There is much talk in social media about tonight's full moon being a blue moon ~ it may or may not be, depending on whose definition you use. The earliest reference is in Middle English (16th century), in which the spelling is belewe which can mean the color, or it can mean "betray".
Why betray? Because a 12-month solar year usually includes 12 full moons, three per season, each with its own unique name. However the lunar cycle is only 28 days long, not 30 or 31, so periodically (every two or three years) an extra full moon occurs within a calendar year. To keep the nomenclature of the other 12 full moons straight, this 13th full moon was belewe or false (or blue). Hence the expression "once in a blue moon" refers to an event that will take place only on rare occasions.
In North America, this definition of an extra full moon in a season which normally had three full moons was formalized as the "Maine rule" in the Maine Farmer's Almanacs, as far back as 1819. In strict astronomical terms, it remains the correct definition.
But things took an unexpected turn in a 1946 Sky and Telescope magazine article in which astronomer James Hugh Pruett unintentionally mistook how blue moon had been used in the Almanac, and mangled the definition to become the second full moon in a given month. His error was perpetuated in the 1980s on a public radio show called StarDate, where Deborah Byrd made reference to Pruett's 1946 misinformation, popularizing the mistake into accidental folklore. The idea stuck.
Another meaning for blue moon actually refers to its color, a rare event resulting from smoke or dust particles in the atmosphere. Forest fires and volcanic eruptions are the most identifiable sources of visibly blue moons.
Below you will find the original positional and old English names for each month's full moon.
- January ~ Early Winter Moon ~ Old Moon
- February ~ Mid Winter Moon ~ Wolf Moon
- March ~ Late Winter Moon ~ Lenten Moon
- April ~ Early Spring Moon ~ Egg Moon
- May ~ Mid Spring Moon ~ Milk Moon
- June ~ Late Spring Moon ~ Flower Moon
- July ~ Early Summer Moon ~ Hay Moon
- August ~ Mid Summer Moon ~ Grain Moon
- September ~ Late Summer Moon ~ Fruit Moon
- October ~ Early Fall Moon ~ Harvest Moon
- November ~ Mid Fall Moon ~ Hunter's Moon
- December ~ Late Fall Moon ~ Oak Moon
It is worth noting that since there are on average 12.37 full moons in a year, a blue moon must occur on average every 2.7 years by either definition. Since tonight's full moon is the second to occur in August, it conforms to the modern folklore version of a blue moon.
For several weeks my western Montana skies have been obscured by seasonal wildfire smoke. Last night when I went outside, the almost-full moon was a pale salmon color due to the smoke. I'll have to rely on my imagination, or on images like the one above (click to enlarge) to witness a truly blue moon.
Check here for an informative Washington Post article. It includes an embedded NASA video on blue moons, as well as a video on making blue moon martinis. Cheers.