This is about as thorough a treatment of the “blue moon” topic as I have seen: http://news.yahoo.com/s/space/20101119/sc_space/thereallystrangestorybehindsundaysbluemoon
Very cool. I both love and hate knowing stuff like this. I love it because I am kind of a geek that way. I hate it because people know that I am kind of a geek that way so they always ask me stuff like, “what does a blue moon mean anyway” and no one is really going to want to hear the real correct answer and it will kill me a little to give the short version.
Huh… I must be thinking about the math wrong, but when I divide a 28-day lunar cycle into the 365 days of the year, I get 13 (and change); so how is it that most years only have 12 full moons? I can see the occasional year only getting 12, but most of them?
They’re missing one thing – why call it a “blue moon” at all?
It doesn’t get emphasized in the various articles about this that the Farmer’s Almanack calls each moon by a different name – the classic “harvest moon”, “strawberry moon”, “hunter’s moon”, etc. The problem is that if you have four moons in a quarter – the original Farmer’s Almanack definition for making a “blue moon” – then you have to give that new moon that showed up a new name. If you don’t, then your cycle of lunar month names will start to drift through the solar year, as happens with the Islamic calendar (Ramadan doesn’t fall at the same time of year every year).
One solution is to re-use the name of the last month, as happens with the Jewish calendar, a lunisolar calendar that performs precisely this correction to keep the months from slipping. This is called an intercalary month:
The Farmer’s Almanack uses the same idea, but instead of repeating names, it uses the name “Blue Moon” for the intercalary month. So it’s not a “blue moon”, the month is called “Blue Moon”.
Incidentally, you might get the idea that these romantic names for the months – “Hunter’s, Strawberry, Harvest” – are some sort of traditional European names, or Pagan names, or translated American Indian names. As far as I can determine they are none of the above. I suspect the Almanack made them up themselves, guided by a few examples like “Harvest Moon” that did exist previously.
As for why the name “blue moon”, as has been pointed out often enough, blue moons do occasionally happen, due to special atmospheric scattering conditions, usually due to forest fires. Such conditions occur even less frequently than every Blue Moon.
I give an answer along the lines of “popular belief is that <popular, yet incorrect answer>. However, that’s not really true. The real answer is kind of long, though – are you sure you want to hear it?”
Try 29.5 instead of 28 and see what you get. 28 is just an approximation.
As usual, the Master spoke on this – 32 years ago.
“Blue moon” meaning “extra moon in a quarter/month” is a modern invention.