From Wikipedia "Blue Moon"
A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year. The extra days accumulate, so that every two or three years (on average about every 2.7154 years[1]), there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times.
Just for the fun of it, I’m going to disregard my own advice to a bemused observer in your last thread and ask what others have asked you numerous times before without getting a straight answer:
What is this supposed to be? Some type of prophetic word? Inspirational/devotional writing? Just poetry? Pure self-indulgence?
For the year 2010, these are the dates of the moons in the northern hemisphere using UTC calculations. These dates use the actual solstices and not the artificial solstices that give each season an equal number of days.
By Jay Michaels
Story Published: Dec 30, 2009 at 8:38 PM CST
When somebody says that something happens 'every once in a blue moon,' usually they mean it doesn't happen very often.
A local astronomer says that back in the 1940's, a researcher for 'Sky and Telescope Magazine' managed to misquote the definition of a blue moon as being a second full moon during a calendar month.
He says the Maine Farmer's Almanac defined the third full moon during a season – as a blue moon.
Faulkner Planetarium Manager Rick Greenawald says, “If there were four full moons during one of those seasons, the third full moon was referred to as the blue moon to keep the other moons in line with their traditional interpretation, harvest, hunters, and so forth.”
He says in seven out of 19 years, a total of 13 full moons will occur in a twelve month period; and he says by using the original method blue moons happen more often than the 'two full moons in a calendar month' reckoning.
As simply as can be described, according to Trefethen's almanac, there are normally three full Moons for each season of the year. But when a particular season ends up containing four full Moons, then the third of that season is called a Blue Moon!
For more than half a century, whenever two full Moons appeared in a single month (which happens on average every 2 1/2 to 3 years), the second has been christened a "Blue Moon." In our lexicon, we describe an unusual event as happening "Once in a Blue Moon." This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare.
On past occasions, usually after vast forest fires or major volcanic eruptions, the Moon has reportedly taken on a bluish or lavender hue. Soot and ash particles, propelled high into the Earth's atmosphere, can sometimes make the Moon appear bluish.
Why "Blue" Moon? For the longest time nobody knew exactly why the second full Moon of a calendar month was designated as a Blue Moon. One explanation connects it with the word "belewe" from the Old English, meaning, "to betray." Perhaps, then, the Moon was "belewe" because it betrayed the usual perception of one full Moon per month. However, in the March 1999 issue of Sky & Telescope magazine, author Phillip Hiscock revealed one somewhat confusing origin of this term. It seems that the modern custom of naming the second full Moon of a month "blue," came from an article published in the March 1946 Sky & Telescope magazine. The article was "Once in a Blue Moon," written by James Hugh Pruett. In this article, Pruett interpreted what he read in a publication known as the Maine Farmers' Almanac (no relation to this Farmers' Almanac, published in Lewiston, Maine), and declared that a second full Moon in a calendar month is a "Blue Moon."
However, after reviewing the Maine Farmer's Almanac, Hiscock found that during the editorship of Henry Porter Trefethen (1932 to 1957), the Maine Farmers' Almanac made occasional reference to a Blue Moon, but derived it from a completely different (and rather convoluted) seasonal rule. As simply as can be described, according to Trefethen's almanac, there are normally three full Moons for each season of the year. But when a particular season ends up containing four full Moons, then the third of that season is called a Blue Moon! To make matters more confusing, the beginning of the seasons listed in Trefethen's almanac were fixed. A fictitious or dynamical mean Sun produced four seasons of equal length with dates which differed slightly from more conventional calculations. So, basically the current use of "Blue Moon" to mean the second full Moon in a month can be traced to a 55-year-old mistake in Sky & Telescope magazine.