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"Once In A Blue Moon" What Is The Meaning Of This Phrase?

Updated on August 10, 2014
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Melvin is an avid reader and a retired chemist after working for a major pharmaceutical company for 32 years.


In our general conversation we often hear the expression "It only happens once in a blue moon." That expression has been around for quite some time. Many people used this expression as a way of saying some event or something will not happen again for a while. That is the event seldom happens.

Another expression some people sometime use is "Twice in a blue moon." This expression refers to events that occur very rarely. The event happens less frequently than "Once in a blue moon."


There are many folklores of how this phrase originated and what it meant many centuries ago. According to one source, the phrase "Blue moon" was first coined in the following proverb written back in 1528, "If they say the moon is blue, We must believe that it is true." Historically, each full moon of the year was given names to help the people and especially the farmers to keep track of time to prepare for certain things. For example one name I often heard was the Harvest moon. My grandfather mentioned this one many times to me since he was a farmer. The harvest moon occurs as a full moon in October just before Halloween. It was around this time of the year when some farmers harvested their crops before the coming winter. Some of the other full moons of year were given names such as Lent Moon ( late winter moon) Growing Moon, Egg Moon (early spring moon), and Snow Moon for obvious reasons.

Here are additional moon names from the past:

In the nineteenth and twentieth centuries, a yellow book published annually called the Maine Farmers' Almanac first published in 1818 had it own definition of a blue moon. This book had some very interesting information to read and one of the main piece of information in it was the weather prediction tables. The almanac defined a blue moon as the third full moon in quarter of a year when there were four full moons in the quarter. Typically a quarter has three full moons.

Astronomical Explanation of Blue Moon

In astronomical terms, a blue moon is the second full moon in a month. In order for a blue to occur, the first full moon has to occur early in the month, usually the first two days since the average lunar cycle is 29.5 days. This means a blue moon can occur in all the months of year except February since it has only 28 days. In some cases if a full moon occurs at the end of January, there will be no full moon in February at all.

So, how often does a blue moon occur? Generally a blue moon occurs every three years and not necessarily in the same month. Also a blue moon does not occur in the same night in the world due to your point of view with respect to the moon position. If a blue moon occurs in New York tonight, it will occur over a span of a few days before or after the occurrence in New York in other parts of the world.

In rare cases, two blue moons will occur in the same year. This celestial event happens approximately every 19 years between occurrences, hence the expression "Twice in a blue moon." This event only happens when there is no full moon in February and the blue moon occurs in the months of January and March; with each having two full moons since February is the shortest month in the year.

Past Blue Moon Occurrences

The last blue moon occurrence based on the astronomical definition took placed on Dec 31, 2009 and the last one that occurred using the Farmers' Almanac definition took place in August 2005. The last blue moon occurred in the United States on August 21, 2013. As I mentioned earlier It would have occurred in other parts of the world in the days before and the days after this date. This happens because it does not occur on the same day in different locations of the world because the moon is viewed from different angles with respect to the observer's position on the Earth.

The last time a blue moon event occurred twice in the same year was 1999, in January and March of that year.

A Real Blue Moon Has Occurred A Few Times

A blue moon was seen for two years following the famous eruption of Krakatoa in 1883, the moon appeared bluish due to the fine particles ejected into the atmosphere from the eruption.

Also in 1950 after the fires in Canada and Sweden, particles from the fires caused the moon to have a blue tinge for a few days.

In order for this phenomenon to occur the particles must be about one micron or 1 millionth of a meter in diameter. This causes light in the red wavelength to be scattered in a direction away from our eyes and direct the light in the blue wavelength toward our eyes.

If you keep your eyes open, you might see a real blue moon one night if you are in the right place at the right time.


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      Wrobb 3 years ago

      The moon tonight was kinda yellow/gold, tried taking a photo but crappy iphone lens was useless, i expected asmuch but all i had

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      Ivo 5 years ago

      Just FYI, the 1528 usage of the term does not really come from a proverb but from a pamphlet violently attacking the English clergy, entitled "Rede Me and Be Not Wrothe" ("Read me and be not angry"): "If they say the moon is belewe / We must believe that it is true" [If they say the moon is blue, we must believe that it is true].

      It was used to demonstrate that people were expected to believe whatever the clergy told them without any proof.

    • melpor profile image

      Melvin Porter 6 years ago from New Jersey, USA

      Kdepree, thanks for reading my hub and leaving a comment.

    • melpor profile image

      Melvin Porter 6 years ago from New Jersey, USA

      Ione77 star, again thanks for stopping by to read my hub and thanks for your comment.

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      Rod Martin Jr 6 years ago from Cebu, Philippines

      Fascinating, Melpor. It's nice to have the background on the language we use. Nicely researched.

      I've seen a "blue" moon -- a full moon just before sunrise. The western sky was several shades of purple, perhaps from DC pollution. This occurred in Rockville, Maryland in the late '60's.

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      Melvin Porter 7 years ago from New Jersey, USA

      Travel_man1971, once again thanks for your comment. I wrote this article to educate myself on this phrase. We all hear these phrases all the time in our conversations but never bother to dig deeper into them to find out what they mean.

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      Ireno Alcala 7 years ago from Bicol, Philippines

      Why didn't I read this 10 months ago? Luckily, I was able to join HubMob and saw your name sir Melpor. Thanks for the hub!

    • melpor profile image

      Melvin Porter 8 years ago from New Jersey, USA

      Thank you for your comment.