The concept of a "Blue Moon" as a synonym for "rare" has existed for centuries. However, the specific calendrical meaning used by modern practitioners was traced by folklorist Philip Hiscock to the Maine Farmers' Almanac of 1937. Interestingly, the almanac’s definition differs significantly from the "second-full-moon-in-a-month" rule frequently cited by modern news outlets.
The Seasonal Pattern (The Maine Rule)
The Almanac’s Blue Moons are tied to the four seasons of the year, consistently falling on the 20th through 23rd day of November, May, February, or August. These dates occur approximately one month prior to the Northern Hemisphere winter and summer solstices, and the spring and fall equinoxes.
While we typically define a calendar year from January 1st to December 31st, the Almanac utilizes the tropical year, measured from one winter solstice (Yule) to the next.
The Standard Year: Usually contains 12 full moons—three for each season (Winter, Spring, Summer, and Fall).
The 13th Moon: Occasionally, a tropical year contains 13 full moons, resulting in one season having four moons instead of three.
Defining the Seasons
The Maine Farmers' Almanac determines the start of seasons using the "Right Ascension of the Mean Sun" (R.A.M.S.), creating seasons of equal duration. This differs from the modern astronomical method of marking seasons by the Sun's celestial longitude (0°, 90°, 180°, and 270°), which produces seasons of unequal length due to Earth’s orbit.
The Almanac also incorporates rules from the 1582 Gregorian calendar reform:
The Vernal Equinox: Fixed at March 21st.
Lent: Begins 46 days before Easter and must contain the Lenten Moon (the last full moon of winter).
The Egg Moon: The first full moon of spring (also called the Easter or Paschal Moon), which must fall within the week before Easter.
Why the Third Moon?
According to the "Maine Rule," when a season contains four full moons, the third moon is designated as the Blue Moon. This naming convention ensures that the other moons—such as the Moon Before Yule and the Moon After Yule—remain properly aligned with the solstices and equinoxes.
The Origin of the "Second Moon" Definition
The modern interpretation—that a Blue Moon is the second full moon in a calendar month—is actually the result of a mid-20th-century misinterpretation:
July 1943: Laurence J. Lafleur discussed the 1937 Maine Farmers' Almanac in Sky & Telescope, noting that the moon occasionally "comes full thirteen times in a year," but he did not specify if this meant a tropical or calendar year.
March 1946: Amateur astronomer James Hugh Pruett wrote an article for Sky & Telescope titled "Once in a Blue Moon." Misinterpreting the Almanac, Pruett wrote: "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."
1980s Popularization: This "second-in-a-month" definition was adopted by the radio program StarDate in 1980 and later by the game Trivial Pursuit in 1986, leading to widespread public adoption.
The Verdict: Is May 2026 a "True" Blue Moon?
For the discerning practitioner, it is important to distinguish between modern folklore and historical almanac tradition. While many will celebrate May 31, 2026, as a Blue Moon, it technically only meets the "Monthly" definition created by a 1946 editorial error in Sky & Telescope. According to the more rigorous "Maine Rule"—which defines a Blue Moon as the third full moon in a season containing four—the May 2026 lunation does not qualify, as the spring season of 2026 only hosts three full moons. To find a "True Seasonal Blue Moon" by the ancestral tropical year standards, we must look ahead to May 20, 2027. Whether you choose to work your magic with the popular "Monthly" moon or wait for the "Seasonal" rarity of 2027, understanding these celestial mechanics allows you to align your practice with the actual rhythms of the cosmos rather than just the trends of the calendar.
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