A blue moon is a phrase with multiple meanings, all of them implying something rare and special. In seeking out the real meaning of blue moon, astronomy and the calendar walk hand in hand with religion, farming, and culture, joined by semantical mistakes.
Currently, a blue moon is defined as the occurrence of two full moons in the same calendar month. The second full moon is the blue moon. All calendar months except for February are 30 or 31 days. This is slightly longer than the 29.5 days the moon takes, on average, to complete its cycle. As a result, one brief stage of the lunar cycle repeats itself within each calendar month. About once every two and a half years, the repeating phase is the full moon.
As of this writing, December 2009, New Year's Eve is slated to be a blue moon in Europe and the Americas. The last full moon came at 7:30 am GMT on December second. The next one will occur at 7:13 pm GMT on the thirty-first, just in time for New Year's Eve revelers to party under the fullness of the blue moon. The time zones in the Americas, ranging from Brazil to Hawaii, fall between three and ten hours behind GMT, keeping this blue moon in December.
Across the International Date Line that cuts through the Pacific, in Australia, New Zealand, and eastern Asia, the moon will not technically be full until the morning of New Year's Day. In those locations, the blue moon will come at the end of January 2010. However, those parts of the world will see a partial lunar eclipse on the thirty-first. Either way, New Year's Eve 2009 will be a rare occurrence as far as the moon is concerned. That brings us to the overarching definition of a blue moon: something uncommon.
The expression "once in a blue moon," in existence for at least a few hundred years, can imply something so rare as to be almost nonexistent. If someone says, "I only go to the movies once in a blue moon," they probably mean they go to the movies no more than once in a year and a half, if that. They may just as easily mean they haven't been to the movies in five years and have no plans to go anytime soon. They could also mean they go to the movies once or twice a year.
Even rarer than two full moons in a month is the scientific phenomenon called blue moon, unrelated to its phase. In this phenomenon, the moon literally does appear blue, and may for months at a time. The color is the result of an unusually high amount of dust particles and smoke in the atmosphere. It has been observed after major volcanic eruptions and particularly devastating forest fires. The 1883 eruption of Krakatoa in Indonesia created a literal blue moon that lasted for two years.
Having two full moons in a month does not, of course, change its color. How and why that came to be called a blue moon is lost in the mists of time and folklore. It can only be speculated.
Originally, a blue moon on the calendar meant, not two full moons in a calendar month, but four full moons in a three month season. This was of utmost importance to the Catholic Church, because it had to reconcile the irregularities between the lunar months and the solar year in order to set the dates for Lent and Easter.
To this day, the dates for Lent and Easter are calculated based on the lunar cycle. Easter is the first Sunday after the first full moon after the first day of spring. Traditionally, the first full moon of spring was called the egg moon. Lent begins with the new moon that starts the last complete lunar cycle of winter, traditionally called the Lent moon. The Lent season is to last a total of forty days.
Most years, this runs smoothly. Lent begins exactly twelve lunar months after the date when it began last year. However, because a solar year contains twelve complete lunar cycles plus ten to twelve days of a thirteenth, this would fall out of sync every two or three years. The starting date for Lent would have to be pushed forward another moon cycle, so as to keep it within forty days of the first full moon of spring. That extra lunar month came to be called the blue moon.
Farmers also needed the blue moon to keep their calendar on track. The traditional names for most of the year's full moons, such as egg moon, growing moon, and harvest moon, were used to determine when to plant crops, when to harvest, and when to expect the chickens, who do not lay eggs in the winter in a cold climate if not tricked into doing so by artificial light, to begin laying again. But just as in the Church calendar, the farmers had to insert an extra moon every two or three years to keep the new season from starting too early.
It is thought that calling this extra moon the blue moon may be a trick of homonyms. In Old English, "blue" sounded like the word for "betrayer." That extra month was a tricky month, sneaking into the calendar as it did. It did not have any solid significance attached to it, unlike the egg moon or the harvest moon. It was the month that no one quite knew what to do with.
Regardless of its original etymology, the usage of "blue moon" to denote a second full moon in one calendar month dates to a widely publicized mistake made in 1946. Sky & Telescope magazine, in an article based on information drawn from the Maine Farmer's Almanac, reported that a blue moon is the second full moon in a month. The Farmer's Almanac never said any such thing. It identified the blue moon as the third full moon in a season in which there were four full moons instead of the usual three.
True to the roots of its name, a blue moon is not really what it sounds like. However, the phrase has incredible cachet, making it a highly popular name for businesses and creative works. A Google search for "blue moon" turns up several restaurants, bars, hotels, a craft shop, and an art gallery or two with that name, as well as a software company and a brand of beer. No less than ten novels currently in print are titled "Blue Moon" (there is no copyright on titles). The songs and poems with that title are likely too many to count.
The real meaning of blue moon is almost as elusive as the roots of the name itself. Currently defined as the second full moon in a calendar month, and idiomatically defined as any rare occurrence, a blue moon originated as a way to mark leap years, only with an extra month instead of an extra day. All of its many meanings are equally valid, even if most of them stem from linguistic accidents.