It’s the Blue Moon . . . the “True” Blue Moon!

If skies are clear for you on Sunday, August 22nd, you’ll want to take a look at the Moon. For on that night, you will be feasting your eyes on the Blue Moon. The real Blue Moon. That is, it won’t actually be blue — it will be its usual dazzling, brilliant self. But it will be the relatively rare full Moon to meet the actual definition.

What do the words “Blue Moon” conjure for you? Maybe they remind you of something that happens infrequently, as in “once in a blue Moon.” For those of you with a melancholy bent, you might listen to songs that invoke the blue Moon as a symbol of sadness or loneliness. Elvis Presley famously implored the “Blue Moon of Kentucky” (written by bluegrass musician Bill Monroe) to keep on shining on the “one that’s gone and left me blue.”

The Moon does actually look blue on the rare occasion when either volcanic eruptions or significant forest fires send heaps of ash and soot particles into the atmosphere. In 1883 the Indonesian volcano Krakatoa blew its top — sunsets were green and the Moon was blue for several years after. And in 1951 massive forest fires in western Canada turned the Moon blue for viewers in northeastern North America. (I wonder if we’ll see that phenomenon this year, what with all the fires currently consuming the Northwest.)

But maybe you’re casting your mind back to Halloween of last year when on October 31st the Moon was full for the second time that month.

That’s generally given as the technical definition of the phrase “Blue Moon.” But it’s not correct. The original definition has gone astray . . . and Sky & Telescope is to blame!

The True Blue Moon

Once upon a time, the Maine Farmers’ Almanac — that august publication that has provided long-term weather forecasts for the U.S. and Canada since 1818 — regularly cataloged “blue Moons.” But these weren’t the second-in-a-month full Moons (like last Halloween’s). Instead, they’re the third full Moon in a season that has four.

The Maine Farmers’ Almanac uses the tropical year, which is measured from one winter solstice to the next). The majority of tropical years have 12 full Moons — or three per season. Many cultures around the world assign names to full Moons that reflect the time of year. Think of the Snow Moon in February, or June’s Strawberry Moon, or the Harvest Moon, the full Moon closest to the autumn equinox (in the Northern Hemisphere). But every now and then a tropical year will have 13 full Moons — this means that one of the four seasons will be graced with four full Moons. Thus, naming the third full moon of a season “Blue” ensures that the other full Moons aren’t out of synch with their names.

So how did the “Blue Moon” adopt the “second-in-a-month” reference?

S&T Blooper

Way back in 1943, when Sky & Telescope was still a fledgling magazine in only its second full year of operation, page 17 of the July issue featured a question-and-answer column. In it, Laurence J. Lafleur of Antioch College, Ohio, referred to the 1937 Maine Farmers’ Almanac when discussing Blue Moons and wrote, “occasionally the moon comes full thirteen times in a year.” But he never explicitly stated that the Blue Moon was the second full Moon of the month.  

Fast-forward to 1946 and page 3 of the March issue of Sky & Telescope. In an article entitled “Once in a Blue Moon,” amateur astronomer James Hugh Pruett referred to the 1937 Maine Farmers’ Almanac and Lafleur’s column. He writes, “But 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 . . . “

By the May 1950 issue of Sky & Telescope, a subsection of the “Observer’s Page” on page 176 features “‘Blue’ Moons in May” wherein Henry Porter Trefethen — the editor of the 1937 Maine Farmers’ Almanac no less — answers a reader’s question on months with two full Moons and provides a list that goes back to 1836. And there you have it. That column likely cemented the association between “Blue Moon” with the second full Moon in a month. (Trefethen himself never linked the two — the “’Blue’ Moons” headline was added after the fact.)

Canadian folklorist Philip Hiscock first became aware of the phrase in 1988 when the month of May that year had a second full Moon. This event sparked the public’s imagination and Hiscock’s curiosity and so he decided to investigate the origin of the phrase. Besides the references mentioned above, Hiscock noted that a popular radio program broadcast on January 31, 1980, was likely responsible for propelling the second-in-a-month definition into public consciousness. He also dug up other curiosities, such as that the popular boardgame Trivial Pursuit incorporated the question “What’s a second full moon in a month called?” in its 1986 edition (if you happen to have the March 1999 issue of Sky & Telescope lying around, turn to page 52 for this story). Texas astronomer-historian Donald W. Olson, along with research librarian Margaret Vaverek at Texas State University, worked with the magazine’s editors at the time to figure out the origin of the mistake, and how the two-full-Moons-in-a-month meaning spread into the English language (see the May 1999 issue, page 36).

Hopefully skies will be clear this weekend, so we can all celebrate August's Sturgeon Moon. We'll have to wait for August 2024 for the next “true” Blue Moon, but we can enjoy an S&T Blue Moon in two years’ time, in August 2023.

Here’s something for the grownups: While waiting to feast your eyes on that dazzling orb this August 22nd, why don’t you wet your whistle with a Blue Moon cocktail. Fill a tall glass with ice, splash in four parts gin and one part blue curaçao, and add a twist of lemon. Cheers!

See What is a Blue Moon in Astronomy? for more details on the way the Farmers’ Almanac assigns the blue Moon status.