December’s Blue Moon? Bah, Humbug!

Industry stats show that by late December the end-of-the-world disaster flick 2012 had grossed $730 million worldwide. This suggests that lots of you have seen it.

But not me. I've got more important things to worry about than this market-driven piece of trumped-up hysteria. After all, an even more alarming calamity awaits us on New Year's Eve: a full Moon — the second one in December.

I couldn't believe that doomsayers had overlooked this dread portent, so I double-checked my facts. Yep, it's all right there on page 52 of December's Sky & Telescope: full Moons occur on December 2nd at 7:30 Universal Time, and again on the 31st at 19:13 UT. Running the numbers, I calculate that those two events take place 29.488 days apart — amazingly close to the Moon's average synodic month of 29.531 days.

And did I mention that late on December 31st there'll also be a partial lunar eclipse, visible from Europe and Asia? And for all this to occur on the final day of 2009, the end of the dread decade of the 00s, the Uh-ohs? Can this all be mere coincidence?

Seriously, I doubt the world will grind to a halt on New Year's Eve. After all, the circumstances were the same 19 years ago, on December 31, 1990 — and there were no global consequences (apart from the debut of the Sci-Fi Channel on cable television).

Chart of Blue Moons
When is the Moon "blue," in a calendrical sense? According to the 1937 Maine Farmer's Almanac, a Blue Moon occurs when a season has four full Moons, rather than the usual three. But according to modern folklore, a Blue Moon is the second full Moon in a calendar month. Click on the chart for a larger view.
In modern usage, the second full Moon in a month has come to be called a "Blue Moon." But it's not! This colorful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope back in March 1946. There author James Hugh Pruett wrote how two full Moons fall in a single month seven times every 19 years. He then stated, "This second in a month, so I interpret it, was called Blue Moon."

Pruett's interpretation might have faded into history and been forgotten, had my old friend Deborah Byrd not picked up on it in January 1980 script for the Star Date radio program. She's since moved on to Earth and Sky and set the record straight. But by then this bit of faux folklore had taken on a life of its own.

It's now clear that "Blue Moon" appeared in a 1937 edition of the Maine Farmer's Almanac to denote an extra full Moon in a given season. You're probably familiar with terms like "Harvest" and "Snow" to describe the full Moons at various times of year. But when a fourth one intrudes in the three-month interval between, say, September's equinox and December's solstice, a gap occurs in this naming scheme. That's why editor Henry Porter Trefethen inserted a Blue Moon (as the third of the four) all those years ago in his almanac.

For the numerologists among you, this month's doubletake is the first since May 2007, and the next won't come until August 2012 (there's that scary date again). As for me, if skies are clear when I'm out celebrating, I'll take a peek at that brilliant orb as it rises over the Boston skyline to see if it's an icy shade of blue. Or maybe I'll just howl.

20 thoughts on “December’s Blue Moon? Bah, Humbug!

  1. Daniel A. Allison

    The term Blue Moon goes back to I believe it was 1884 when Karacatoa (sic?) blew up and the moon appeared to be blue for a period on several months due to smoke and ash.

  2. Geoff Gaherty

    Deborah Byrd’s goof would probably be forgotten too, if it hadn’t been picked up by the board game Trivial Pursuit Genius Edition, as Terry Dickinson points out in the latest issue of SkyNews.

  3. Mike Heneghan

    How common (uncommon) are two blue moons in a row. Didn’t it happen last year? If a blue moon occurs in very late January, then there is no full moon in February, and there will be a blue moon in March.

  4. Steve Greer

    I’m good with howling. Anytime the moon is full is a poor time for armatures sky lookers. It’s not even the best time to look at the moon. But the recent spate of articles (in the last few years) about the inaccuracy of calling the 2nd full moon in a month a “Blue Moon” seems to me like something to write about when there’s not much else happening. Let’s have a moratorium on this subject.

  5. Larry Robinson

    These are the real definitions:

    - Second Moon – Second full moon in month
    - Blue Moon – A second moon making 4 full moons in quarter
    - Lost Moon – No full moon in February
    - Rare Moon – Two second moons and lost moon in same quarter

  6. Wallace Cason

    Between 1981 and 1985 I subscribed to Sky and Telescope. In it I read an article saying that a blue moon occurred when the penumbra of earth covered the moon, i.e. a BLUE MOON was an eclipse of the earth – the sun getting blocked by earth so the shadow (penumbra) from the earth would cause the moon to look blue, or shadowed. Sky & Telescope gave the date of this and I looked that night and SAW the earth’s shadow give a blue cast to the moon for several hours. I much prefer this to the dictionary definition, because what I saw was literally a BLUE MOON. By the way, I was in Santa Marta, Colombia, South America when it happened. It was awesome.

