Two full moons will grace the December sky this year. This happens occasionally: most months have 30 or 31 days. The lunar month – the time it takes the moon to orbit the earth (also known as the lunation or its synodic cycle) – is 29 days, 12 hours, 44 minutes and 2.78 seconds. This time difference between the lunar month and the calendar month causes the two cycles to slowly weave in and out of sync. Every once in a while the synodic cycle of the moon falls completely within a calendar month, and two full moons appear in that given month. This happens about once every two or three years. The second full moon in a calendar month is commonly known as a ‘Blue Moon.’

This popular consensus on the astronomical meaning of a ‘Blue Moon’ is recent, only really finding it’s way into our language within the past 30 years or so.

The back story on the expression ‘Blue Moon’ is wonderfully complicated. Old English scholars point to the possible double meaning of ‘belewe’ in the earliest reference to ‘Blue Moon,’ which appeared in a short pamphlet entitled Rede Me and Be Not Wrothe. This short work was published in 1528. It was, to my understanding, a kind of critical essay mocking the English Clergy. The exact line is:

If they say the moon is blue / We must believe that it is true.

The word ‘belewe’ had a double meaning in Old English. It could have meant either the color blue, or ‘to betray.’

I think the writer was a man of wit, working ‘belewe’ here as a clever double-entendre, and perhaps introducing the idiomatic expression ‘blue moon’ for the first time. Although the meaning was clearly different then.

Here is the backstory. The church was the keeper of all arcane knowledge in those dark times, including the calculation for the date of Easter.

The date for Easter was (is) based on the date of a particular full moon. Easter must fall on the Sunday following the first full moon after the ecclesiastical vernal equinox, which always falls on March 21. (The Church divided the year neatly into 4 seasons, each of exactly 3 month, with the equinoxes and solstices always falling on the 21st. Astronomers, however, always define the change of seasons as when the sun crosses the celestial equator, or when it reaches it’s northernmost or southernmost extreme.)

Many important Christian days, such as Saint days, are fixed on the calendar, always on the same date, but Easter is variable because of the holy day’s origin during early Christianity’s Jewish Period. It referenced the Jewish lunisolar calendar, as did many other important events. The tradition of doing the calculation rather than fixing the date was kept by the Christians as its dogma evolved from Judaism.)

It’s a little too complicated, and off subject, to go into it here, but the calculations are kind of interesting and you can look at them here).

The short version of the story is that the church had to say which moon was the Lenten Moon, and which was the Paschal (Easter) Moon, because it wasn’t always obvious. Seven times over the course of every 19 years there would an extra moon somewhere in a season.

The three full moons across each “normal” season all had well established names that helped people organize their lives. The moons marked the change of seasons, and told them when to plant, reap, fast, celebrate, be hopeful. To everything there is a season as it is so beautifully written in the book of Ecclesiastes. But the ‘belewe’ moon betrayed this celestial almanac. Since there was no regular name for this moon, because it appeared in different seasons, those who kept the names and the knowledge would call this third full moon in a season of four moons the ‘belewe’ moon. The fist, second and last moon of a season kept their traditional names, and the rhythm of the seasons remained intact.

I’m no Old English scholar (or even student) but it seems to me that this early pamphlet confirms that the everyday use of the phrase ‘belewe moon’ was the liturgical reference to the third full moon in a season of four full moons. And that the ironic use of ‘belewe moon,’ meaning something that is absurd or impossible, was also indicated, perhaps for the first time, here in this same short work. Why else would the author speak thus? He was already being critical of the clergy, so there was obviously no fear involved, no need to cleverly mask the meaning in double-entendre. I call it wit. We’ll never know for sure.

This ecclisiastical definition of a ‘blue moon’ is followed by the Maine Farmer’s Almanac and is noted in publications as far back at 1817. This ‘rule’ is now generally referred to as the ‘Maine rule.’

And there’s more! Due to a misinterpretation of a 1943 Sky & Telescope article about the Maine Rule, a second definition of “blue moon” entered the vernacular. In an understandable mis-reading of this article an author simplified the rule to define a blue moon as the second full moon in any given month. One of the Stardate editors picked up on this definition from Sky&Telescope (hey, if you can’t trust S&T who can you trust?) and further popularized it in national radio broadcasts sometime in the late 1970s. The error has been noted by both Stardate and S&T, but the definition is so much a part of popular culture now it’s even made its way into Trivial Pursuit. But maybe not the genius edition.