Blue Moon – Meaning ?


Blue Moon – Meaning ?

We often use the term ‘once in a blue moon’ to describe a situation or event that is rare.  But where did this saying come from ?

We metaphorically use this expression of BLUE MOON as it relates to one of the following rare lunar events:

1. Two full moons fall within one calendar month, thus the 2nd full moon is called a BLUE MOON.  This is the most popular definition of a Blue Moon; or

2. BLUE MOON is referred to as the 3rd of 4 full moons in a single season (with a season being defined as the time period between a solstice & equinox or equinox & solstice).

Whilst the term ‘once in a blue moon’ doesn’t really relate to the colour of moon, the moon can take on a blueish appearance at times due to changes in atmospheric conditions such as smokey haze or dust particles.

BLUE MOON – 2nd Full Moon in Calendar Month

How did the 2nd Full Moon in a Calendar Month come to be called a BLUE MOON ?
The March 1946 edition of  Sky and Telescope magazine, contained an article by James Hugh Pruett entitled “Once in a Blue Moon”.  In this article where he inadvertently misinterpreted/simplified the 1937 Maine Farmers Almanac definition of BLUE MOON:  “Seven times in 19 years there were – and still are – 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.”   As a side note here, interestingly 1937 only had 12 full moons !

EarthSky’s Deborah Byrd happened upon a copy of this old 1946 issue of Sky and Telescope in the stacks of the Peridier Library at the University of Texas Astronomy Department in the late 1970s. Afterward, she began using the term Blue Moon to describe the second full moon in a calendar month on the radio. Later, this definition of Blue Moon was also popularized by a book for children by Margot McLoon-Basta and Alice Sigel, called “Kids’ World Almanac of Records and Facts,” published in New York by World Almanac Publications, in 1985. The second-full-moon-in-a-month definition was also used in the board game Trivial Pursuit.

Thus whilst this definition of BLUE MOON is not technically correct, it is the most popular definition and most commonly used.

BLUE MOON – 3rd of 4 Full Moons in a Season

Purists, swear by their definition of BLUE MOON being the third full moon of four in a season. The Maine Farmer’s Almanac defined a Blue Moon as an extra full moon that occurred in a season (and their definition of season related to winter, spring, fall, summer).  A season normally has 3 full moons, however if there are 4 full moons in a season then the third full moon is called the BLUE MOON and the 4th moon is referred to as the last of that season.


July 2015 – Second full moon in month

May 2016 – Third full moon in a season of four full moons

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