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Blue Moons - - - Photo of the moon - - - original photo from an unknown source

Blue Moons

Think you know what a "Blue Moon" is?

Bet you're wrong!

See if your explanation is the same as this wrong explanation from Information Please (InfoPlease.com):


"Incorrect" Information From Information Please

The second time the Moon is full within a particular month, it is called a "blue moon". This condition occurs once every few years when the date of the first full moon is at or near the beginning of the month so that the following full moon comes before the end of the month. The expression "once in a blue moon," meaning "very seldom," stems from this phenomenon. Over the years a "blue moon" appears to have meant any rarely occurring kind of moon. A "blue moon" can also refer to a moon that appears to be blue in color, which is caused by unusual atmospheric conditions.

Full moons occur about once every 29.53 days, or 12.3683 times a year. Therefore, months containing two full moons occur on the average every 2.72 years. Approximately once every 19 years, one year will have two months with two full moons because February will have no full moon at all. February can never have two new or full moons because the shortest time between them is 29.27 days.

The rare phenomena of two "blue moons" occurring in the same year happened twice in 1999. The first was on January 31 and the second on March 31.

This particular article has been removed from the Information Please site. But I link to other articles on the site below.


Why Is That Wrong and How Did It Get That Way?

This "definition", two full moons in the same month, is the most common "wrong" answer. How did it get this way?

It turns out that the same magazine responsible for the error has also been responsible for the discovery of the error.

In the May 1999 issue of Sky and Telescope Donald W. Olson, Richard Tresch Fienberg, and Roger W. Sinnott explain how a  53-year-old mistake in that same magazine changed pop culture and the English language in unexpected ways.

Here is the story.

In the July 1943 issue, on page 17, Laurence J. Lafleur (19071966) of Antioch College, Ohio, discussed Blue Moons in a question-and-answer column. He cited the 1937 Maine Farmer's Almanac as his source. Lafleur quoted the almanac August 1937 calendar page word for word. This commentary notes that the Moon occasionally "comes full thirteen times in a year. Lafleur didn't elaborate more.

The error began three years later. James Hug Pruett (18861955) wrote an article: "Once in a Blue Moon" for the March 1946 Sky & Telescope (page 3). Pruett was an amateur astronomer and a frequent contributor to Sky & Telescope. 

In his Blue Moons article 1937 Maine Farmer's Almanac.  He also repeated some of Lafleur's earlier comments. Unfortunately, he went on to say, "Seven times in 19 years there were  and still are  13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon."

"So I interpret it." It was a caveat. No one seemed to notice.

If was the first time anyone gave this definition. It was the first time the "wrong" answer was used.

It was partially right. Years normally have 12 full moons.  But every now and then a year has 13. This "extra" moon is a Blue Moon. But, which particular moon is the Blue Moon. In fact, when does a year start? Do you use the calendar year? Or do you use a different kind of year. But . . . I'm getting ahead of myself.

If Pruett had looked carefully at the 1937 Maine Farmer's Almanac he would have noticed that the Blue Moon fell on August 21st.  It couldn't possibly have been the second full moon that month. In fact, 1937 (the calendar year) had only 12 full Moons. 

Sky & Telescope began to perpetuate the error. On page 176 of the May 1950 issue, it used Pruett's definition in a note entitled "Blue Moons in May". In an irony particularly suitable to the story, H. Porter Trefethen of Winthrop, Maine provided the data on lunar phases for this note. Trefethen was the editor of the Maine Farmer's Almanac

Still, it wasn't Trefethen's fault. He never called the second full moon in a month a Blue Moon. The error was probably perpetuated by Sky & Telescope's founding editor, Charles A. Federer, Jr. 

When Sky & Telescope contacted Federer (he is 90 years old and lives in Florida) he agreed that he probably wrote the headline ("Blue Moons in May") thinking of Pruett's article and without consulting Trefethen.

On January 31, 1980, Deborah Byrd read the wrong definition on the air on National Public Radio's show StarDate. Examination of the script for the show (isn't it amazing the "stuff" we keep, like old scripts) shows that the script had a footnote (not read on the air) citing Pruett's 1946 article as the source. Deborah wrote it again in a December 1990 issue of Astronomy magazine.

