‘Once in a blue moon’ is every 2.5 years
by Art Merrill
A brief history of a new folklore
We’re having a blue moon in May. That doesn’t mean what it used to, and while most folks think the whole blue moon thing is steeped in ancient folklore, truth is, it’s no more ancient than your granny.
Anyone can tell you the expression “once in a blue moon” means something is infrequent or rare and, yes, that definition goes back at least 400 years. But the idea that it also refers to two full moons within the same calendar month only goes back to 1946 – and is the result of a magazine writer’s error.
Do the math
A “blue moon” used to be associated with a natural event, the changing of the seasons; with its 20th-century redefinition, however, it’s more a mathematical curiosity that gives math geeks happy shivers and sends them running for their pencil sharpeners to plot out charts and sequences.
“If you make a list of every year between 1600 and 9999 in which two Blue Moons occur, then take the year modulo 19 and assign a letter to it so that zero maps to A, 1 to B and so forth, up to S, then you find that there are sequences of successive double Blue Moon years which have the same letter,” writes Ph.D. mathematician/astronomer David Harper. “Eventually, another letter appears to herald the start of a new sequence. This is always eight letters earlier in the sequence. For example, the E sequence, which runs from 1771 to 2113, gives way to the P sequence which begins in 1915 and ends in 2485. The sequences almost always overlap in this way, but there are never more than two parallel sequences at any one time. Each sequence runs for between 17 and 31 Metonic cycles, i.e. between 323 years and 589 years. A given sequence letter recurs after about 6,700 years, but the sequence is not repeated exactly.” Etc., etc.
All this mathematical rapture is the beneficiary of the difference between the moon’s natural cycles and the rigid abstraction of our Gregorian calendar system. The average interval between full moons is about 29.5 days; our calendar months, as you know, may have 28, 29 (leap year), 30 or 31 days (an average of 30.5 days). This disparity causes the date of the full moon to skitter across the calendrical year and drive the anal minds to predict the frequency of the blue moon. They’ve found that in a century (1,200 months) there will be an average of 41 calendar months that will have two full moons in the same month. According to that schedule, we can conclude that “once in a blue moon” means “once every two and a half years.”
Incidentally, in a 100-year period four or five calendar years will host two blue moons. And there can never be a blue moon in February.
Do the history
More interestingly, to those of us who only memorized calculus formulas long enough to pass exams and who retain just enough math skill to figure the square footage of our living rooms for new carpet, Dr. Harper also points us to Sky & Telescope magazine to learn how “blue moon” unintentionally came to mean “two full moons in the same calendar month.”
You can read the story for yourself, but to summarize here, the Maine Farmer’s Almanac used to define a “blue moon” as the third of four full moons in a season – that is, a natural “season” as determined but the sun’s position at actual equinox and solstice, beginning with the traditional “new year” at winter solstice. Each of the four seasons usually hosts three full moons but, again, the moon’s natural cycle is not the same as the sun’s natural cycle, so there are essentially 13 full moons within the 12 months of a solar year. When a season had four full moons, the almanac called the third one the “blue moon.”
Why didn’t the fourth full moon get the title? Because the almanac followed the rule decreed by the Catholic Church’s Gregorian calendar reform of 1582 that the vernal equinox (spring) always fall on March 21, regardless of the sun’s actual position, that Easter must include the first full moon of spring, and that Lent, which begins 46 days before Easter, must include the last full moon of winter. Since reckoning must be taken on the last full moon of the season (“The Moon After Yule”), the almanac relegates the extra full moon to the third position.
If that seems about as clear as calculus, blame the Catholic Church’s desire for political control of formerly pagan Europeans, who celebrated the natural cycle of the seasons. The Church usurped those celebrations, inserting its own holidays and saints on the same dates – that’s why Christmas is near winter solstice and Easter is at vernal equinox when there is no historical or religious basis for those dates having anything at all to do with Christ’s birth or resurrection.
The changeover to the Gregorian calendar is a fascinating subject in itself. Changing from the Julian calendar (named for Julius Caesar) to the Gregorian (Pope Gregory XIII) required “losing” 10 days; so, everyone who adopted the new calendar (Spain, Portugal and Italy) went to sleep on 4 October 1582 and woke up on 15 October 1582. While there is some history out there that claims illiterate peasants believed they had literally “lost” 10 days from their lives, a few grains of salt may be in order. I wonder if that, as well as stories about riots in the streets, isn’t just political rhetoric coined by opposition Tories: England didn’t adopt the Gregorian calendar until 1752, and only after the government made special provisions that it not interfere with tax payment dates. How that might have influenced the American Revolution, I couldn’t say. Alaska changed over in 1867, after we got it from the Russians, who didn’t go Gregory until 1918. The last country to accept the calendar was Greece, in 1923.
Anyway, in 1946 a Sky & Telescope magazine writer misinterpreted the Maine Farmer’s Almanac definition, assigning the erroneous definition of “two full moons in a calendar month.” Other writers and radio journalists picked up and parroted the error over the years, and today we have a new folklore.
There are already some new traditions and portents attached to the second appearance of a full moon within the artificial and totally arbitrary boundaries that Gregorian months define. I found one Wiccan practitioner (Wicca is a nature-based religion) online who claims that the blue moon is “a time of enormous influence for prophecy and divination.” Interestingly, she notes (correctly) that for every blue moon there is also a month with a second new moon, which she calls the “Black Moon” and says is a time for working extra-powerful “magik.” But Wicca is not dogmatic, and it would be incorrect to assume that all Wiccans believe that.
Do the Internet
Our first full moon of May is on the 2nd. Our blue moon for 2007 is at 7:04pm Arizona time on May 31. If you’re in London it’s on June 30 and if in Auckland it’s July 30. If you want to know why, then visit Dr. Harper’s website at www.obliquity.com/astro/bluemoon.html. For more detail on how a Sky & Telescope magazine writer inadvertently invented our new blue moon folklore, go to http://www.skytonight.com/. I found the Wicca info at www.witchvox.com (type “blue moon” into the search bar). For more on the Gregorian calendar, try www.infoplease.com/spot/gregorian1.html, Wikipedia, and http://scienceworld.wolfram.com/astronomy/GregorianCalendar.html, where you can get the cool Julian-to-Gregorian conversion formula JD=367Y-INT(7(Y+INT((M+9)/12))/4-INT(3(INT((Y+(M-9)/7)/100)+1)/4)+INT(275M/9)+D+1721028.5+UT/24. Remember to subtract 8 hours to get Arizona time.
(You can contact Art at editor@readitnews.com.
