Another is a description of the apparent colour of the moon, which is usually a yellow shade and very rarely a shade of blue. Full moons are given names in folklore, and a blue moon is the name for a rare full moon. The modern blue moon definition applies to a second, extra full moon that occurs in a calendar month.
The older definition of blue moon is for an extra full moon that occurs in a quarter of the year, which would normally have three full moons, but sometimes has four. Oddly, it's the third full moon, in a season that has four full moons, which is counted as the "extra" and named the Blue Moon.
Visibly Blue Moons: The most obvious meaning of blue moon is when the moon (not necessarily a full moon) appears to an observer to be unusually blue in colour, which is a rare event. This effect can be caused by smoke or dust particles in the atmosphere, such as happened after forest fires in Sweden in 1950 and Canada in 1951.
After the eruption of Krakatoa in 1883, moons to appeared blue for nearly two years.
Farmer's Almanac Blue Moons: The older meaning of blue moon to name an extra full moon, as was used in the Maine Farmer's Almanac (the third full moon in a quarter of the year). The division of the year into quarters for this purpose has the dividing line set between March 21 and March 22. This is to do with the rule for setting the date for the Christian Holy Day of Easter, which depends on the last full moon.
This meaning of blue moon was lost when the editors of the original Farmer's Almanac died. It was recovered only when researchers for Sky & Telescope magazine noticed that the Maine Farmer's Almanac from 1829 to 1937 reported blue moons that did not fit the meaning of the term calendar blue moon as described…
Calendar Blue 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. This definition of blue moon originated from a mistake in an article in the 1946 Sky & Telescope magazine, which failed in an attempt to infer the earlier definition used in the original Farmer's Almanac. It was helped to popularity when a Trivial Pursuit question used this as a source for one of its questions. Sky & Telescope discovered the error nearly 60 years later and the magazine printed a retraction and correction. But by the time the correction came, the calendar definition had already come into common use. Because it is so much easier to understand, the "mistaken" calendar-based meaning has stuck.
Calendar blue moons occur infrequently, thus using the saying once in a blue moon to describe a rare event. The infrequency of extra moons in a month is because the lengths of the Gregorian calendar months are all very close to the length of the 29.5306 day period of the moon's phases: the synodic month, or lunation. Second full moons are possible because every month except February is longer than this period by 1 or 2 days, but the odds are low. One calendar blue moon occurs on average every 2.72 years. The last calendar blue moon was December 31, 2009. Because February will have no full moon in 2018, January and March will each have a calendar blue moon that year.
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