People have been using this phrase to imply “hardly ever” for at least 400 years.
very rarely.
“he comes round once in a blue moon”
Original Meaning
It seems to have its origins in a rare meteorological phenomenon in which the moon’s light passes through dust or smoke particles, creating a bluish tinge similar to that caused by cigarette smoke.
A New Meaning
However within the in the last 150 years or so the phrase has somehow acquired a second meaning :
![](https://extra.guernseydonkey.com/wp-content/themes/Extra-Child/images/quotes60x50-grey-left.png)
A month in which two full moons appear- a calendrical quirk made possible by the fact that the average length of a month is just a smidgen longer than the period between successive full moons.
![](https://extra.guernseydonkey.com/wp-content/themes/Extra-Child/images/quotes60x50-grey-right.png)
Quite why this new meaning has anything to do with the colour blue isn’t clear but it does at least allow us to give a precise definition of the phrase :
![](https://extra.guernseydonkey.com/wp-content/themes/Extra-Child/images/quotes60x50-grey-left.png)
a blue moon puts in an appearance once in a period of slightly over 2 years.
![](https://extra.guernseydonkey.com/wp-content/themes/Extra-Child/images/quotes60x50-grey-right.png)