, half moon, new moon etc) is 29.53 days, and there are 365.24 days in an Earth year. Dividing one by another gives us an average of 12.37 lunar cycles in a year, with a rigid 12 calendar months a year to squeeze in to. So the end result is every 2–3 years (7 times in 19 years to be more precise) we have an extra full moon in our night sky to contend with in a calendar year.

Those who track these sorts of things (farmers etc) have names for every Moon in the cycle, which are wildly inconsistent from source to source and entirely arbitrary (“Beaver Moon” is/was a name given to the full moon in November, for example). So this infrequent but predictable spare moon needed a name as well, and “Blue Moon” just seems to be the one that stuck. Possibly because it already existed as an expression, with them leaning on the idiom’s meaning to some extent, but it is impossible to be certain.

Much like leap years, the lunar cycle is entirely predictable and to call it a rarity would be a massive overstatement.

Our natural tendencies to distort language and give things multiple meanings have inexplicably caused the expression and the lunar cycle to become mixed up, as has our nasty habit of building folklore and retroactively applying meaning to quite meaningless things.

To add to that, nowadays a blue moon is also a beer, a Jeff Bezos funded spacecraft, the name given to various animals, and something old singers croon over as a symbol of loneliness and sadness.

But let’s just be clear here once and for all: a Blue Moon is the 13th full moon in a calendar year (the second in an astronomical season), which doesn’t happen all the time, but it certainly does not happen “once in a blue moon” either. Got that? Good.

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Sources

• https://skyandtelescope.org/observing/once-in-a-blue-moon/
• https://www.phrases.org.uk/meanings/once-in-a-blue-moon.html
• https://www.smithsonianmag.com/science-nature/once-in-a-blue-moon-and-other-idioms-that-dont-make-scientific-sense-76485092/
• https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa

• https://en.wikipedia.org/wiki/Blue_moon