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What is a Blue Moon?

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What is a Blue Moon?

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.” This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare.

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 second full Moon of any month doesn’t turn blue in color.

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 the Old English, meaning, “to betray.” Perhaps, then, the Moon was “belewe” because it betrayed the usual perception of one full Moon per month.

However, in the March 1999 issue of Sky & Telescope magazine, author Phillip Hiscock revealed one somewhat confusing origin of this term. It seems that the modern custom of naming the second full Moon of a month “blue,” came from an article published in the March 1946 Sky & Telescope magazine. The article was “Once in a Blue Moon,” written by James Hugh Pruett. In this article, Pruett interpreted what he read in a publication known as the Maine Farmers’ Almanac (no relation to this Farmers’ Almanac, published in Lewiston, Maine), and declared that a second full Moon in a calendar month is a “Blue Moon.”

(Continued Below)

A Blue Moon By Another Definition

However, after reviewing the Maine Farmer’s Almanac, Hiscock found that during the editorship of Henry Porter Trefethen (1932 to 1957), the Maine Farmers’ Almanac made occasional reference to a Blue Moon, but derived it from a completely different (and rather convoluted) seasonal rule. As simply as can be described, according to Trefethen’s almanac, 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 of that season is called a Blue Moon!

To make matters more confusing, the beginning of the seasons listed in Trefethen’s almanac were fixed. A fictitious or dynamical mean Sun produced four seasons of equal length with dates which differed slightly from more conventional calculations. So, basically the current use of “Blue Moon” to mean the second full Moon in a month can be traced to a 55-year-old mistake in Sky & Telescope magazine!

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19 comments

1 The second blue moon of 2018 is about to arrive — here's what that means and how rare they really are – Black Owned And Operated { 03.31.18 at 11:23 am }

[…] older — and still valid — definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

2 The second blue moon of 2018 is coming Saturday night - Science Global News { 03.31.18 at 11:22 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

3 The second blue moon of 2018 is coming Saturday night - Science News 24-7 { 03.31.18 at 10:48 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

4 The second blue moon of 2018 is coming Saturday night - LiveBrain.com { 03.31.18 at 10:48 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

5 The second blue moon of 2018 is about to arrive — here’s what that means and how rare they really are % { 03.31.18 at 10:46 am }

[…] older – and nonetheless legitimate – definition of a blue moon comes from the Maine Farmers’ Almanac. Traditionally, that publication outlined a blue moon because the third full moon in an […]

6 The second blue moon of 2018 is coming Saturday night - Science News { 03.31.18 at 10:45 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

7 The second blue moon of 2018 is coming Saturday night ⋆ Discovery { 03.31.18 at 10:43 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

8 The second blue moon of 2018 is coming Saturday night - Weird Facts About Life { 03.31.18 at 10:42 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

9 The second blue moon of 2018 is coming Saturday night - Science Thrill { 03.31.18 at 10:42 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

10 The second blue moon of 2018 is coming Saturday night - Science Informers { 03.31.18 at 10:41 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

11 The second blue moon of 2018 is coming Saturday night - Science News Hub { 03.31.18 at 10:41 am }

[…] older – and still valid – definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

12 What is a blue moon? | Android Lover { 03.31.18 at 10:29 am }

[…] older — and still valid — definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

13 The second blue moon of 2018 is about to arrive — here’s what that means and how rare they really are – Business Insider | | All Breaking News { 03.28.18 at 3:06 pm }

[…] older — and still valid — definition of a blue moon comes from the Maine Farmers' Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

14 The second blue moon of 2018 is about to arrive — here's what that means and how rare they really are | Everyday News Update { 03.28.18 at 12:21 pm }

[…] older — and still valid — definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

15 What is a blue moon? - WallstreetRag { 03.28.18 at 11:53 am }

[…] older — and still valid — definition of a blue moon comes from the Maine Farmers’ Almanac. Historically, that publication defined a blue moon as the third full moon in an astronomical […]

16 Todo lo que deben saber de la “Luna azul” que brillará este viernes – Guatemala { 12.28.16 at 11:41 am }

[…] A pesar de llamarse “Luna Azul”, el color del satélite no cambiará. Se cree que su nombre podría derivarse del término “belewe moon”, que podría traducirse como “Luna traidora”, esto porque una luna llena adicional podría extender el ayuno de la cuaresma, de acuerdo a la revista Farmer’s Almanac.  […]

17 Blå måne | Blogleder { 07.30.15 at 9:45 am }
18 Black Supermoon Will Be Visible Wednesday Night | Fernanda Civit { 02.18.15 at 3:03 pm }

[…] The origins of the term are obscure, but probably an imitation of blue moon, which in turn seems to be an adjustment of the obsolete “belewe.” […]

19 Farmer’s Almanac Definition | Our Farm { 01.12.15 at 9:36 pm }

[…] What is a Blue Moon? – Farmers’ Almanac – If you notice a hole in the upper left-hand corner of your Farmers’ Almanac, don’t return it to the store! That hole isn’t a defect; it’s a part of history…. […]

If you notice a hole in the upper left-hand corner of your Farmers' Almanac, don't return it to the store! That hole isn't a defect; it's a part of history. Starting with the first edition of the Farmers' Almanac in 1818, readers used to nail holes into the corners to hang it up in their homes, barns, and outhouses (to provide both reading material and toilet paper). In 1919, the Almanac's publishers began pre-drilling holes in the corners to make it even easier for readers to keep all of that invaluable information (and paper) handy.

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