Why wasn't the Blue Moon blue?
As we completed August's lunar cycle, many of you commented that the Super Blue Moon did not look blue. Because occasionally the moon does appear blue - and I've cheekily used a photo from my February 2019 trip to Thailand here not the one I took in Sidcup last month, which was more orange - I thought you might be interested to know the etymology of the name.
August's full moon was also a supermoon, which references the moon's orbit in relation to Earth. Supermoons appear bigger and brighter because they are so close to Earth. However, confusingly, the term blue moon has nothing to do with colour and instead refers to the frequency. A blue moon is either a second full moon in a calendar month or, by the astronomical definition, the third full Moon in an astronomical season of four. August 2024’s blue moon fell into the latter category.
Cultures around the world, have given names to each of the full Moons, each typically happening in its own month - for example the 'Wolf Moon' is usually the full Moon occurring in January. With the cycle of the phases of the Moon lasting approximately one month, and there being 12 months in a year, we typically have 12 full moons each year.
However, the phases of the Moon actually take 29.5 days to complete, meaning 354 days total for 12 full cycles. This falls some way short of the 365/366 days in a calendar year: therefore, roughly every two and a half years a 13th full moon is seen. This additional full moon doesn't fit with the normal naming scheme and so is instead referred to as a ‘blue moon’. In addition, the older, more traditional meaning of a blue Moon is related to the third full Moon in an astronomical season of four, also referencing that it’s an uncommon event.
Because a blue moon is rare, the idiom “once in a blue moon” is now used to describe any rare event. But where did the colour blue come from? Though the Moon can appear with a bluish tinge under certain atmospheric conditions, for instance, if volcanic eruptions or fires release particles in the atmosphere of just the right size to scatter red light preferentially - tbh I have no idea why the moon appeared blue on my trip to Thailand, but it was beautiful - you now know the term blue moon refers to frequency, so why use the word blue to describe this event?
Like many words the exact roots are tricky to unpick - those of you who've completed the BWY Foundation Course with me will know translating and interpreting ancient Sanskrit descriptions of yoga practices is not straightforward - but the current known earliest recorded English usage of the term blue moon is found in an anti-clerical pamphlet, published in 1528 containing the passage:
"O churche men are wyly foxes [...] Yf they say the mone is blewe
We must beleve that it is true
Admittynge their interpretacion."
Thought to be using an example of an absurd statement, "the moon is blue", to make the point that priests require laymen to believe in statements even if they're obviously false, it's believed that the term blue moon originated from the 16th-century expression “the Moon is blue,” meaning something impossible.
Later, in 1883-5, following the eruption of the Krakatoa volcano in Indonesia, people around the world reported seeing strangely coloured sunsets and a Moon that appeared blue. The impossible became possible, and from here, it's understood that this gave rise to the phrase “once in a blue moon,” meaning a rare rather than impossible occurrence.
Sources
https://en.wikipedia.org/wiki/Blue_moon
https://www.britannica.com/question/Why-is-a-blue-moon-called-a-blue-moon
https://www.rmg.co.uk/stories/topics/what-blue-moon-how-often-does-it-occur