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Once in a Blue Moon

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What is a “Blue Moon?”

No, the Moon won’t be blue in color when you see one; a “Blue Moon” is the name given to the second full Moon in a single month.

Blue Moons occur, on average, once every 2 1/2 to 3 years. Perhaps this is why we traditionally describe an unusual event as happening “Once in a Blue Moon,” an expression that was first noted in 1821.

Why “Blue”?
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.

(Continued Below)

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.”

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.

But Can a Moon Really Be Blue?
Sort of. At times, 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.

So be sure to check your most recent edition of the Farmers’ Almanac for when you can see a “Blue Moon!”

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1 Ron Sharpe { 02.27.15 at 12:09 pm }

I was trying to find out if March 30 2015 Was a good time to have back surgery L- one and L-2

2 get download { 11.16.14 at 8:33 pm }

I have recently started a web site, the information you provide on this web site has helped me greatly. Thank you for all of your time & work.
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3 Jaime McLeod { 09.26.12 at 9:06 am }

Molly, It’s different every month. You can find moonrise and set times for every night of the year in our book.

4 Molly { 09.23.12 at 1:13 pm }

This is a really sweet website!!!

5 Jaime McLeod { 09.04.12 at 10:14 am }

A Blue Moon is not actually blue in color. The story above explains that.

6 Denise Hodges Fisher { 09.01.12 at 8:52 am }

Last nite i sawthe “blue” moon but it was hazy so it wasnt ,blue” today and tomorrow its supposed to be very rainy and cloudy ( thanks to Isaac!)

7 Jaime McLeod { 08.31.12 at 9:43 am }

There is no place in our Almanac where we refer to it as a Red Moon. You must be looking at someone else’s Almanac (there are several competing publications with a similar title on the market).

8 Al Jordan { 08.31.12 at 8:38 am }

If this Blue Moon is called a Red Moon in the Almanac, isn’t it actually a Purple Moon then?

9 Tianah { 08.30.12 at 9:43 am }

I have a Farmer’s Almanc Planner, and in it it spoke of a “Full red moon” for August 31st, can it be blue and red?

10 repete { 08.27.12 at 8:01 am }

It is widely believed that the moon has a relationship with fertility due to the corresponding human menstrual cycle, which averages 28 days. However, no connection between lunar rhythms and menstrual onset has been conclusively shown to exist, and the similarity in length between the two cycles is most likely coincidental.
It is sometimes claimed that surgeons used to refuse to operate on the full moon because of the increased risk of death of the patient through blood loss.In October 2009, British politician David Tredinnick asserted that during a full moon “surgeons will not operate because blood clotting is not effective and the police have to put more people on the street.”.A spokesman for the Royal College of Surgeons said they would “laugh their heads off” at the suggestion they could not operate at the full moon. (from Wikipedia)

11 ROBIN { 08.25.12 at 2:09 pm }

Don’t forget to charge your crystals and quartz that night, leave them out.

12 sonya { 08.25.12 at 6:32 am }

BLACK MOON???? there is none in Astrology …

13 bassmaster357 { 08.24.12 at 6:44 pm }

I love the blue moon tide it’s great for fishing

14 caroljean56 { 08.24.12 at 11:21 am }

Has anyone else done the “old wives” tale? Shake an empty purse at the full moon…for financial prosperity for the next month???

15 Sum Guy { 08.23.12 at 6:37 pm }

The second New Moon of the month is called a “Black Moon”.
Nobody talks about the Black Moon.
Black Moons get no respect.

16 Marie { 08.23.12 at 7:28 am }

To Shirley–think about your question. It takes 28 days for a moon cycle. The most days in a month is 31! You only get a “blue moon” if the first full moon is very early in the month and the second one is the very last days of the month.

17 Jaime McLeod { 08.23.12 at 9:12 am }

No, Shirley, three full moons in a month is not possible.

18 SHIRLEY { 08.23.12 at 1:54 am }


19 sandra { 08.22.12 at 6:09 pm }

I love the moon of every months, i love the Blue Moon, I love the Super Moon.

20 Pat Speicher { 08.22.12 at 5:48 pm }

I love August moon because its also my birthday moon August 28th 1948

21 Bill Bennett { 08.22.12 at 1:57 pm }

Ray Eberle comes to mind and the Glen Miller Band way back when. He had so many songs that made the moon and it’s light so special. It would have been a lot different without the moon at night. There is a star in the sky for every time a person has gazed at the moon.

22 debbie { 08.22.12 at 1:08 pm }

this is why i love reading the almanac it is full of stuff we’all ponder about like why is there two full moons in a month and is it a good omen or bad ?.

23 Paul Brown { 08.22.12 at 11:27 am }

Blue moon oer Kentucky, keep on smilin’……

24 J. Langille { 08.22.12 at 10:18 am }

That’s my birthday this year, thinking it could make for a rather creative birthday cake idea…

25 Jaime McLeod { 08.22.12 at 10:30 am }

S. Woolridge,
That may be one way the term is used in that culture, but that is not the official meaning.

26 S. Wooldridge { 08.22.12 at 10:03 am }

I was told by an Indian that if you could see the Moon in the daytime, that was called a Blue Moon. Any truth to that?

27 Jaime McLeod { 08.22.12 at 10:33 am }

Joe – That also happens about every 2 1/2 to 3 years (generally, if there are two full moons in a single month, there will be four that season).

28 Joe Holzgrefe { 08.22.12 at 9:32 am }

What is the frequency of 4 full moons in a season?

29 joyce { 08.22.12 at 9:09 am }

i can hear the song in my head now.. blue moon, i saw you standing there, funny how something can make you think of things…

30 K. Sullivan { 08.21.12 at 2:30 am }

I love this kind of information. Isn’t nature glorious? Isn’t mankind’s attempt to explain these sorts of phenomena strangely creative?

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