You have probably heard the term “once in a blue moon” to describe something that doesn’t happen very often. But what is a blue moon? While it’s true that there are times when certain conditions make the moon appear with a bluish tint, that’s not what a “blue moon” actually is. Instead, a blue moon is a full moon that happens at a specific time.
When Do We See a Blue Moon? Two Definitions of the Blue Moon
In folklore, the blue moon occurs on the third full moon in a season with four full moons. Normally, each season — winter, spring, summer, fall — has only three full moons. During times when planting and harvesting and other activities were based around the seasons, which coincided in many ways with lunar cycles, there were only three moons expected in each season. When four moons showed up in a season, it tended to cause some calculation problems. So the third full moon was called a blue moon, and the fourth full moon retained its “regular” designation. (By this definition, the next blue moon doesn’t arrive until August of 2013.)
In the last century, though, many started considering a blue moon as the second full moon in a month. Normally, each month has only one full moon. However, the lunar cycle is 28 days, while we (in the west) are on a solar calendar that features months with more than 28 days. As a result, it is possible, every two or three years, for a full moon to occur in the first few days of a month, and then again in the last couple of days of the month. This year, 2012, the blue moon by this definition falls on August 31.
Most of the time, only one month in the year has a blue moon. However, there are times when a year contains two blue moons. This normally happens when the cycles line up so that there are two full moons in January, a month with 31 days, and two full moons in March, another month with 31 days. In between, February, the shortest month with 28 days, just misses having a full moon. So there is no full moon in February on years when there are two blue moons.
There is a debate over which definition is better than the other. Most people, though, consider the blue moon as the second full moon in a month.
The Betrayer Moon
What about the name “blue”? Why call it a blue moon when it clearly isn’t blue. There are two different origins of the term in circulation:
- The first uses the term “belewe” in the early 1500s. This was a spelling of blue in English at the time. The term was used in a pamphlet against the English clergy, scoffing that congregants were required to believe it if their religious leaders told them the moon was blue.
- However, there is also the contention that, in Middle English, the word “belewe” meant betray. In some cases, a blue moon showed up around Easter, and since the moon was used as a guide as to the season. The “belewe moon” was a “betrayer moon”, and the faithful needed the clergy to tell them which was the true lunar guide for Lent and Easter.