Where did that saying come from - “once in a blue moon"?

We, as a species, often use words, sayings, phrases, idioms, quotes, clichés, proverbs, maxims, quips, adages, epigrams and epithets without actually knowing what they mean. Sometimes this happens so widely and consistently as to butcher the original meaning, and billions of people end up wandering around saying things that are wildly inaccurate and linguistically nonsensical. I digress; back to the subject. I was researching a post over at Blogstronomy and came across the phrase 'blue moon,' or, more usually 'once in a blue moon.' Of course, I know what it means:

This is not a "blue Moon", just the Moon photographed in the daytime.
Image by Susan Cipriano from Pixabay

'Once in a blue moon' is used to describe something that doesn't happen very often.

For example "Tommaths is read once in a blue moon."

But where did it come from?

Most years will have twelve full moons, one in each month. The length of a lunar month can be a little hard to pin down (depending on how you're measuring it) but in the context of this post we're looking at the time between one full Moon and the next, which is (on average*) 29.5 days. In a non-leap year, then, that means that there are $365/29.5 \approx 12.37$ lunar months in each year, so every now and then we get a 'leap' full moon**. That's a 0.37 month discrepancy compared with our nice, predictable calendar or exactly 12 months, so we get a bonus full-Moon every ($1/0.37 \approx$) 2.7 years. 

But who's to say which full moon is the 'extra' one?

• Back in olden times***, if there were more than the traditional three full moons in any season (a season being a given set of three months), then the third full moon would be the 'blue' one. The next blue moon using this definition will be on August 22nd, 2021.
• A more modern method (i.e. traditionally incorrect but now widely used because it's a lot simpler) is to say that if there are two full moons in any one (calendar) month, it's the second one that is "blue". This is thought to have stemmed from a misinterpretation of the Farmer's Almanac by a 1946 edition of Sky & Telescope magazine. The next blue moon using this definition is at the end of the month that this sentence is being written (and is what prompted the update of this article): 31st October, 2020 (because we also saw a full Moon at the very beginning of the month).

But why "blue"?

Many an individual has carefully marked the date of the next blue Moon and rushed out at night only to be disappointed that the Moon appears just as it always does and is not, in fact, actually blue. It is, therefore, important to note that "blue" Moons are not actually coloured blue.

The earliest known usage of the term "blue moon" is in a 1528 pamphlet catchily entitled "Rede me and be nott wrothe, for I say no thynge but trothe," a document aimed at disparaging King Henry VIII's almoner, Cardinal Thomas Wolsey. The relevant part reads "Yf they say the mone is blewe / We must beleve that it is true," with the broader context implying something along the lines of 'If religious leaders say weird things we're just supposed to believe them," with no obvious reference to timekeeping or astronomy.

Another (largely unsupported) speculation is that a "blue Moon" was originally a "belewe Moon," with "belewe" being an Old-English word meaning "to betray": the extra full Moon betrays the expected order of things.

But, and this is important, can the Moon ever actually be the colour blue?

In the right conditions, our moon can (and has) appear(ed) blue to the naked eye. If there is the right concentration of smoke or dust particles of the right size (about one micrometre in diameter), light at longer wavelengths (that's the red end of the visible spectrum) may be scattered more than the shorter wavelengths, meaning that the shorter wavelengths of light are more likely to reach your eyes, shifting the observed colour of the moon towards the bluer end of the visible light spectrum. As for any other moons... I guess that depends on what they're made of*****.

* The implication here is that this time varies month by month, and that's absolutely true.

** As in 'leap' year or 'leap' second. I made up that terminology. It's not official. "Leap Moons" are not (usually) marked by a cow jumping over it.

*** This is a standard phrase that means 'I don't know when, I can't be bothered to look it up, but it was quite a while ago now.' We could also say "when I was young and you were even younger". 

**** Actually "blewe moon".

***** Stilton, maybe? That would be blue-ish.