  7. Wallace Cason

    Between 1981 and 1985 I subscribed to Sky and Telescope. In it I read an article saying that a blue moon occurred when the penumbra of earth covered the moon, i.e. a BLUE MOON was an eclipse of the earth – the sun getting blocked by earth so the shadow (penumbra) from the earth would cause the moon to look blue, or shadowed. Sky & Telescope gave the date of this and I looked that night and SAW the earth’s shadow give a blue cast to the moon for several hours. I much prefer this to the dictionary definition, because what I saw was literally a BLUE MOON. By the way, I was in Santa Marta, Colombia, South America when it happened. It was awesome.

  8. Jonathan Hayes

    The old saying is “once in a blue moon” meaning an occurence so improbable as to be extremely unlikely to happen at all. I don’t know the origin, but feel confident that that exprssion predates 1937. The song of the same name made it a symbol of lost love.

    If Kelly wants to howl, well, why not? We should all have a good howl now and then.

  9. Jonathan Hayes

    The old saying is “once in a blue moon” meaning an occurence so improbable as to be extremely unlikely to happen at all. I don’t know the origin, but feel confident that that exprssion predates 1937. The song of the same name made it a symbol of lost love.

    If Kelly wants to howl, well, why not? We should all have a good howl now and then.

  10. Wallace Cason

    Sometime between 1981 and 1985, I read a Sky and Telescope article about a literal blue moon, caused by the moon passing through the penumbra of the earth (i.e. an eclipse of the earth, from the moon’s point of view). Although I do not question the dictionary definition of blue moon, nor any part of the above discussion, please note — I SAW A MOON THAT TURNED BLUE while living in Colombia, South America, upon the advice of S&T!! I think it was a gimmick to call it a blue moon, but very appropriate since when the shadow of the earth passed over the face of the full moon, the moon did indeed have a distinct blue cast for several hours — not sure of the exact length of time — but I saw it with my own eyes, thanks to S&T!

  11. Wallace Cason

    Sometime between 1981 and 1985, I read a Sky and Telescope article about a literal blue moon, caused by the moon passing through the penumbra of the earth (i.e. an eclipse of the earth, from the moon’s point of view). Although I do not question the dictionary definition of blue moon, nor any part of the above discussion, please note — I SAW A MOON THAT TURNED BLUE while living in Colombia, South America, upon the advice of S&T!! I think it was a gimmick to call it a blue moon, but very appropriate since when the shadow of the earth passed over the face of the full moon, the moon did indeed have a distinct blue cast for several hours — not sure of the exact length of time — but I saw it with my own eyes, thanks to S&T!

  12. Janice Little

    If you go with the original definition of a Blue Moon (the third full moon in a season with four full moons) when will the next actual Blue Moon occur? According to information I’ve been reading it’s supposed to be November 2010. But the full moon in December actually occurs on the first day of winter, so how does that work???

  13. Enrico the Great

    If this event has no real significance, why discuss it? Just say it has curio value, an “Astronomical Morsel” if you will. It is still interesting.
    On a different note, can we keep the knowing references to popular culture to a minimum? They detract from an otherwise high quality website and magazine. Referencing the Sci-Fi channel as this article does brings real astronomy down to the level of cheap entertainment.
    I watch and enjoy Star Trek, but I cringe when I see it referenced in the magazine (thankfully a rare occurence). I enjoy good schlocky sci-fi, but I just don’t want to read about in Sky and Telescope or its website. Here I wanty REAL ASTRONOMY!!!

  14. Dave Altman

    Great article, as usual, but for one detail:

    “… the final day of 2009, the end of the dread decade of the 00s”

    We went through this ten years ago. There is no Year 0. The First Decade, the First Century, and the First Millenium all started with the Year 1 and ended in a year ending in 0. Next year, 2010, is the last year of the “00s”, not the first year of the Teens.

  15. Deborah Byrd

    Hi Kelly and all,

    I have to admit … I don’t understand S&T’s continued focus on the “mistake” or “blooper” aspect of this story. It’s folklore! There’s nothing right or wrong about it. Surely all folklore evolves to whatever the folk declares it to be. The name Blue Moon to describe the second full moon in a month has become established. It’s the 21st century definition. People enjoy it. It’s something that brings people TO astronomy.

    The focus on complicated definitions for blue moons just turns people off. Why do that?

    Let people have their Blue Moon tonight. Astronomy as a whole will benefit.

    Happy New Year to all!

    Deborah

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