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Question What is a blue moon? Is the moon ever really blue?


“Once in a blue moon” is a common expression that has been used for a long time, and which means ‘not very often,’ or ‘very rarely.’ It often refers to an extra full moon; however, it has been used to describe the way the moon actually looked, when for different reasons it had turned a blueish color.

A sailboat floats near the eastern cliffs of Navy Base Point Loma, California in view of the first super blue blood moon in 35 years. Petty Officer 1st Class Charles White, U.S. Navy photographer, Jan. 2018. U.S. Navy Media Gallery

Have you ever heard the expression “once in a blue moon?” It means ‘not very often,’ or ‘very rarely,’ but exactly how often does a so-called blue moon occur?  Does the moon ever really appear blue?

The expression “blue moon” has been around for a very long time, and it almost always refers to a full moon of a very special kind, but sometimes it is really a description of the moon, which for different reasons turns blue.  When the famous volcano Krakatoa exploded and lots of dust was sent into the atmosphere, it made for some very unusual skies, including blue moons!

James T. Powers in The Blue Moon. Theatrical poster, 1906. Prints & Photographs Division, Library of Congress.

In America ‘blue moons’ were calculated every year and listed in the Maine Farmers’ Almanac starting in the 19th century and continuing on into the early 20th century.  Instead of using the calendar year that we are used to, which runs from January 1st through December 31st, the almanac used what is called the ‘tropical year,’ which extends from one winter solstice, usually around December 20 (the “Yule”), to the next. Most tropical years contain 12 full Moons — three each in winter, spring, summer, and fall — and each of those full moons is named for an activity appropriate to the time of year (Easter Moon, Harvest Moon, Yule Moon, etc.). But every once in a while a tropical year contains 13 full Moons, so one season has four full moons instead of just three, hence the ‘blue moon.’ The almanac also names all the full moons, including blue moons, at the beginning of each tropical year calendar.  But the interesting twist is that the blue moon is the third full moon in the season that has four.  Why the third moon rather than the fourth, you might ask?  That is because other full moons with other special names have to fall at their correct times of the year.  This rule made it all work out on the calendar according to the almanac.

Waxing moon seen from Lamar Valley, Yellowstone National Park. Neal Herbert, photographer, 2015. National Park Service, NP Media Gallery.

Today the expression ‘blue moon’ has another meaning which started out as a misinterpretation published in a magazine article!

The second type of blue moon, which has become the most widely used, simply means the second full moon of a calendar month. Back in 1946, a very popular astronomy magazine published what turned out to be an incorrect explanation of the blue moon, and this is how it happened:

In a question-and-answer column in Sky & Telescope‘s July 1943 issue, Laurence J. Lafleur of Antioch College, Ohio, discussed ‘Blue Moons,’ citing the 1937 Maine Farmers’ Almanac as his source, but he didn’t explain all the complicated calendar stuff.  Then, three years later, in March 1946, a man named James Hugh Pruett from Eugene, Oregon, who also wrote for Sky & Telescope, wrote an article called “Once in a Blue Moon.” In his piece, Mr. Pruett also mentioned the 1937 Maine Farmers’ Almanac and repeated some of Lafleur’s earlier comments. But then he went on to say that “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.” 

The Harvest Moon. Currier & Ives, 1860-1870. Prints & Photographs Division, Library of Congress.

This meaning has today become the commonly accepted one.  It was further fixed into modern use when it was broadcast on the popular radio program StarDate on January 31, 1980. The author of that program was Deborah Byrd, who also writes for the radio program Earth & Sky, and whose Web site (see below) gives her perspective on this modern contribution to lunar folklore.

So, ‘blue moon’ as most of us today know it, is modern American folklore, but with a long interesting history involving calendars and the measuring of the year.  Still, no matter what meaning you give it, blue moons are pretty rare, and everyone knows what you mean when you say “once in a blue moon!”

Of Further Interest

The Maine farmer’s almanack (or Farmers Maine almanac) is part of the American Almanac Collection at the Library of Congress.

How to find the lady in the moon. 2 views of the moon, “as photographed by the great Lick Telescope”, and showing “the lady in the moon idealized,” c.1894. Prints & Photographs Division, Library of Congress.

Published: 11/19/2019. Author: Science Reference Section, Library of Congress

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