A True-Blue “Blue Moon”

August's full Moon has come and gone. While it wasn't the second one occurring this month, it was a "Blue Moon" according to a definition dating to the 1930s.

"Blue" Moon rising
This month's full ("blue") Moon rises over the Propylaea to the Temple of Poseidon Erechtheus, which is part of the Acropolis in Athens, Greece. Click here for a larger, full-frame version.
The Moon was full early today at 3:45 Universal Time, which was last night for North Americans. Ordinarily, its timing would have passed by largely unnoticed. But, at last check, Google has tallied more than 50,000 mentions of this celestial moment as a "Blue Moon."

Wait, what? Isn't that supposed to mean the second full Moon in a given month? That's the definition that we often hear in the news these days. For example, there were two full Moons in August 2012.

The situation is confusing, and Sky & Telescope is largely to blame — thanks to a calendrical goof that worked its way into the magazine's pages back in March 1946.

Nearly a decade earlier, "Blue Moon" had been used in the Maine Farmers' Almanac to mean the third full Moon in a season containing four of them (rather than the usual three). This occasional add-on was needed to keep the tradition sequence of names — Wolf Moon, Strawberry Moon, Harvest Moon, and so on — in sync with the calendar. In 2013 full Moons fall (reckoned by Universal Time) on June 23rd, July 22nd, August 21st, and September 19th. That's four occurrences between June's solstice and September's equinox. So, like a round of celestial musical chairs, August's is the misfit and hence a "Blue Moon."

But in a 1946 S&T article, amateur astronomer James Hugh Pruett (1886–1955) made an incorrect assumption about how these extra orbs are handled. That's where the two-full-Moons-in-a-month notion got started.

It might have remained an obscure celestial aside, except that it resurfaced during a 1980 broadcast of the "StarDate" radio program. Then it showed up in the popular board game Trivial Pursuit. The genie was out of the bottle, as celestial sleuths Don Olson, Rick Fienberg, and Roger Sinnott detail here.

Calendar of "Blue Moons" 1999-2020
When is the Moon "blue," in a calendrical sense? According to the Maine almanac, a Blue Moon occurs when a season has four full Moons, rather than the usual three. This type of Blue Moon is found only in February, May, August, and November, one month before the next equinox or solstice. According to modern folklore, a Blue Moon is the second full Moon in a calendar month. This type of Blue Moon can occur in any month but February, which is always shorter than the time between successive full Moons.
Sky & Telesope diagram
In the graph at right, the occasions reckoned by the Maine Farmer's Almanac rule are actually less common than the two-per-month events (which double up in 1999 and 2018, for example). So saying "once in a blue Moon" to mean a rare occurrence will be more meaningful if we return to the MFA definition. Interestingly, the "Maine rule" dictates that Blue Moons can occur only in February, May, August, and November — whereas two full Moons can never occur in February.

The original meaning of the term is lost to antiquity, as Philip Hiscock explains, though it's been used for centuries. Conceivably the phrase "once in a blue Moon" derived from those truly rare times when the Moon takes on a pale blue tint due to atmospheric effects (such as high-altitude ash from a volcanic eruption).

Frankly, I like how the Maine Farmer's Almanac meaning ties into seasonal cycles. As pioneering aviator Antoine de Saint-Exupéry (or was it Carl Sandburg?) once said, "The time for action is now. It’s never too late to do something." So let the campaign to get those Blue Moons back where they belong start here!

18 thoughts on “A True-Blue “Blue Moon”

  1. Charlie Miller

    So, if the period between Solstices and Equinoxes is referred to as a "celestial season", then the rule becomes: "When a celestial season contains four full Moons, the third full Moon shall be known as the ‘Blue Moon’."

    Mmmm — two in one month is a lot simpler and it is an already well established and accepted definition.

