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What is a Blue Moon And When Is The Next One?

What's the real definition of a Blue Moon? Was it based on a mistake? And when is the next one? We have the dates!

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

  1. August 19, 2024
  2. May 20, 2027
  3. August 24, 2029
  4. August 21, 2032
  5. 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.

To Recap

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!

Weigh In

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!

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This is a brilliant article! Admittedly, I had always followed the two full moons in a calendar month rule. However, I had then heard some old lore (possibly Celtic because of the lore surrounding a Sidh Moon/ Faerie Moon which supposedly refers to the second New Moon in a month) which stated if a calendar year had 13 Full Moons, the last Full Moon (thus in December) was considered the Blue Moon.
Considering what one of the other comments said about the Full Moon having bluish overtones in winter – it kind of makes sense too. Old lore is fascinating! I love how stories & traditions develop, and I love researching the roots of long-held ideas.
Thank you!

rod rod the rv gawd

The Original saying, “once in a Blue Moon” was in reference to the Full Moon that occurred on the Winter Soltice. Being the middle of Winter in the northern hemisphere it was cold, thus the color Blue. Now, the “Once in a Blue Moon” was a rough measure of time of 17 1/2 years between these events. The term today was twisted to the 2nd full moon in a month by the Star Gazer on PBS back in the seventies. It’s really just the 2nd full moon in a month.

Susan Higgins

Hi Rod, it seems your information conflicts with ours, which came from an astronomer, so we stand by it.

Wayne Keith

I always have and will continue to use the “twice in one month” definition. It’s what I’ve always heard since I was a boy. I never heard of the 4 in a season deal until just recently, and I’ve been a sky and starwatcher since the 60s

Jodie Masteller

I really like the article. I appreciate the information. Your article information will be passed on. Great job!

Susan Higgins

Thank you, Jodie! We’re glad you found it informative.

Megan C Bradbury

Interesting. Always wondered where that expression originated.

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