December full moon, blue moon?

The second of this month’s two full moons is scheduled for today at 2:14 p.m. EST. This circumstance (two full moons in a calendar month) was commonly referred to as a blue moon when I was growing up, but here is the dope from Wikipedia, fact-checked by yours truly (please don’t ever accept Wiki’s version of events without checking things out from real, edited, copy):

• The Farmer’s Almanac defined 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.
• Recent popular usage defined a blue moon as the second full moon in a month, stemming from an interpretation error made in 1946 that was discovered in 1999.^[1]

Wiki’s superscript reference 1, there, is to a 1999 Sky & Telescope article that provides a lovely historical analysis of the problem. And although it’s a bit, well, technical, the basic definition (which I paraphrase below) is pretty simple:

A blue moon is the third full moon in a season with four full moons (rather than the usual three).

Not too complicated, right? There’s an extra moon in a season (not a month), and that extra moon is the blue moon. Of course, some of you might wonder why it’s the third full moon, rather than the fourth, that would be the “extra,” or “blue,” moon. You get a gold star.*

For the answer to that questions, there is a simple explanation and a more complicated explanation. Let’s start with the simple one. The third full moon is “extra” because the last full moon of the season is the one that needs its traditional name: think “Moon before Yule” and “Moon after Yule”: if the moon before Yule were all of a sudden “blue” instead, the moon names would be hopelessly out of whack!

OK, now for the more accurate, but more complicated, version. I cite Sinnott et al., from the aforementioned article in Sky & Tel:

The [Maine Farmer’s Almanac] follows certain rules laid down as part of the Gregorian calendar reform in 1582. The ecclesiastical vernal (spring) equinox always falls on March 21st, regardless of the position of the Sun. Lent begins on Ash Wednesday, 46 days before Easter, and must contain the Lenten Moon, considered to be the last full Moon of winter. The first full Moon of spring is called the Egg Moon (or Easter Moon, or Paschal Moon) and must fall within the week before Easter.

At last we have the “Maine rule” for Blue Moons: Seasonal Moon names are assigned near the spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent. The beginnings of summer, fall, and winter are determined by the dynamical mean Sun. When a season contains four full Moons, the third is called a Blue Moon.

The article also provides a lovely graphic, showing all the blue moons, in both the “easy” definition (two in a calendar month) and the “Maine” rule (the third in a four-moon season), from 1999-2020. For copyright reasons, I do not reproduce that image here, but encourage you to follow the link to the article to see it for yourself.

Of course, now that we have two competing definitions of blue moons, one of which is Easily Understandable (“two full moons in the same calendar month”), the other of which is pretty darn arcane (“the third full moon in a four-full-moon season, defined according to an arcane religious computus that isn’t aligned with our solar calendar”), well, which one do you think will have the most staying power?

If you’re curious to answer that rhetorical question, check out the so-called Blue Moon calculator here, which lists this month, December 2009, as a blue moon month. This site also declares that there are no blue moons in 2010. I can’t say that David Harper and Lynn Marie Stockman, who run that website, are wrong. They’re using the clear and easy-to-understand rule for determining a blue moon that I used to use. They even address the question explicitly on the blue moon home page., with a list of “traditional” blue moons for the 20th and 21st centuries.

I am just going to say this: anyone with a calendar and a list of full moons can determine when the next blue moon is going to be, using this rule, without needing to consult a website or an almanac.

A truly blue moon, though, one which occurs only in the months of February, May, August, or November is quite a bit rarer. And February can never be a blue moon month by the “two-full-moons-in-a-calendar-month” rule. (That limitation of blue moon months to Feb, May, Aug, and Nov should help explain why this December’s second full moon isn’t blue: it’s after the solstice, so it’s really the first full moon of winter 2010, rather than the last full moon of autumn 2009. I never said this was simple!)

If you’d like an even more complicated explanation of the moons in a year, complete with computer code, try this article.

*Disclaimer: I am not actually awarding you a gold star. If you would like to claim your star, please go the nearest office or teacher’s supply store and purchase one, and apply it wherever you see fit.

Related Images: