Every Halloween, there always seems to be an incredible feeling of excitement and an indescribable ooky-spookiness in the autumn air.

Mount St. Joseph News


Blue moon!

You saw me standing alone,

Without a dream in my heart,

Without a love of my own!

—“Blue Moon,” Lorenz & Hart


Every Halloween, there always seems to be an incredible feeling of excitement and an indescribable ooky-spookiness in the autumn air. The veil between this world and the next is at its thinnest that night after all, and people still shudder with superstitious glee as they imagine what might be lurking in the shadows. The poet Robert Burns imagined that “...Witches, Devils, and other mischief-making beings, are all abroad on their baneful midnight errands; particularly, those aerial people, the Fairies, are said, on that night, to hold a grand Anniversary.” This year the fairies will have one of the grandest anniversaries they’ve ever had, for they will be able to celebrate it in the wondrous light of the Blue Moon on Oct. 31. It will be the second full moon this month since Oct. 1, a rather rare phenomenon.


Curiously, the Blue Moon is not blue at all. The moon does occasionally appear to be different colors, though: if you had been present at the massive eruption of the Indonesian volcano Krakatoa in 1883, you might have observed that the moon was either blue or green, but this was only a trick of the light caused by enormous clouds of ash. The Blue Moon is actually so named because of its relative rarity—it was a popular English idiom long before it became known as the second full moon occurring in one month.


The first idiomatic uses of “blue moon” can be found in the sixteenth century, in reference to something that was of obvious absurdity. A little later on, this meaning shifted to “never,” denoting that something was absolutely unlikely to happen. Then, just 150 years ago, the idiom took on its modern usage of “rarely” or “practically never,” most commonly expressed as “once in a blue moon.” While this saying has remained in use, the Blue Moon is most usually associated today with the second full moon that occurs in the same month. This is largely due to a scholarly misinterpretation of a farmers’ almanac that has since become a lasting part of American folklore.


Going back to the nineteenth century, farmers’ almanacs made yearly calculations of when the full moons would occur based on the “tropical year.” The tropical year runs from the winter solstice to the next winter solstice (in contrast to the calendar year), producing seasons of equal duration. It usually has 12 full moons, with three in each of the four seasons; all 12 are named according to what’s happening seasonally at the time they occur, influenced by the same rules used for determining the dates of the Lenten season. For example, the Harvest Moon occurs in autumn, when it is said that the field is lit up well enough that it may be harvested late into the night (the full moonlight helps crops to keep better as well).


It is essential that the named full moons line up with their assigned time of the year, which leads to a problem when roughly every two-and-a-half years or so, there is a thirteenth full moon. This leads to one season with four full moons instead of three. If left alone, this would push the full moons out of alignment with the seasons in relation to the solstices and equinoxes, as well as the folkloric names that have been assigned to them. To remedy this, a rule was established that, no matter the season in which the four moons occurred, the third of the four would always be considered the odd one out, and labelled as “blue” due to its rarity. So, the “Almanac” did not define the Blue Moon as the second full moon in a month, but the third of four in a season.


In a 1943 “star quiz” published by Laurence J. Lafleur in the popular magazine “Sky & Telescope,” one of the answers was “blue moon,” citing a 1937 “Maine Farmers’ Almanac” as the source. Lafleur made no mention of the fact that the ‘Almanac’ based its calculations on the tropical year. In 1946, James Hugh Pruett wrote another article for “Sky & Telescope” entitled “Once in a Blue Moon,” in which he also cited the 1937 “Almanac,” along with Lafleur’s comments. He didn’t know about the tropical year, and mistakenly concluded that seven times in 19 years there are 13 full moons instead of 12, and that the second full moon in a month is known as the Blue Moon. This, once again, was a misinterpretation of the information presented by the 1937 “Almanac,” which defined a Blue Moon as the third of four full moons occurring in a season. In 1980, the popular radio show “StarDate” perpetuated this idea of the Blue Moon as the second full moon in a month, which essentially immortalized it in the popular imagination.


What could be more fitting than a full moon on Halloween, and a rare Blue Moon at that? The mere fact of the Blue Moon’s rarity and overall special nature reminds us of the connection we have with the natural processes that are constantly swirling around us, and gives us pause as we remember fond times gone by. With Halloween being on a Saturday this year, it will be the perfect time for us to do those things we usually only have time for once in a blue moon.


*Photo by Don Denney, used with permission


Sources Consulted

Ayto, John. Oxford Dictionary of Idioms, Kindle ed., Oxford University Press, 2020.

“Blue Moon.” NASA, 7 July 2004, https://www.nasa.gov/vision/universe/watchtheskies/07jul_bluemoon.html.

Burns, Robert. Poems, Chiefly in the Scottish Dialect. E-book, William Creech, 1787, https://archive.org/details/poemschieflyins00burn/page/154/mode/2up?q=halloween.

Hiscock, Philip. “Folklore of the Blue Moon.” International Planetarium Society, 1999, https://www.ips-planetarium.org/page/a_hiscock1999.

Olson, Donald W., et al. “What is a Blue Moon in Astronomy?.” Sky & Telescope, 27 July 2006, https://skyandtelescope.org/observing/celestial-objects-to-watch/what-is-a-blue-moon/

“What Is a Blue Moon? Is the Moon Ever Really Blue?.” Library of Congress, 2020, https://www.loc.gov/everyday-mysteries/item/what-is-a-blue-moon-is-it-ever-really-blue

Zoltán, Ildikó. “‘The Fourth Dimension Time’ in English Idioms.” Studia Universitatis Petru Maior - Philologia, vol. 13, 2012, pp. 350-357. OneSearch, https://msj.idm.oclc.org/login?url=https://search.ebscohost.com/login.aspx?direct=true&db=ufh&AN=89863545&scope=site