Different traditions and conventions place the extra "blue" full moon at different times in the year
- In calculating the dates for Lent and Easter, the Clergy identify the “Lent Moon.” It is thought that historically when the moon’s timing was too early, they named an earlier moon as a “betrayer moon” (belewe moon) because it betrayed the usual perception of one full Moon per month, allowing the “Lent Moon” to come at its expected time.
- Folklore gave each full moon a name according to its time of year. A full moon that came too early had no folk name, and was called a Blue Moon, retaining the correct seasonal timings for future full moons.
- The Farmers’ Almanac defined the Blue Moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a Blue Moon.
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 Moon,” 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, and declared that a second full Moon in a calendar month is a “Blue Moon.”
“Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called 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 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. Full moon names are given to each full moon in a season: For example, the first full moon of summer is called the Early Summer Moon, the second is called the Midsummer Moon, and the last is called the Late Summer Moon. But when a particular season ends up containing four full Moons, then the third of that season is called a Blue Moon so that the last can continue to be called the late 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.
Widespread adoption of the definition of a “Blue Moon” as the second full moon in a month followed its use on the popular radio program StarDate on January 31, 1980.
The most literal meaning of Blue Moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is also a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951, and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanos have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991.
The term “once in a Blue Moon” actually has nothing to do with the moon’s color. Instead, the term refers to the relatively rare occurrence of what is called a “Blue Moon.” This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare.
A Blue Moon cannot happen in February because the calendar month never has enough days. Furthermore, sometimes the occurrence of a blue moon depends on your time zone. During 1993 a blue moon occurred in either in August or September depending on where you lived. If you lived east of the line that runs through the Atlantic Ocean, the blue moon occurred in September, but west of that line it occurred in August.