Today, Byrd writes for the radio program earth & Sky.  Their website (see below) has some additional links for information.

The Kids' World Almanac of Records and Facts (New York, 1985: World Almanac Publications) picked up on the definition.

Then, Trivial Pursuit, using The Kids' World Almanac of Records and Facts as its source, used the definition in the Genesis II edition in 1986.

Since then it has been talked about, written about and miscommunicated in newspapers, magazines, and radio and TV stories.

Ok. So that's how we got the "wrong" definition.

What is the "right" definition.

Turns out the "right" definition is substantially more complicated.

The year contains four seasons. And the year usually contains 12 full moons: three each season. The first full moon in each season is named for an activity appropriate to that time of year:  for example, the Harvest Moon in autumn, the egg Moon or easter Moon or Paschal Moon in spring.. The last moon of the prior season is named Moon Before (e.g., Moon Before Yule). The second moon of the same season is named Moon After (e.g., Moon After Yule). 

So, when the year has 13 full moons that means one of the seasons has 4 full moons. The first, second and last all have names.  The third needed one: the Blue Moon.

This is the Maine Rule: If a season has four full moons, the third full moon of the season is a Blue Moon.

Now it turns out that this definition wasn't ever explicitly stated in the almanac. It required that the authors do a bit of sleuthing. 

First they had to locate old issues of the almanac. They managed to locate 42 copies dating back to 1819. The first mention of Blue Moon they could find is in the 1937 edition.

The second step was to apply the Maine Rule (the third of four full moons in a season) to see if the results match the Blue Moons listed in the almanac.

Unfortunately there are different definitions of year and season.  There are the definitions regular people use and there are the special definitions astronomers use.

First, the definition of year needs to be the "tropical year" going from one winter solstice to the next. Then, instead of using the earth's actual motion (in an elliptical orbit) you need to use the "mean sun" which is what would happen if the earth moved in a circular orbit. Then you . . . well . . . to get all these peculiar calendar details you should read the article. They are the kinds of things amateur astronomers will really love.

Suffice it to say that when they did all these calendar tricks and applied the "Maine Rule" (third full moon in a season) that their calculated Blue Moons matched the ones listed in the almanac.  Voila!

So what does this mean? It means that the Blue Moons in January of 1999 and in March of 1999 aren't "really" Blue Moons, at least according to the Maine Rule.

The "next" Blue Moon was on February 19, 2000.

Still, the "wrong" definition is so prevalent now, it may become the "right" definition. As the article says: "With two decades of popular usage behind it, the second-full-Moon-in-a-month (mis)interpretation is like a genie that can't be forced back into its bottle."


Sky & Telescope (May.1999), "What's a Blue Moon?"Sky & Telescope (May.1999) , "What's a Blue Moon?": This is the article that reveals all the details about the misinterpretation.

Sky & Telescope (Mar.1999), "Once in a Blue Moon Moon"Sky & Telescope (Mar.1999) , "Once in a Blue Moon Moon": A March article (written by Philip Hiscock) that began the search for the truth in Sky & Telescope.

Folklore of the Blue Moon, by Philip HiscockFolklore of the Blue Moon, by Philip Hiscock: This is another article by Philip Hiscock (who wrote the article in the Mar.1999 issue of Sky & Telescope). It has much of the same information. But it also has some additional information that I found interesting.  

Earth & Sky (15.Mar.1999), Blue Moon Number Two by Deborah Byrdearth & Sky (15.Mar.1999) , Blue Moon Number Two by Deborah Byrd: Deborah Byrd's latest article on the Blue Moon issue. It doesn't mention the Sky & Telescope revelations.  

1999  Twice in a Clue Moon; Only Once Every 19 Years1999 Twice in a Clue Moon; Only Once every 19 Years: An Information Please article about the "two blue moons" in 1999. It hedges the explanation by claiming that it is using "the modern definition" of blue moon.  

Once in a Blue Moon; What is a Blue MoonOnce in a Blue Moon; What is a Blue Moon: An Information Please article with more information that the article I quoted above. This one, for example, presents "both definitions" of blue moon.  

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