“Once in a blue moon” is one of those phrases that just roll off of our tongues without much prior analysis from our brains. Somewhere in the back of our consciousness is the knowledge that the phrase means “every once in a great while.” But where did this saying come from?
It just so happens that a blue moon is a real astronomical event. It also just so happens that astronomers are somewhat divided on the precise event.
There are two events that the term “blue moon” is sometimes used to describe:
- A second full moon in any month.
- A third full moon in any season containing four full moons instead of the usual three.
The first definition seems to be more common than the second (perhaps because it’s easier to remember). However, it also appears to date back no farther than 1946, when an amateur astronomer mistakenly connected blue moons with a comment in a copy of the Maine’s Farmer Almanac which said that the moon sometimes “comes full thirteen times in a year.”
The second definition, however, is much older. It dates to an old custom of naming the full moon of each month. “Harvest moon” is one of the last relics remaining of this practice, and it refers to the convenient full moon that gave light in harvest time in some parts of the world.
Some of the moon names we are less familiar with today include:
- Lenten Moon: The last full moon of winter, falling sometime within the observance of Lent.
- Paschal, Easter, or Egg Moon: The first full moon of spring, falling sometime within the week before Easter.
- Moon Before Yule: This full moon usually fell in December.
- Moon After Yule: This full moon usually fell in January.
And there were many others. Different cultures used different names to describe the full moons, and many of these names revolved around agricultural activities.
Once in about every three years, however, an event occurred which threw this handy calendar out of alignment—there would be 13 full moons in a year instead of 12!
To solve this problem, the third full moon in the offending season would be called “blue,” the fourth moon would follow the regular calendar of names, and the months could progress as usual.
Why the extra full moon was called blue is something of a mystery. The Native Americans referred to pink moons (the moon of the month when a flower called moss pink or mountain phlox was prevalent) and red moons (the moon of the month when the sky is filled with reddish haze). But blue? The explanation seems to be lost in the mists of time.
So now you have the origin of the phrase “once in a blue moon.” A blue moon is actually the third full moon in a season with four full moons instead of three, an astronomical event occurring approximately once in three years.