Mistakes like this are pretty rare. They only happen, like, once in a…well, hold that thought.

The ultimate source of the problem is the lazy nature of the moon. The moon orbits the earth every 29.5 days or so, which is a little less than once a month. In most years, this means that there are 12 full moons in the year, and everything works out just fine. In medieval times the moon was a handy device for planning things, and these 12 full moons all had well-known formal names like “Hunter’s Moon” and “Harvest Moon” to help everyone get in sync. However, from time to time a year would come along that had 13 full moons in it and then there would be a big problem. With 13 full moons, someone had to decide where on the calendar to denote this extra full moon. And, ever since the Roman Council of Nicaea of 325 AD, that someone was the Pope, who was responsible for determining such things as the date of Easter and the specifics of the annual calendar.

Long story short, the Pope gets to tell the peasants where this odd 13th moon goes, and therefore how to plan and live their annual lives. OK, fair enough, everyone’s cool with that for over a thousand years. But then, in the 16th century, during the Reformation, the Church of England breaks away from Rome. In 1528, two Protestant friars from Greenwich publish a pamphlet attacking the Catholic Church, and highlight their resentment of Rome’s control of their calendar, writing “Yf they say the mone is blewe / We must beleve that it is true.” Now, “mone” is the Old English word for both “month” and “moon”, and “blewe” means “betrayer” but is pronounced like “blue.” Which means, to an Englishman of the time, a moon that is “blue” betrays the normal calendar, and is evidence of evil Roman control of the world.

Specifically, Rome declared that, whenever there was a solar quarter (equinox to solstice, or solstice to equinox) that contained 4 full moons instead of the usual 3, the 3rd one would be considered the “odd” one, the “betrayer,” the “blewe” one.  Put more simply, to an English speaker, the Blue Moon is the 3rd full moon in any solar quarter than has 4 full moons.

But hold on there! I just heard a bunch of you say that you thought a Blue Moon was the second full moon in any given month, which is very different. Ah, yes, and therein lies the mistake. The traditional Roman determination of the Blue Moon lasted through the Reformation into the modern day, and was formally adopted by the American farming system in 1817. (In that year, the Maine Farmer’s Almanac published a list of the names of the 13 full moons of the year – using the Roman method – and from that time that method became known in the United States as the “Maine rule.”) But then, in 1943, a scientist writing in Sky and Telescope magazine completely blewe it, and incorrectly explained that a Blue Moon was the second full moon in any month. Unfortunately, this faulty explanation was picked up by a popular radio program in the 1970’s, and then the error made its way into that great pre-Google source of all truths, the first printing of Trivial Pursuit.  From there it was widely disseminated to an entire generation of English speakers. And just like that, a millennium of religious doctrine was overturned by a board game.

Today, July 31, there will be a Blue Moon, based on the modern definition. And it won’t actually be visibly blue, which is a good thing. Moons are only blue when there is significant debris in the atmosphere, such as in 1950, when huge forest fires in Canada poured smoke around the world, or in 1883, when Krakatoa blew up.

Nope, this will be a regular old Blue Moon, plain white, not recognized by Rome, and not evidence of any planetary disaster.

But still pretty rare, and pretty cool.