If you miss the "blue moon" tomorrow night (Thu., 5/31/07), don’t cry. Just fly to Europe. The full moon there on June 30 will be the second full moon in a month, hence by the controversial definition, it is a "blue moon." Or you can just wait until December of 2009, when there will be a "blue moon" in Europe as well as all of North America.
All in all, it’s kind of interesting that so many people seem so fascinated in this definition of a "blue moon," because there is no astronomical significance to it at all. They are neither rare nor blue. By the now-standard defintion, a "blue moon" is the second full moon in a given month. These moons look no different from any other moon and they are certainly not blue in color (except under unusual atmospheric conditions which see below). The whole concept has been explained extensively, including here on Earth & Sky: May 31 radio / May 31 Tonight’s Sky.
Traditionally, the idea of a "blue moon" implied a very rare occurance, perhaps a "once in a lifetime" event. But this is not true. One study I have seen indicates a frequency of about 29 months, which means that there is a "blue moon" on the average of every two years five months. See Once in a Blue Moon.) From that average, there should have been about 23 such "blue moons" in my lifetime so far. I have counted the number of "blue moon" occurances (thanks to Astronomical Tables of the Sun, Moon, and Planets by Jean Meeus) and in fact there were 22, as seen from Denver, with at least one additional time that was very close but not right on. That’s not really all that rare. It is, of course, considerably more common than a year in which February has 29 days.
And, again, standard "blue moons" are not blue in color. They look the same as any other moon. On occasion, the moon can appear blue, but not due to any quirk of the calendar. There is good reason to believe that the orignial term came from the bluish tinge the moon gets when viewed through smoke or ash high in the atmosphere. In a number of cases, ash from volcanoes such as Krakatoa in 1883, turned the moon blue. I can’t remember for certain, but I seem to recall seeing a blue moon in the early 1980s due to an ash cloud thrown up by an eruption of Mount St. Helens. And in the early 1950s widespread parts of Canada and even Europe saw a distinctly blue moon through the smoke of a large forest fire in British Columbia. You can listen to a Canadian Broadcasting Company radio report from the time here: CBC Blue Moon.
In the long run, whether they have astronomical significance or not, if these not-so-rare blue moons get people outside and actively observing the sky, I guess they are a pretty good thing. We should have one every month!
Image from my backyard on 28 May 2007. Not quite a blue moon, but a waxing gibbous moon, somewhat overexposed, through cirrus ice clouds, with a Colorado blue spruce in the foreground.
Larry, do you have any ideas about the linguistic origin of the term “blue moon”?
I’ve seen white moons, gray moons, orange moons, brown moons, and moons with rings around them, but I’ve never seen a blue moon. I have, however, seen a blue Sun. Smoke from big fires in California a few years ago blew eastward over Arizona, where I live, turning the sky gray and the Sun (when it could be seen) blue.
Hi Larry-
I think that a meteorologist readjusted the definition for blue moon in the 60s. However, like David’s comment, forest fires and volcanic eruptions can cause blue moons. The way that I explain it to my students is that light likes to scatter off of things that are close to its wavelength. The sky is blue because blue wavelengths are closer to the size of atoms than red ones. when you get micron sized particles in the air, the red wavelengths are more likely to scatter than the blue ones, leaving an originally white ray of light bluish. Cheers- Grant
Could u put any link to definition of what u call blue moon? i didnt got it..
Grant and David,
I’ll certainly go with the smoke and/or ash origin, as the popular definition today just doesn’t have much punch to it. The earliest reference I can find is from Webster’s Ninth New Collegiate Dictionary, which states that the first known reference in literature was in 1821, although it doesn’t say just exactly what the reference was. Wordsworth’s “Dictionary of Phrase and Fable” defines it simply as “very rarely indeed.”
Now, as for the second full moon in a given month being “very rarely indeed,” I don’t think so. But in my 56 years, I think I can remember seeing a blue-tinged moon once, that time having to do with volcanic ash. To me, once in 56 years is a lot rarer that once in 29 months, so I’m going with the smoke and ash story. (But it seems to me that during the fires around here a few years ago, the smoke gave the moon more of a milky orange color, although there may have been kind of blue tinge around that, hmmmm.)
My guess is that the term actually goes back much farther than 1821. It sounds very Shakespearean to me. I also wonder if the original meaning might have been something other than “rare” — maybe denoting something unexpected, such as “out of the blue.” We may never know.
Larry
P.S. I just noticed that Dr. Tony Phillips on the Science@NASA site has said that the term has been around for at least 400 years:
http://science.nasa.gov/headlines/y2007/30may_bluemoon.htm
Hi all,
I happen to know something about the current origin of the term blue moon, as it’s used to mean the second blue moon of a single month.
The following is an excerpt from an article by folklorist Phillip Hiscock: Folklore of the “Blue Moon”
http://www.ips-planetarium.org/planetarian/articles/folkloreBlueMoon.html
“Deborah Byrd’s late 1970’s radio program, ‘Star Date,’ may in fact have been responsible for spreading the new (or reborn) term. During research for that program she ran across this 1943 Sky & Telescope star quiz which included a question about two full moons in a calendar month, the answer to which was ‘blue moon.’ The term clearly was not commonly known in 1943, and the author of the quiz, L. J. Lafleur, attributed it to a 19th century Maine almanac. I’ve searched for a Maine almanac with the term in it but to no avail–if any readers have such a thing, I’d be very grateful to receive a copy! On her ‘Star Date’ show Byrd used the ‘blue moon’ term; that probably started the ball rolling.”
I actually remember the moment when - in the 1970s - I was standing in the astronomy library at the University of Texas, flipping through old back issues of Sky & Telescope magazine, found an article called “Once in a Blue Moon” and decided to use some info from it for a radio show. As it turns out, that S&T article contained an “error,” or at least what’s now described an an error. Not the second full moon in a single month, but the fourth full moon in a season. Whatever.
I don’t understand why people care about that so-called “error” in Sky & Telescope … an error that I, of course, passed along. This is folklore! And we’re the folk! The current definition of a blue moon has all the credibility in the world, to my mind. Surely as much credibility as naming patterns of stars in the sky after mythological heroes, animals and gods.
But then … I have a bias for the new blue moon definition …
Deborah
Michal,
By popular definition, a “blue moon” is the second full moon in any given month. This can happen month because the time from full moon to full moon (29.53 days) is less than a standard 30 or 31 day month. So if the moon is full at the beginning of a regular month, 29.53 days later might still be in the same month, hence making that second full moon, “blue.” This actually occurs on the average about once every 29 months, but again that is just an average. In 1999, both January and March had “blue moons.”
February is the only month that cannot have a “blue moon” because of its shortness (even in a Leap Year). In fact it sometimes may not even have a full moon. I haven’t figured that out yet, but I suspect that a February without a full moon is considerably more rare than a “blue moon.”
LS