Is there ever really a blue moon?

It's time we had a talk about this.
If you want to get technical, blue moons do happen once in a while. The moon can be made to appear blue by a cloud of sulfur particles floating in the upper atmosphere. The moon looks bluish when it's seen from the right angle through the chemical haze.

It takes a good-sized conflagration to shoot enough sulfur into the air, though. Usually, nothing this side of a volcanic eruption or a major forest fire will do it.  One such blue moon occurred on September 26, 1950. A forest fire in northern British Columbia sent up enough sulfur to make the moon seem bluish when seen from England.

The moon isn't literally blue very often. So you may be tempted to conclude that "once in a blue moon" means "rarely."  Not quite. Judging from the earliest citations of the term, it originally meant "never."

The first appearance of "blue moon" is in a work entitled Rede Me and Be Not Wroth (1528): "Yf they say the mone is blewe/We must believe that it is true."  According to the Oxford English Dictionary, "if the moon is blue" is equivalent to saying "if the moon were made of green cheese." In other words, it's meant to indicate a patent absurdity.

But you know how things go. Over time the meaning of "once in a blue moon" got watered down. First the expression went from meaning "never" to "once in a long while." Maybe it was that forest fire in 1950.

Today things are even worse. There is a popular conviction, which the media have done much to reinforce, that a blue moon is a second full moon occurring within a calendar month. This occurs every two or three years. So now "once in a blue moon" means "not all that often, but more often than the Olympics."  The most recent blue moons occurred in January 1999 and again just two months later in March--a highly unusual circumstance that garnered a lot of attention from the press.

There is no astronomical significance to this type of blue moon. It is solely a consequence of the fact that the lunar month is shorter than most calendar months.

You occasionally get people who tell you that since ancient times a blue moon has been a good omen or a bad omen or who knows what.  Nonsense.  Ancient peoples didn't use our modern calendar. They typically used a lunar calendar, in which you had exactly one full moon a month.

As far as I can tell, this blue moon = 2nd full moon thing is an invention of the last 25 years.  Ranks right up there with the belief that the seasons "officially" start on the solstices and equinoxes. But don't get me started on that.

From: http://www.straightdope.com/