Weather Eye is often indebted to Dr Ian Elliott of Dun sink Observatory for his regular gems of astronomical serendipity. His latest dossier, forwarded complete with photocopies of articles written more than half a century ago, concerns the origin of the term "blue moon".
The popular understanding of this expression is that such an event occurs when two full moons fall in the same calendar month. On such occasions the second full moon is said to be a "blue moon". There were two, allegedly, this year in Ireland, in January and in March.
The expression "once in a blue moon", meaning "very seldom", has been around for a very long time. Indeed references to "blue moons" in literature have been traced back more than 400 years, and there have been many songs about them; perhaps the most famous is Rodgers and Hart's Blue Moon, written in 1934 and later popularised by Elvis Presley, but there are many, many more.
But it seems that the association of the term with two full moons in the same month is of fairly recent origin. It dates, to be precise, from July 1943, when this explanation first appeared in a quiz column in the American astronomical journal Sky and Telescope. The author was L.J. Lafleur, and he was, he said, quoting from another publication, the Maine Farmer's Alma- nac for 1937.
Mr Lafleur, however, had got it wrong. What the Maine Farm- er's Almanac actually said, albeit not very clearly, was that a blue moon occurs when a season has four full moons rather than the usual three, which is quite a different thing.
And where this rather obscure publication got its information, no one knows. In any event, Lafleur's erroneous interpretation that a blue moon is the second full moon in any calendar month was borrowed by a Mr Pruett for an article in the same journal, Sky and Telescope, in 1946, and 35 years later, in 1980, one Deborah Byrd used the information from this latter article in a radio talk.
From there the notion was adopted by the wider media and, although based upon a misconception, it has been the generally accepted version ever since.
The best minds in astronomy have toiled, and failed, to find any meaning related to the calendar for the term "blue moon" that pre-dates 1937. Moreover, if the Maine Farmer's Almanac 1937 definition is correct, it means that we in Ireland did not, as was alleged, have two blue moons this year at all; we had only one - the fourth full moon of spring which occurred in the closing minutes of the month of March.