A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Is there ever really a blue moon?

January 1, 1977

Dear Cecil:

There's an old cliche about things that happen "once in a blue moon." Has there ever been a blue moon, or is this just a variation on hell freezing over? If blue moons have happened before, when will the next one be?

Cecil replies:

It's time we had a talk about this.

If you want to get technical, blue moons do happen once in a while. The moon can be made to appear blue by a cloud of sulfur particles floating in the upper atmosphere. The moon looks bluish when it's seen from the right angle through the chemical haze.

It takes a good-sized conflagration to shoot enough sulfur into the air, though. Usually, nothing this side of a volcanic eruption or a major forest fire will do it.  One such blue moon occurred on September 26, 1950. A forest fire in northern British Columbia sent up enough sulfur to make the moon seem bluish when seen from England.

The moon isn't literally blue very often. So you may be tempted to conclude that "once in a blue moon" means "rarely."  Not quite. Judging from the earliest citations of the term, it originally meant "never."

The first appearance of "blue moon" is in a work entitled Rede Me and Be Not Wroth (1528): "Yf they say the mone is blewe/We must believe that it is true."  According to the Oxford English Dictionary, "if the moon is blue" is equivalent to saying "if the moon were made of green cheese." In other words, it's meant to indicate a patent absurdity.

But you know how things go. Over time the meaning of "once in a blue moon" got watered down. First the expression went from meaning "never" to "once in a long while." Maybe it was that forest fire in 1950.

Today things are even worse. There is a popular conviction, which the media have done much to reinforce, that a blue moon is a second full moon occurring within a calendar month. This occurs every two or three years. So now "once in a blue moon" means "not all that often, but more often than the Olympics."  The most recent blue moons occurred in January 1999 and again just two months later in March--a highly unusual circumstance that garnered a lot of attention from the press.

There is no astronomical significance to this type of blue moon. It is solely a consequence of the fact that the lunar month is shorter than most calendar months.

You occasionally get people who tell you that since ancient times a blue moon has been a good omen or a bad omen or who knows what.  Nonsense.  Ancient peoples didn't use our modern calendar. They typically used a lunar calendar, in which you had exactly one full moon a month.

As far as I can tell, this blue moon = 2nd full moon thing is an invention of the last 25 years.  Ranks right up there with the belief that the seasons "officially" start on the solstices and equinoxes. But don't get me started on that.

ANOTHER THEORY

Dear Cecil:

Sky & Telescope seems to disagree with you!!!! --Chas. Jones, La Grande, OR

Cecil replies:

You dope, the Sky & Telescope article (actually a companion article, Once in a Blue Moon, written by folklorist Philip Hiscock) confirms what I said, namely that (1) "blue moon" originally meant "never," and (2) the idea that blue moon = 2nd full moon in a month (hereinafter called the second-moon theory) is a modern invention.  But you bring up an interesting twist.

In a fine bit of scholarly detective work, Hiscock traced the origins of the second-moon theory as follows: Trivial Pursuit Genus II edition (1986). You just knew these guys had to be involved. They got it from: The Kids' World Almanac of Records and Facts by Margo McLoone-Basta and Alice Siegel, 1985. They apparently got it from: "Star Date," National Public Radio program by Deborah Byrd, late January 1980. She got it from: The March 1946 Sky & Telescope (I know, getting kind of circular here), which got it from: A Q&A column in the July 1943 Sky & Telescope, which got it from: The 1937 Maine Farmers' Almanac.

Now it gets interesting. The Maine Farmers' Almanac identified a blue moon, but it was not the second full moon in a month.  The second-moon theory arose from a misinterpretation by the author of the 1946 S&T piece, James Hugh Pruett. (See the article for details.) In other words, current belief about blue moons is all due to a writer's mistake!

How did the almanac define blue moon? It's when a season has four full moons instead of three. The three full moons occurring in each season have traditional names--Lenten Moon, Paschal Moon, etc.  When there are four full moons in a season (which occurs sporadically), the third (not the fourth) is called the blue moon.

This interpretation of "blue moon" is also a late invention--as I say, the original meaning of "once in a blue moon" was "never."  Still, it relates the lunar month to the solar year, which is kinda cool, and doesn't rely on the arbitrary divisions of the calendar.  But let's face it--once something gets into Trivial Pursuit, it acquires the granite solidity of Truth as far as the public is concerned. So we'd better learn to live with the second-moon theory, even though it was a mistake.

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