Once in a Blue Moon

A client of mind just reminded me that this summer (August 31st) the occasion of the Blue Moon will be upon us. Wow, it kind sneaks up on you sometimes! The last casting I had for my Blue Moon Wish Spell was two years ago!

So I thought I might entertain you with a little Blue Moon lore since my client brought it up and I have always adored the celebration of the Blue Moon.

What is a Blue Moon?

There are in fact two definitions for a blue moon. According to the more recent definition, a blue moon is the second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month (the average span between two moons is 29.5 days).

The Other Kind of Blue Moon

The older definition, which is recorded in early issues of the Maine Farmer’s Almanac, states that the blue moon is the third full moon in a season that has four full moons. The full Moon on Nov. 21, 2010, was this type of blue moon; it was the third of four full moons between the fall equinox and the winter solstice. Why would one want to identify the third full moon in a season of four full moons? The answer is complex, and has to do with the Christian ecclesiastical calendar.

Some years have an extra full moon—13 instead of 12. Since the identity of the moons was important in the ecclesiastical calendar (the Paschal Moon, for example, used to be crucial for determining the date of Easter), a year with a 13th moon skewed the calendar, since there were names for only 12 moons. By identifying the extra, 13th moon as a blue moon, the ecclesiastical calendar was able to stay on track.

How Often Does a Blue Moon Occur?

Over the next 20 years there will be about 15 blue moons, with an almost equal number of both types of blue moons occurring. No blue moon of any kind will occur in the years 2011, 2014, and 2017.

The more recent phenomenon, where the blue moon is considered to be the second full moon in a calendar month, last occurred on December 31, 2009. Two full moons in one month may occur in any month out of the year except for February, which is shorter than the lunar cycle.

The other, older blue moon event, which happens when there are four full moons in a season, last occurred in May 2008 and will again on Nov. 21, 2010. Since this type of blue moon is reckoned according to the seasons, it can only occur in February, May, August, or November, about a month before the equinox or the solstice.

Twice in a Blue Moon

The rare phenomenon of two blue moons (using the more recent definition) occurring in the same year happens approximately once every 19 years. 1999 was the last time a blue moon appeared twice, in January and March.

The months of the double blue moons are almost always January and March. That is because the short month that falls in between them, February, is a key ingredient in this once-every-19-year phenomenon. For January and March to each have two full moons, it’s necessary for February to have none at all. Since February is usually 28 days long, and the average span between full moons is 29.5 days, if a full moon occurs at the end of January, it’s possible for the next full moon to skip February entirely and fall in the beginning of March.

 

 

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