Fostering curiosity in kids (and their parents) since 2011

“Why do they call it a blue moon?”

Two waterbirds at night in front of an enormous full moon

(Photo: Janet Skarzynski)

Driving home late on April 4th, The Eight-Year-Old noticed that the moon was full.

Mommyo, incorrectly: “It’s early in the month. I wonder if we’ll have a blue moon in April.”

Thanks to her on-again, off-again obsession with space, The Eight-Year-Old already knew that the modern definition of a blue moon is simply the second full moon in a given calendar month.

The Eight-Year-Old, perplexedly: “Why do they call it a blue moon, Mommyo?”

In my search for this answer, I’ve come across all sorts of fun facts.

Fun Fact #1: I was wrong.

There will be no blue moon in April. The next blue moon is on July 31, 2015.

Fun Fact #2: The definition of a blue moon originally had to do with the number of full moons in a given season, not in a given month.

Seasons are three months long. With full moons occurring roughly every 29.5 days, most seasons boast only three full moons. Every once in a while, a season will have four full months, not three. Before the 1940s, the third of those full moons in a four-full-moon season was called a blue moon. This original definition was set by the Maine Farmer’s Almanac in 1937.

It didn’t last very long.

The definition was changed to the more modern meaning after a series of articles appeared in Sky and Telescope magazine in the 1940s and 1950s. Basically a writer misunderstand the Farmer’s Almanac definition–changing it to mean the second full moon in a given calendar month–and no one caught the error for almost 50 years, by which time the new definition of a blue moon was firmly entrenched in everyone’s mind.  You can read the full story here.

A blue moon rises over the Acropolis in Athen, Greece. (Photo: Anthony Ayiomamitis via Sky and Telescope)
A blue moon rises over the Acropolis in Athens, Greece. (Photo: Anthony Ayiomamitis via Sky and Telescope)

Fun Fact #3: Every 19 years, there’s no full moon at all in February.

This triggers a set of calendrical events which ends up giving us two blue moons (according to the modern definition) in a year. The last time this happened was in 1999. Those double blue moons almost always occur in January and March.

Fun Fact #4: Every full moon in the year has a name. 

I’d heard of the Harvest Moon (September or October) before, but today I learned for the first time that every full moon in the year has a name.  According to the Farmer’s Almanac, a full moon in January is called a Wolf Moon. February’s full moon is called a Snow Moon, and June’s full moon is called the Strawberry Moon. You can find the full list here, along with the rationale behind the various names.

Fun Fact #5: The Christian church has its own names for some of those full moons.

For example, the ecclesiastical calendar calls first moon of Spring the Paschal moon. That’s the full moon used to set the date of Easter every year.

Easter is defined as falling on the first Sunday after the first full moon after the vernal equinox. To make things a bit more predictable from year to year, the Western Christian Church stopped using the actual astronomical date of the vernal equinox a while ago. Instead, it declared March 21 to be the date of the vernal equinox, and calculates Easter based on that.

Eastern Orthodox churches use a different calendar (the Julian calendar) to calculate Easter, which is why the two Easters are celebrated on different Sundays.

Fun Fact #6: According to the Oxford English Dictionary, the first reference to a blue moon comes from a proverb recorded in 1528

The reference doesn’t seem to have anything to do with a blue moon as we understand it today.

If they say the moon is blue,
Then we must believe that it’s true.

Proverb from 1528

Since the moon at that time never actually appeared blue, calling the moon blue was obviously an absurd thing to do. It was like calling black white or waiting for pigs to fly.

Only in the 19th century, did folks start talking of a blue moon not as an absurd impossibility, but as a rare event. That when the phrase “once in a blue moon” started coming into more frequent usage.

Fun Fact #7: Sometimes, the moon actually does appear blue.

All it takes is filling the air with dust and/or smoke particles of the right size. Namely, the size spewed by volcanoes like Krakatoa. After Krakatoa exploded in 1883, people reported seeing a blue moon nearly every night.  (Forest fires have also been known to turn the moon blue.)

Victor C. Rogus took this picture of the August 21, 2013 blue moon a few seconds after the moment the U.S. Naval Observatory declared the moon to be officially full. (Photo: Victor C. Rogus via
Victor C. Rogus took this picture of the August 21, 2013 blue moon a few seconds after the moment the U.S. Naval Observatory declared the moon to be officially full (20:45:00 for those of you whose minds work more precisely than mine). (Photo: Victor C. Rogus via

So why is the second full moon in a month called a blue moon? 

Clearly, I could go on with these fun facts for a while. I feel like I’ve only scratched the surface of blue moon lore. But none of these fun facts get us much closer to the answer to The Eight-Year-Old’s original question.

Here are two theories on why the moon is called blue culled from my reading this morning:

1) The Christian Ecclesiastical calendar has two important named full moons during the period of Lent and Easter.

The first is the first full moon during the season of Lent. They call it the Lenten Moon. The second, is the Paschal full moon used to set the date for Easter itself. In the years where a third full moon occurs during the period of Lent and Easter, the church needed to call it something to distinguish it from the more important full moons used to set the ecclesiastical calendar. So the monks in the Dark Ages who were grappling with this problem called those moons blue. More accurately, they called them “betrayer” moons, which in the language of the time was spelled “belewe,” and pronounced “blue” (more or less).

2) The 19th century proverb about “once in a blue moon” meaning a rare event, became linked in our minds with the relative rarity with which a double full moon in a calendar month or a quadruple full moon in a season occurs.

The Farmer’s Almanac simply formalized the connection by calling those extra full moons blue in its 1937 edition.

Choose one to believe at your own risk. Personally, I’m more inclined to believe #2.

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