What Exactly Is a Blue Moon?

The term 'blue moon' was figurative a long time before astronomers assigned various definitions. Learn why blue moons aren't blue and what they can mean in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/is-blue-moon-really-blue.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, a production of I Heart Radio,
Hey brain Stuff Lauren Bogobam. Here in nineteen fifty four,
a young Elvis Presley used the country standard Blue Moon
of Kentucky to help kickstart his career. By then, blue
moons had become a fixture in popular culture. This is
still the case today. Just look at blue Moon brand

(00:24):
beer or single blue Moon by back. But just because
a term is widely employed doesn't mean it's widely understood.
If you've ever wondered what blue moons are in the
astronomical sense of the phrase, this episode is for you.
The term blue moon dates back to at least the
sixteenth century, and since then it's been given several different definitions,

(00:45):
many of which are contradictory. At first, blue moon was
slang for something that was flat out impossible, but over
time the meaning of this idiom changed to refer to
things that were either rare or highly unlikely. This, explained
means the modern phrase once in a blue moon, and nowadays,
when a person says that something occurs once in a
blue moon, they mean it doesn't happen very often, but

(01:08):
that it's not impossible. And in the eighteen hundreds, this
expression received yet another meaning. It takes the moon twenty
nine point five three days to complete a full rotation
around the Earth. In the process, the appearance of the
moon goes through all of its phases, from new to full. Therefore,
each calendar quarter or season spring, summer, fall, and winter

(01:32):
typically sees three full moons apiece, assuming one full moon
each month, but every so often a single season will
get an extra moon, and during the nineteenth century some
stargazers began to refer to the third full moon in
a season, which sees four of them altogether, as a
blue moon. The Main Farmer's Almanac popularized this definition in

(01:55):
time for a quick aside. You might be wondering why
the third full moon in a season with four was
singled out here. Why didn't people just call that fourth
one the blue moon? The answer boils down to naming conventions. Again,
most years see twelve full moons in total. Many cultures
have given names to those that appear at specific points

(02:15):
in the year. For example, in some American traditions, the
last full moon of the winter is called the worm moon.
If the winter season in any given year were to
see four full moons, calling the final one a blue
moon would disrupt this linguistic status quo right then back
to the Main Farmers Almanac. From ninety two to nineteen

(02:37):
fifty seven, this now defunct publication championed this definition of
blue moons. The waters were further muddied in nineteen forty six,
but when an astronomer named James Hugh Pruett wrote an
article about blue moons for the magazine Sky and Telescope.
In it, he misinterpreted an excerpt from the nineteen thirty
seven edition of the Main Farmers Almanac. This led him

(03:00):
to conclude erroneously that a blue moon is the second
full moon in any given calendar month. Pru It's blunder
went on to have a life of its own. One
episode of the radio program started repeated his mistaken idea
about what a blue moon is. After that, this new
definition turned up on a trivial pursuit card and in

(03:22):
a children's nonfiction book. Yesterday's missprint can become tomorrow's accepted wisdom.
Most people now subscribe to prue It's definition. Thanks to
this kerfuffle, astronomy fans are left with two competing schools
of thought about what constitutes a blue moon. The rival
definitions now go by different names. The third full moon

(03:44):
in a season with four of them is called a
seasonal blue moon. Meanwhile, the second full moon in a
calendar month is called a monthly blue moon. The latter
phenomenon occurs once every two to three years. You'll note
that neither definition has anything to do with the Moon's
actual coloration. By virtue of its surface geology, Earth's natural

(04:06):
satellite usually looks gray, and during lunar eclipses, the refraction
of sunlight can give it a rusty red appearance. But
does the Moon ever turn blue? Well, yes, but only
under certain circumstances. In the past, there have been documented
instances of smoke and ash from massive forest fires and

(04:26):
volcanic eruptions, sending up particles into the atmosphere that filter
out red light. When this happens, the moon takes on
a blue appearance. In short, if you ever see a
moon that is literally blue, it's because something very destructive
is happening down on Earth. Today's episode is based on

(04:49):
the article why a blue Moon's Not really blue on
how Stuff works dot Com, written by Mark Vancini. Brain
Stuff is production of by Heart Radio in partnership with
how Stuff Works dot com and it's produced by Tyler
Clay Or more podcasts, My Heart Radio, visit the i
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