The Jefferson Agrarian

Think you know? If you’re like us, you’re probably thinking that it’s the second full moon in the same month, like the one occurring on this New Year’s Eve, December 31.

Well, that turns out to be simplistic rubbish, and you can thank National Public Radio’s popular program StarDate for perpetuating the misnomer. According to their website, “Because the time between two full Moons doesn’t quite equal a whole month, approximately every three years there are two full Moons in one calendar month. Over the past few decades, the second full Moon has been come to be known as a “blue Moon.” The next time two full Moons occur in the same month (as seen from the United States) will be December 2009.”

POP CULTURE VERSION?
Over the past few decades? Well, StarDate has been on since the 1970s, so it seems that they’re out to dumb things down for us. The March 1946 Sky and Telescope article “Once in a Blue Moon” by James Hugh Pruett first misinterpreted the 1937 Maine Farmers’ Almanac. “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.” Widespread adoption of the definition of a “blue moon” as the second full moon in a month followed its use on StarDate on January 31, 1980.

FARMER’S VERSION
In the 19th and early 20th centuries, the Maine Farmers’ Almanac listed blue moon dates for farmers. These correspond to the third full moon in a quarter of the year when there were four full moons (normally a quarter year has three full moons). Names are given to each moon in a season: For example, the first moon of summer is called the early summer moon, the second is called the midsummer moon, and the last is called the late summer moon. When a season has four moons the third is called the blue moon so that the last can continue to be called the late moon.

The division of the year into quarters starts with the nominal vernal equinox on or around March 21. This is close to the astronomical season but follows the Christian computus used for calculations of Easter, which places each equinox evenly between the summer and winter solstices to calculate seasons rather than using the actual equinox.

Some naming conventions keep the moon’s seasonal name for its entire cycle, from its appearance as a new moon through the full moon to the next new moon. In this convention a blue moon starts with a new moon and continues until the next new moon starts the late season moon.

FOLKLORE VERSION
Historically, moons were given folk names, twelve each year, to help people to prepare for different times of the year and the related weather and crop needs. Names varied with locality and culture, often with descriptive names such as harvest moon, growing moon, snow moon, and egg moon. Most years have 12 moons (giving 12 names), but in the years with thirteen full moons the monthly “seasons” would be expected to come too early – for example, hens would not recommence laying their eggs by the fourth full moon since it was still too cold – so the early moon was named a “blue moon”. This then re-aligned the rest of the year’s moons and “seasons”.

The origin of the term “blue moon” is steeped in folklore, and its meaning has changed and acquired new nuances over time. Some folktales say that when there is a full blue moon, the moon had a face and talked to those in its light.

Visibly Blue Moon
The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. The particles in the atmosphere have to be about one micrometre in diameter; under these circumstances, long-wavelength light, which appears red to a viewer, is scattered out of the line of sight and short-wavelength light, which appears blue to a viewer, is selectively transmitted into a viewer’s eyes.