If you are hoping that something extremely rare or unlikely will happen—for example something that is wont to happen “once in a blue moon,” well, August will be your prime opportunity, as August 31 will give us a “blue moon.” Or will it? It may surprise you to know that the conventional definition of a blue moon—the second full moon in a calendar month—is actually based on a misreading of an old almanac. Astronomically, there are two basic definitions of a “blue moon”: one—which is pretty rare (hence the phrase)—is when the moon physically looks blue, which can be caused by ash, soot, and other atmospheric particulates. The gunk in the atmosphere causes the moonlight to appear blue to Earth-bound observers. These types of blue moons are occasionally seen after major volcanic eruptions. When Krakatoa erupted in 1883, the moon looked blue for almost two years. Wikipedia tells us that “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 second, more common—and, perhaps, erroneous—definition of “blue moon” (the second full moon in a single month) is a bit more complicated. Most of the time, there is but one full moon in a given month. But, on rare occasions—almost as rare as (seemingly...) three fortnightly paychecks in one month—there is a second full moon, the purported blue moon. Let’s back up a second. The origin of the origin of the phrase “blue moon” dates from a 1528 treatise called “Rede Me and Be Not Wroth” by William Roy and Jerome Barlowe, which contained the couplet, “Yf they say the mone is blewe/We must believe that it is true.” According to the Oxford English Dictionary, the context here is to indicate a complete absurdity—akin to saying “if the moon where made of green cheese.” So, originally, the phrase “once in a blue moon” meant “never,” rather than just “very rarely.” The first recorded use of the phrase to refer to something that is rare is 1821. Somehow over the course of the 20th century, the notion of a blue moon as a rarity became conflated with the notion of two full moons in a single month—which, by the way, happens every two or three years, so is actually not that rare, and actually has no special astronomical meaning or significance; it’s simply a consequence of the lunar calendar differing from the solar calendar. (There also would not have been any ancient “omen-related” meaning attached to this, as ancient cultures strictly relied on a lunar calendar which, by definition, precludes there being more than one full moon in a month.) Interestingly—and here’s where  suggest you take a break and grab a six-pack of Blue Moon beer—a Sky & Telescope article has uncovered the fact that the definition of a blue moon as “two full moons in one month” was actually an error, and one propagated in that very same magazine. In the early 1940s, the calendrical meaning of the term blue moon was traced to the Maine Farmers Almanac, specifically, the 1937 edition of this almanac. More recent researchers, looking at editions from 1819 to 1962, found many many references to blue moons, none of which happened to correspond to the second full moon of the month. However, they did find a seasonal pattern to the so-called blue moons. It involves how the Maine Farmers Almanac measured the year—not from January 1 to December 31, but rather from one winter solstice to the next. This is referred to as a “tropical year” and in a tropical year, each of the four seasons has three full moons (three months, three full moons). This works out to 12 full moons in a tropical year. However, for the same reason that today’s calendars can occasionally have two full moons in a month, every so often—once in a blue moon, say—a tropical year could have 13 full moons, or have one season in that year with a fourth full moon. Just to mess with our heads, it was the third, and not the fourth, full moon that was considered the blue one. Don’t ask me why. So, in 1943, Sky & Telescope—the definitive source for these things, or so it would seem—featured a Q&A column on the subject of blue moons, and revisited the Maine Farmers Almanac and responded simply that once in a while, there are 13 full moons in a year. Cryptically, the author did not mention if he was talking about a calendar year or a tropical year—but then nor did he say anything about two full moons in a month. Where things get sticky is three years later when, also writing in Sky & Telescope, a frequent contributor named James Hugh Pruett wrote on the subject of blue moons (man, they wrote about topic until they were blue in the face!), referencing the prior S&T article and the Maine almanac but, for some reason, including the passage:

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.

Even though the blue moon cited in the 1937 almanac was not the second full moon of the month—and even though 1937 only had its normal allotment of 12 full moons, the Pruett definition of a blue moon seems to have become the “official” one. A careless misinterpretation of an old almanac has now become cultural lore. Someday, I hope my own misinterpretations—of which there are myriad examples, no doubt—take on such immortality. Now, true sticklers may insist that the original “blue moon is the fourth—er, I mean third—in a season” definition is the correct one, and it does sound more than a little like the old “should we celebrate the millennium in 2000 or 2001?” issue that launched a thousand op-ed pieces back in the day. But, since there is no practical consequence either way, and anything that gets people interested in astronomy isn’t a bad thing, I say let’s call August 31’s full moon a blue moon and be done with it.