Blue Moon

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The blue moon occurs when a second full moon falls in a calendar month. Image by Andy Williams/Taxi/Getty Images

How many times have you heard the phrase "once in a blue moon"? Well, it's more than just an expression - a blue moon is the name given to an actual phenomenon. Here's how it works.

The Science Behind the Blue Moon

A full lunar cycle is a little over 28 days long. However, a calendar year is 365 days, which means that during some years, you may end up with thirteen full moons instead of twelve, depending on where in the month the lunar cycle falls.

This is because during each calendar year, you end up with twelve full 28-day cycles, and a leftover accumulation of eleven or twelve days at the beginning and end of the year. Those days add up, and so about once every 28 calendar months, you end up with an extra full moon during the month. Obviously, that can only happen if the first full moon falls in the first three days of the month, and then the second takes place at the end.

Deborah Byrd and Bruce McClure of Astronomy Essentials say, "The idea of a Blue Moon as the second full moon in a month stemmed from the March 1946 issue of Sky and Telescope magazine, which contained an article called “Once in a Blue Moon” by James Hugh Pruett. Pruett was referring to the 1937 Maine Farmer’s Almanac, but he inadvertently simplified the definition. He wrote: 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." 

So, although the term "blue moon" is now applied to the second full moon to appear in a calendar month, it originally was given to an extra full moon that happened in a season (remember, if a season only has three months on the calendar between the equinoxes and solstices, that fourth moon before the next season is a bonus).

This second definition is a lot harder to keep track of, because most people just don't pay attention to the seasons, and it generally happens about every two and a half years.

Of note, some modern Pagans apply the phrase "Black Moon" to the second full moon in a calendar month, while the Blue Moon is specifically used to describe an extra full moon in a season. As if this wasn't confusing enough, some people use the term "Blue Moon" to describe the thirteenth full moon in a calendar year.

The Blue Moon in Folklore and Magic

In folklore, the monthly moon phases were each given names that helped people prepare for various types of weather and crop rotations. Although these names varied depending on culture and location, they generally identified the sort of weather or other natural phenomenon that might take place in a given month. 

The moon itself is typically associated with women's mysteries, intuition, and the divine aspects of the sacred feminine. Some modern magical traditions associate the Blue Moon with the growth of knowledge and wisdom within the phases of a woman's life. Specifically, it is sometimes representative of the elder years, once a woman has passed far beyond the status of early cronehood; some groups refer to this as the Grandmother aspect of the Goddess.

Still other groups see this as a time - because of its rarity - of heightened clarity and connection to the Divine. Workings done during a Blue Moon can sometimes have a magical boost if you're doing spirit communication, or working on developing your own psychic abilities.

Although there is no formal significance attached to the blue moon in modern Wiccan and Pagan religions, you can certainly treat it as an especially magical time. Think of it as a lunar bonus round. In some traditions, special ceremonies may be held -- some covens only perform initiations at the time of a blue moon. Regardless of how you see the Blue Moon, take advantage of that extra lunar energy, and see if you can give your magical endeavors a bit of a boost!