Blue Moon: The Facts
When the subject of 'what is a blue moon' turns up in a scientific newsgroup, it is obvious that the 'two full moons in one calendar month' definition is becoming widely known, and is often given, in my view erroneously, as the ONLY correct answer to the question (and we now  know that this is in any case wrong and was never the definition intended).
This note attempts to clarify the situation... note that it includes extracts from other's work which I duly acknowledge and give the www links for reference:
Extract from the alt.english.usage FAQ: "blue moon". (notes by Philip Hiscock)
"The phrase "blue moon" has been around a long time, well over 400 years, but during that time its meaning has shifted around a lot. I have counted six different meanings which have been carried by the term, and at least four of them are still current today". (items 1 to 5 deleted)http://www.alt-usage-english.org/excerpts/fxbluemo.html
"6.Finally, in the 1980s, a sixth meaning was popularized (chiefly by the game Trivial Pursuit): the second full moon in a month. The earliest reference cited for this is The Maine Farmers' Almanac for 1937. Rumour has it that when there were two full moons in a calendar month, calendars would put the first in red, the second in blue".
However, from the March, 1999 issue of 'Sky and Telescope', it appears that even this was wrong! Apparently, the use of the term 'blue moon' relates to its seasonal occurrence, not its monthly occurrence. According to the almanac, a "Blue Moon" was said to occur when a season (defined by using the solstices and equinoxes) has four full Moons, rather than the usual three. It is then the third of these 4 full moons that is 'blue'. In other words, it would have been the last full moon in that season, but for the fact that, in some astronomical years (not every year) we need to pack 13 moon cycles in a 12 month year (winter solstice to winter solstice). The (now known as totally erroneous) definition that the 2nd full moon in a calendar month was 'blue' is thought to derive from a 1980's popular radio programme, which in turn based its answers on mis-interpretation of original articles in 'Sky and Telescope'.
So, to collate our understanding of 'blue moon' usage in terms of various newsgroups:
- In a meteorological/atmospheric physics ng (e.g. sci.geo.meteorology, and probably alt.talk.weather): The correct definition of a blue moon is the physical explanation of why, on rare occasions, the moon appears blue....... Caused by scattering of moon-light by smoke particulates; red-end of spectrum scattered more than blue, hence light seen from moon biased towards blue: a blue moon.
- In an astronomical ng (e.g. sci.astro): given that the definition of two full moons in a month is now apparently common knowledge in North America, and will become elsewhere in the astronomical community as moon prediction programs include algorithms for its prediction, then, as in the FAQ for sci.astro etc:"... the second full moon in a month. ". However, it has now (1999) been found that this definition is, and always was, WRONG! Strictly, from the source, a 'blue moon' is the third of four full moons in an astronomically derived 'season'. It is not clear why this should be coloured 'blue' but again the use of different colours in a publication to pick such an event out (which would not have happened every year) is an attractive explanation.
- In a general purpose ng, also specialist language ng (e.g. alt.english.usage): both definitions are now appropriate, though it would be pertinent to note in connection with the 'once in a blue moon' saying, that its meaning will/has changed;
1. To those who know the scientific definition, it means, what it has always meant, something that occurs very rarely or in extreme cases, never... i.e. in the 'pigs might fly' category. The event is not predictable, and you might go a lifetime without experiencing the event.
2. To those who grew up with the 'two full moons in a calendar month' definition (many parts of US and some areas in Canada), and those who play 'Trivial Pursuit', it will mean that an event happens now-and-then, but with longish gaps in between; is predictable, most/all people will experience such events in their lifetime, at irregular intervals. What happens in the future, now that we know it was really meant as the third full moon in a season with four... I suspect that we are stuck with the former!