For more than half a century, whenever two full Moons appeared in a single month (which happens on average every 2 1/2 to 3 years), the second has been christened a “Blue Moon.” (In our lexicon, we describe an unusual event as happening “Once in a Blue Moon.”) However, there’s another definition of “Blue Moon” that has to do with the number of full Moons throughout a single season, which is the case in August 2021. But is this the correct definition?
Why “Blue” Moon?
For the longest time, nobody knew exactly why the second full Moon of a calendar month was designated as a “Blue Moon.” One explanation connects it with the word belewe from Old English, meaning, “to betray.” Perhaps, then, the Moon was “belewe” because it betrayed the usual perception of one full Moon per month? That makes sense!
However, in the March 1999 issue of Sky & Telescope magazine, writer Phillip Hiscock decided to get to the bottom of the somewhat confusing origin of the term. It seems that the modern custom of naming the second full Moon of any given month a “Blue Moon” was actually based on a misinterpretation outlined in an article in the March 1946 Sky & Telescope magazine. That article was titled, “Once in a Blue Moon,” written by James Hugh Pruett, who incorrectly summarized what he read in the Maine Farmers’ Almanac (no relation to this Farmers’ Almanac based in Lewiston, Maine). Pruett declared that a second full Moon in a calendar month is a “Blue Moon.” However, Pruett’s summary was an incorrect interpretation of what the almanac was saying!
Blue Moon Seasonal Definition
After reviewing what was actually written in the Maine Farmer’s Almanac on the subject, Hiscock found that the Maine Farmers’ Almanac‘s definition of a “Blue Moon” was derived from a completely different (and rather convoluted) seasonal rule. As simply as can be described, the almanac stated that there are normally three full Moons for each season of the year. But when a particular season ends up containing four full Moons, then the third is called a Blue Moon. If you’re wondering how Pruett could make such an error in his interpretation, you’re not alone.
In most years, there are twelve full Moons—one for each month, and three for each of the 4 seasons. In 2021, there are 12 full moons, yet four of them occur within the boundaries of our current summer season: June 24, July 23, August 22, and September 20. Thus, according to that old almanac rule stated above, the August 22 full Moon is the third of four Moons this summer of 2021 so it is branded a “Blue Moon.”
When Will the Next Blue Moon Appear?
The following is a list of Blue Moons following the “seasonal” rule (after the August 22 full Moon):
Blue Moon: Seasonal Rule
- August 19, 2024
- May 20, 2027
- August 24, 2029
- August 21, 2032
- May 22, 2035
Blue Moon: Two Full Moons In One Month Rule
The next Blue Moon by the “two-in-one-month” definition is August 31, 2023.
There are two instances when we call a full Moon a “Blue Moon:”
- when there are four full Moons in a single season; the third is a Blue Moon, and
- when there are two full Moons in a single calendar month, the second is a Blue Moon.
Does The Moon Really Turn Blue?
On past occasions, usually after vast forest fires or major volcanic eruptions, the Moon has reportedly taken on a bluish or lavender hue. Soot and ash particles propelled high into the Earth’s atmosphere, can sometimes make the Moon appear bluish. But the full Moon of any month or season doesn’t turn blue in color.
Wondering when the next full Moon is? Check out the names, dates, and times of all the year’s full Moons!
Based on the above information, what do you think should be done to correct the definition of Blue Moon? Share your thoughts with us in the comments below!