The term “Blue Corn Moon” refers to a full moon that meets two separate naming criteria: the traditional name for the lunar cycle and a designation for its frequency. This celestial event is not a single, official astronomical body but rather a convergence of an ancient harvest title with a modern rule for an extra full moon. The combination of these two definitions is rare, which can make the concept difficult to understand for those unfamiliar with lunar naming conventions. The name itself often leads to the misconception that the moon will be visibly blue, but the designation relates only to timing.
The Origin of the “Corn Moon” Name
The name “Corn Moon” originates from the traditional naming practices of Indigenous peoples in North America, particularly the Algonquin tribes. These names served as a practical calendar, marking the changing seasons and agricultural activities of the year. The Corn Moon specifically corresponds to the time in late summer or early autumn when corn and other crops were ready for harvesting.
This full moon typically occurs in September, coinciding with the peak of the harvest season in the Northern Hemisphere. In some years, the September full moon is known instead as the “Harvest Moon,” which is the name given to the full moon that falls closest to the autumnal equinox. When the Harvest Moon is in October, the September full moon is then consistently designated as the Corn Moon, signifying the time to gather the mature crops.
Defining the “Blue Moon” Phenomenon
The designation “Blue Moon” relates to two distinct interpretations of having an extra full moon in a given period. The original definition is the “seasonal Blue Moon,” which refers to the third full moon in an astronomical season that contains four full moons. A season, defined by the solstices and equinoxes, usually holds three full moons, but the approximately 29.5-day lunar cycle occasionally introduces a fourth.
The more common definition today is the “monthly Blue Moon,” which is simply the second full moon that occurs within a single calendar month. Because the lunar cycle is slightly shorter than most calendar months, a full moon at the beginning of a month leaves enough time for a second full moon to occur before the month ends. This popular, monthly definition arose from a misinterpretation of the original seasonal rule published in a 1946 edition of Sky & Telescope magazine.
When a Corn Moon Becomes “Blue” and Its Actual Appearance
A “Blue Corn Moon” occurs when the full moon corresponding to the corn harvest season also satisfies one of the two frequency requirements for a Blue Moon. For example, a monthly Blue Corn Moon would happen if the full moon in September, or the one closest to it, was the second full moon within that calendar month. This convergence is infrequent, happening roughly every two or three years, regardless of which definition of “Blue Moon” is used.
Despite the name, the full moon does not appear blue. Visually, a Blue Corn Moon looks like any other full moon, typically appearing white, yellow, or sometimes orange when viewed low on the horizon due to atmospheric scattering. The moon can, on extremely rare occasions, genuinely appear blue, but this is caused by specific atmospheric conditions unrelated to the lunar cycle names. Particles of dust or smoke must be approximately one micron in size to scatter red light and allow blue light to pass through to the observer, creating the unusual blue hue.