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

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

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

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.