There was a bit of fanfare related to the full moon on August 31. It was the second full moon to grace our north american skies in the month of August.
Many people referred to this phenomenon by the name “blue moon.” This name was popularized to its modern definition in the early 80’s when a widely listened to radio program used bad information from an astronomy magazine (Sky and Telescope) published in the 40’s. That magazine had incorrectly interpreted a centuries old naming cycle presented in the Maine Farmers’ Almanac. The almanac listed moon names by season with the year divided into our familiar quarters as per the table below.
| Positional name |
Associated Month |
English name |
| Early Winter |
January |
Wolf Moon |
| Mid Winter |
February |
Snow Moon |
| Late Winter |
March |
Storm Moon |
| Early Spring |
April |
Seed Moon |
| Mid Spring |
May |
Milk Moon |
| Late Spring |
June |
Mead Moon |
| Early Summer |
July |
Hay Moon |
| Mid Summer |
August |
Corn Moon |
| Late Summer |
September |
Harvest Moon |
| Early Fall |
October |
Hunter Moon |
| Mid Fall |
November |
Beaver Moon |
| Late Fall |
December |
Oak Moon |
As the lunar cycle puts full moons about 29.5 days apart, having two in one month doesn’t happen too often. It happens only seven times every nineteen years.
Farmers were not the first to notice this and have the need for creative accounting. I’ll get back to them in a bit. First the source of the name: “blue moon”.
A month with two moons has been a source of centuries long havoc in christian mythologies. The dates for easter (the death and resurrection of jesus), were set by the council of Nicea in 325 CE as the first Sunday after the full moon following the northern hemisphere’s vernal equinox. Unfortunately they didn’t account for (or perhaps they rejected) the millennia old geometric calculations showing equinox varies from March 19-21 (usually 20). Instead the church fixed the date of the equinox to March 21. In addition, they were still using the Julian calendar which overestimates the length of our year by eleven minutes. And so, for the about a thousand years, the calendar date of the equinox slipped bit-by-bit. By the time Pope Gregory tried to fix things in 1582 CE with his humbly named “Gregorian calendar” the equinox had slipped to March 11.
In the intervening 1200 years, it was important to be celebrating moon-centric christian holidays on the correct days. When an inconvenient full moon would mess with the math it would simply be declared a “betrayer moon” by the clergy or, in the vernacular of the day, a “belewe moon”. It first showed up in print around 1524 CE.
So now we have a name. Back to the farmers and their almanac. When a year would have 13 moons, that extra one had to fit into their neat and orderly table somehow. The formula was simple and less subject to papal confusions. Each season typically has three moons, each with a name. If a season has four moons, the third of four becomes the “blue moon” and nothing else changes.
The 40’s era magazine article misinterpreted this to mean that the second moon in a month would be a blue moon and that got popularized.
Our “blue moon” on August 31 actually fits both criteria so you can stop worrying that you looked silly for celebrating it. Unless you live east of GMT+10:30. Then the full moon didn’t actually happen on August 31, it happened on September 1. You had the old school blue moon, but your new age blue moon will happen on September 30.
Anyone up for a trip down under to take in some double blue month action?
Eat Logical Sentences 