  2. Frank ReedFrank Reed

    Yet another entry in the "Blue Moon" legend… Of course, the astronomical/astrological term "blue moon" (whether as the second Full Moon in a month or as the third Full Moon in a season of… oh whatever!) never had any standing whatsoever. It was a tiny footnote in history known only to a small group of people. The small article in the predecessor to S&T back in 1946 undoubtedly had some small influence, but the expression STILL was never used in astronomy –it was quite unknown among astronomers until the 1980s. Meanwhile in the common English language (English ONLY! –another reminder that this is NOT a significant astronomical term), the expression "blue moon" had been used as a ‘colorful’ expression for a very rare or near-impossible event for at least 200 years. The subject of this article was IN NO WAY the origin or the etymology of the expression "once in a blue moon" though the desire to create a ‘folk etymology’ for the common English expression is the root of interest in the much later astronomical expression.

  3. Frank ReedFrank Reed

    Furthermore, it is not true that the astronomical/astrological expression vanished for decades between 1946 and the 1980s. In fact, while never heard among astronomers before it entered the popular lexicon in the 1980s, the astronomical "definition" as the second Full Moon in a month was found on a regular basis in small specialty columns in some small town newspapers through the 1950s/60s/70s: in trivia columns. It was kept alive in widely-available "trivia guides" –source-books for those little filler newspaper columns. And while astronomically-minded commentators get a big kick out of attributing its later rise to an episode of "StarTalk" (another opportunity to put themselves front and center in this story), far more important was the release of an edition of the game "Trivial Pursuit" which had stolen/borrowed from a huge trivia encyclopedia that had formerly been the leading trivia filler column source book (leading to a major copyright law case which the encylopedist lost, btw). The expression "blue moon" shot into the public spotlight when that game became the giant board game hit of the 1980s. The rest is history. If folks want to slap a label on some meaningless number of Full Moons and say, "this one’s a Blue Moon!", well we’re as stuck with that as we are with "Supermoons". But it is, of course, absolutely wrong to suggest that this is the origin or etymology of the phrase "once in a blue moon". That suggestion is poor history.

  4. kdconodkdconod

    I disagree with both definitions – one is a mistake the other isn’t really ‘traditional’ or folklore really because as far as we know it only ever appeared in one obscure Maine almanac. Practically no one knew about it until the 2006 S&T article.

    I always thought the 2006 S&T article was a cop-out. A mistake was made and that was the time to correct it — but the authors passed it off as "modern folklore". So a mistake is OK as long as everyone repeats and believes the wrong answer is right? That’s like the Indiana politicians who wanted to round off pi to 3!

    The only true Blue Moon is one that actually appears blue due to smoke or dust high in the atmosphere (after eruptions of Mount St. Helens (1980) and Mount Pinatubo (1991) are two recent examples). This is also the only definition that jives with the phrase "once in a blue moon" which dates back to at least the 16th century.

  5. Justin SJustin

    The calendar is strictly a human construction. A “blue moon” where one is on August 1st and the other on August 31st may occur in New York City, but then it may be September 1st in Manila and so no “blue moon” occurs there.. But the equinoxes and solstices that delimit the seasons are specific astronomical events. They happen at a particular moment no matter what time zone an observer is in or where they are in relation to the international date line. So the traditional definition of “blue moon” is decidedly superior.

  6. jay ryan

    Kelly wrote:
    "So let the campaign to get those Blue Moons back where they belong start here!"

    Good luck with that, Kelly! For better or worse, the cat is out of the bag, I don’t think anyone will change the S&T "two in a month" rule. I saw lots of non-astronomers bellyaching online yesterday about "astronomers always changing the rules." Trying to revert to the MFA style will be as popular with the public as the de-planetization of Pluto!

