Where does the idiom ‘once in a blue moon’ come from?
We often use the term ‘once in a blue moon’ to indicate a rare occurrence. This common idiom is often associated with the second full moon in a calendar month, that is referred to as the ‘blue moon.’ However, this is not the entire story and the phrase ‘blue moon’ itself has existed over 400 years, while its calendrical meaning has only become popular in last 25 years. So, what is the real origin story behind the phrase ‘once in a blue moon?’ Let us find out.
When and how did the phrase ‘once in a blue moon’ originate from?
In the 16th century, an absurd notion about the moon had surfaced, that is, the moon was blue. However, scientists in those times declared the idea as bizarre, which is when the phrase ‘blue moon’ started to mean as something that is never going to happen or is too infrequent to notice.
However, throughout history, there have been instances when the moon actually did turn blue. For instance, in 1883, in and around the explosion of the Indonesian volcano Krakatoa, the moon was observed to have turned blue. In 1927, when India was going through a long-stretch of drought and the monsoons were terribly late, then too, the moon was seen to be blue. Also, in 1951, the moon appeared to be blue in western Canada and north-eastern North America.
Therefore, by mid-19th century, the scientist community had concluded that although blue moons were rare, they were real and visible from time to time. This is when the phrase ‘once in a blue moon’ also came into being. The phrase meant what it exactly means today, that is, a fairly infrequent event that is not quite regular enough to pinpoint.
When was the idiom first used?
In 1821, in a non-fiction named ‘Real Life in London’ by Pierce Egan, the phrase ‘once in a blue moon’ was first used and it meant something that was impossible or too rare to ever happen.
What is the modern meaning of ‘blue moon?’
A blue moon is often referred to as the extra full moon that appears in a subdivision of a year, namely, the second of two full moons that occurs in the same calendar month or the third full moon in a season that contains four rather than the three usual full moons. These happen because the moon’s phases are not in sync with our regular annual calendars and all the months except February are longer than the usual lunar cycle of 29 and a half days. This is why, blue moons are seen about seven times in every 19 years. Since the months have different lengths, the phenomenon moves around it.