Blue Moon

The origin of the term blue moon is steeped in folklore, and its meaning has changed and acquired new and interesting meanings and nuances over time. The earliest known recorded usage was in 1528, in a pamphlet entitled Rede Me and Be Not Wrothe: [If they say the moon is blue, we must believe that it is true]. "They would make men believe ... that the moon is made of green cheese".

The most obvious meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden in 1950 and Canada in 1951 and, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years.

The older meaning of blue moon to name an extra full moon, as was used in the Maine Farmer's Almanac, was the third full moon in a quarter of the year when there were four full moons – normally a quarter year has three full moons.

In recent times, people have taken to calling a full moon a blue moon based on the Gregorian calendar. By this use of the term, a blue moon is the second of two full moons to occur in the same calendar month.

Calendar blue moons occur infrequently, and the saying once in a blue moon is used to describe a rare event. On average, this happens once every 2.72 years. Additionally, in some years there is no full moon in February at all, since February is slightly shorter than the time from one full moon to the next. This condition, known as black moon, gives additional 'blue' moons in the preceding and following months.

Since February is one or two days shorter than the moon's cycle, very occasionally it has no full moon – there is a full moon at the end of January, and the next one is at the beginning of March. What this means is that both January and March will have blue moons. This happens, on average, once every thirty-five years.

The next blue moon will be December 31, 2009. Because February, will have no full moon in 2018, January and March will each have a calendar blue moon that year.