  7. Michael-MartinMike Martin

    I live on the very tip of Eastern Long Island(North Fork. At Approx 2100 brain viewed a Waning Gibbous Moon, Magnitude -12.1. The location was 104* ENE and 10.28* above the horizon.
    I observed the Moon for several minutes and came to the conclusion that it looked strange to me. I summoned my Fanily to the front porch for further analysis and opinion. They also confirmed my observation. We all came to the same conclusion which was" The Moon looked Orange". My daughter elBorare further she said" Daddy, it looks like Halloween". We all went inside and checked the calendar, and it wasn’t Halloween. Now if the 4 of us can agree that the Moon was Orange. Then why not a "Blue Moon"
    Clear Skies All. Mike

  8. Michael-MartinMike Martin

    Sorry Folks, hit the submit button to soon. Can We all agree to disagree on the Topic "Blue Moon Fact or Fiction and Myth or Legend"". Let the general public believe in Fiction or Myth. And the rest of Us believe in Fact or Legend. In closing, if a Plane can be red, and a Star blue, then how about "A Blue Moon". Mike

  9. Jim DeCamp

    Whether or not the two-in-a-month type is more common than the four-in-a-season in that chart, over the long run, they have to even out. There are twelve months and four seasons in every year. In nineteen years there are always seven more lunations than months, the extra seven full moons have to go somewhere. In that period there will be always be precisely seven seasons with four full moons, and seven months with two full moons.

  10. Peter WilsonPeter

    There’s a blue moon every month, 3 – 5 days after it is new. Use a 6” scope or larger, and focus on the Earth-shine, keeping the sun-lit portion outside the field-of-view. Under these conditions, you can see the Earth-shine is decidedly blue.

  11. Kelly BeattyKelly Beatty

    Jim… well, not quite. the graph covers a little more than a full metonic cycle, and in the 19-year span of 1999-2017, inclusive, you’ll find 8 two-in-a-month events and 7 third-in-four events. February is the reason for the difference: it’s nearly a synodic month long, so it permits a pair of two-in-a-month cycles in the adjacent January and March (and in that year there’s no full Moon at all in February).

  12. Dennis

    The Farmer’s Almanac definition seems to apply to a proper-ish name, while the 1946 S&T definition is more of a label.

    Perhaps the seasonal is "the Blue Moon" while the calendar monthly is "a blue moon"?

  13. Jim DeCamp

    My previous comment about the Metonic cycle did not account for the possibility that some months out of the 228 calendar months in the 19 year Metonic cycle do not have *any* full moons. Those months would be called "February". In years where there is no full moon in February, January and March are certain to have "Sky & Telescope 1946" style blue moons. The likelihood of February lacking a full moon is approximately 1- (97/400) x (29/29.53) – (303/400) x (28/29.53) ~ 0.0436, or about once in 23 years. The likelihood of such February in any Metonic cycle are about 19/23 or about 5 out of 6.

    The two-in-a-month rule produces, on average, a little less than one extra moon blue moon every 23 years. For the 19 tropical years beginning on 12/21/2000 thru Dec 22, 2019, there are indeed seven Maine Almanac blue moons, and eight S&T46 blue moons.

    Can anyone figure out which months will have both a S&T46 and a Maine Almanac blue moon? (You only have to consider the months of May, August and November.) I’ll report back if I figure it out.

  14. Jeff

    Under the MFA rule the Blue Moon cannot occur any later than the 23rd of a month. The corollary is therefore that a Blue Moon can never be the second full moon of the month.

  15. Jim DeCamp

    My previous post about the Metonic Cycle contained the assumption that every month had at least one full moon. Obviously, that is not true, since once in a while February does not contain a full moon. Whenever this happens, January and March will usually have two full moons each. A moonless February occurs about once every 23 years or so, therefore one would expect about one additional blue moon every 23 years, using the two-in-the-same month rule vs. the third-in-a-season with four.

  16. Marjorie A. Walz

    One of the first lab experiments a psychology student does is to hold a white card against a blue background and note that the white card appears pink. The same white card against a red background is subjectively blue. I saw a new moon against a "Pinatubo pink" sky, and the same effect gave it a striking, unforgetable blue cast. I think this is the real origin of the "blue moon" idea.

  17. Fred ShumanFred from Laurel, Md

    Jim – that would be a good point, except that it *is* possible to have a month without a full moon (not too uncommon for February), but *not* to have an astronomical season with fewer than 3.

    So there will be more "doubling up" in the long run, for individual months, than for seasons, resulting in more blue moons in the long run, by the two-in-a-month definition.

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