What is “Once in a blue moon”?

Answer 1

"Once in a blue moon" meand*exceedingly* rare (in fact, something you might NEVER see!)... much rare than the "second full moon in a month" explanation suggests.

In fact. . . sorry to all those who have accepted the notion, but the "second full moon in a month" explanation is incorrect in two respects.

First, the astronomical use of the expression "blue moon" is very recent and is NOT the source of the old expression "once in a blue moon" , which appeared in print as early as 1528 -- and seems to have more to do with a LITERAL "blue moon" (a rare occurrence, at best)

Second, the "second full moon" explanation was itself based on a misunderstanding of the astronomical use of the term!

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The Astronomical Mistake

This "second full moon" explanation, popularized by the board game "Trivial Pursuit" is actually based on a mistake in a 1946 article in Sky & Telescope magazine. In 1999 the magazine printed two articles on the subject by way of retraction,
A) explaining the REAL meaning of the astronomical expression (which is somewhat more complicated than the catchy "second full moon" idea),
"What's a Blue Moon?"
explains the correct astronomical usge -- when a season contained four full moons, the third was known as a "blue moon" -- and how the 1946 mistake came about
http://skytonight.com/observing/objects/moon/3304131.html?page=1&c=y

and
B) tracing just how the mistake was made and has spread into popular lore (via radio, Trivial Pursuit and newspaper articles)
"Once in a Blue Moon:Fact and fantasy about blue Moons" by Philip Hiscock
(opening snip)
"According to old folklore," some people say, the second full Moon in a calendar month is called a "blue Moon." They go on to explain that this is the origin of the expression "once in a blue Moon." But it isn't true! The term "blue Moon" has been around a long time, well over 400 years, but its calendrical meaning has become widespread only in the last 20 years."
http://skytonight.com/observing/objects/moon/3305141.html

Compare
http://www.wilsonsalmanac.com/bluemoon.html

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The REAL Origins

But all the astronomical uses (even the correct one!) are very recent, and have nothing to do with the origins of the expression "Once in a blue moon"

An sample explanation of the meaning
E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Blue Moon: Once in a blue moon. Very rarely indeed. 1
On December 10th, 1883, we had a “blue moon.” The winter was unusually mild.
http://www.bartleby.com/81/2105.html

Here is an explanation of just what the older "blue moon" of the expresion was:

"Once in a Blue Moon" is a phrase that refers to something that in all likelihood will never happen at all or only far off in the future. . . . The expression itself derives from the extremely rare, actually blue-tinged Moon caused by atmospheric layers of forest-fire smoke or volcanic dust at just the right height and visual angle, a rare occurrence."
http://www.ipl.org/div/farq/bluemoonFARQ.html

(lso note that the original "blue moon" need not be a FULL moon.)


See also the summary & links at:
http://en.wikipedia.org/wiki/Blue_moon

Answer 2

Very rarely. The reason it is called that is because on rare occasions, there are two full moons in the same month. The second one is called a "blue moon." Since it doesn't happen very often, something which occurs only occasionally was referred to as happening "once in a blue moon."

Answer 3

A blue moon is the second full moon in a month. It doesn't happen very often, so once in a blue moon is a very rare occurrence.

Answer 4

a blue moon is what they call it when there are two full moons in the same month. since this does not happen very often, it is rare so to speak, the saying once in a blue moon means that it is rare for something to happen..

Answer 5

A "blue moon" is a second full moon in the same month, which happens very rarely. So if something happens once in a blue moon, it is a very rare occurrence.

Answer 6

Two full moons in the same month.

The average interval between Full Moons is about 29.5 days, while the length of an average month is roughly 30.5 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen.

On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every two-and-a-half years.

Answer 7

A blue moon is when there are two full moons in a month.
Usually happens once every few years.
So if something is going to happen with small chance of comming about it is said to be Once in a blue moon.

Answer 8

Once in a very great while. On story I heard is that this came from when Krakatoa exploded in the 1800's, and the dust circling the earth afterwards made the moon take on a blue cast. After it went away, no more "Blue Moon!"

Answer 9

Anything that happens RARELY can be said to happen once in a blue moon

Answer 10

A BLUE MOON IS THE SECOND OF TWO FULL MOONS IN ONE MONTH. IT DOESN'T HAPPEN VERY OFTEN. SO IF SOMEONE SAYS, I TAKE A BATH ONCE IN A BLUE MOON'' THAT MEANS THEY DON'T TAKE A BATH VERY OFTEN.