And then there suddenly appeared before me
The only one my
arms will hold.
I heard somebody whisper
“Please adore me”
And when I looked the Moon had turned to gold.
—”Blue Moon,” Rodgers and Hart
There are as many names for the Full Moons of each month as there are cultures and this is not a complete listing. Not one of the Full Moons listed above—nor is any Full Moon from any culture—known as a Blue Moon.
WHAT IS A BLUE MOON?
Just what is a Blue Moon? Not what most people believe when you ask them for a definition. According to common lore, a Blue Moon is often described as the second Full Moon in a month. Since the Moon has a 29.5 day synodic period of phases, it is possible to have a second Full Moon in some months, but they are not so rare as to warrant a phrase “Once in a Blue Moon.” In fact, there are two Full Moons in a month about once every two years—hardly a rarity. This particular usage of the phrase appears to stem from a misinterpretation of the original meaning and can be traced to the Trivial Pursuit game of the 1980s and a children’s book published the year before the Genus II edition of the game was released.
Of Modern Origin
The earliest known usage of the phrase “blue moon” appears to be 1528, in a pamphlet entitled Rede Me and Be Not Wrothe in which the author writes “Yf they say the mone is belewe We must beleve that it is true.” The expression seems to refers to something so absurd as to be unbelievable or someone so argumentative that he would claim something clearly false is true. There is also a double-meaning in the phrase since belewe translates from the Old English as betrayer and refers to the misplaced Full Moon before Lent—a betrayer third Full Moon—when there are normally four Full Moons in a season. Thus, the third Full Moon in a season that has four is the Blue or Betrayer Full Moon.
Why the third Full Moon? Because the name of the fourth Full Moon must be in accordance with the seasonal change since the name of that Full Moon relates to the impending equinox or solstice. A complete description of how Blue Moons are determined is quite convoluted, but is an interesting example of how calendar reform, religious observances (namely Lent and Easter), and astronomical events are joined together. Read Sky & Telescope’s article.
The modern usage of “Blue Moon” appears to have its origin in the Maine Farmer’s Almanac. Editors at Sky & Telescope, with the assistance of several librarians, obtained 40 copies of the Almanac dating back to the early 1800s and found numerous citations that referred to Blue Moons, but not one of them referred to the second Full Moon of the month. In fact, the Blue Moons always occurred on the 20th – 23rd days of February, May, August, or November! These Full Moons always occur about one month before a seasonal change. A further nuance discovered with additional research revealed that the almanac definition relied on the use of the tropical year, which is measured from one winter solstice (Yule) to the next, instead of using the traditional calendar year. Most tropical years have 12 Full Moons—three per season—but occasionally there are 13 Full Moons with one season having four Full Moons.
Is the Moon Really Made of Green Cheese?
Basically, we have to define exactly what “green” is. In this case, green is not simply the color that has a wavelength of approximately 5,500 Ångstroms, but rather, green refers to something young—in other words, unseasoned or unripened. Therefore, green cheese is unripened, young cheese.
The origin of the comparison appears to come from a phrase coined in the sixteenth century, attributed variously to John Heywood in his Proverbes (1546), François Rabelais (c. 1490–1553), and John Frith in 1529. “Green” refers not to the color, but to a young (i.e. green) or immature rounded cheese with a mottled surface and color similar to that of the Moon. As written in Frith’s translation A Pistle to the Christian Reader, “They woulde make men beleue … that ye Moone is made of grene chese” seems to imply that a person is foolish enough to believe anything he is told if he believes the Moon is made of green cheese.
Can the Moon Really Look Blue?
According the the Science@NASA website, there was a time when people saw blue moons almost every night, except some nights when they were green.
In 1883 the Indonesian volcano Krakatoa exploded with the force of a 100-megaton nuclear bomb. Fully 600 km away, people heard the explosion as if a cannon had been fired. Plumes of ash rose to the very top of Earth’s atmosphere. And the Moon turned blue. Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide—the right size to strongly scatter the red end of the Sun’s spectrum while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green after the red end of the reflected sunlight was scattered.
Blue moons persisted for years after the eruption and people even saw lavender suns. The ash caused “such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration,” according to volcanologist Scott Rowland at the University of Hawaii. The unsual sights even prompted Edward Munch to paint “The Scream.”
Other volcanoes have produced blue moons, too. People saw blue moons in 1983, for instance, after the eruption of the El Chichon volcano in Mexico and there are reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991. The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micron) suspended in the air with no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as can forest fires.
Blue Moons and Lavendar Suns by Sue Ann Bowling, Ph.D.