HI, John! John wrote a little while ago to alert me to the fact that this calendar month has two full moons - the second of which occurs today, July 30th. It seems that someone once suggested to him that the saying "once in a blue moon" has its origins in rare occurrences like this, and that full moon number two is always blue. "What a pleasure it would be for me and many others," John, an artful flatterer, goes on, "if Weather Eye that day were headed `Once in a Blue Moon'".
It was believed at one time that 13 full moons in the same year was a portent of extensive flooding. Moreover, the worst of these inundations could be narrowed down to the month in which the two full moons occurred. Nonsense, of course - and neither, as one might expect, is there any reason why the second should be blue: when a real "blue moon" occurs, meteorologists prefer to seek another explanation.
The moon high in the sky is normally white or yellow, and near the horizon, it often appears orange or even red - both latter colours a consequence of the effect on the reflected sunlight of a long passage through the atmosphere. Very occasionally, however, when many dust particles of a fortuitously specific size are suspended in the air, the full moon, for a month or two, appears a delicate shade of blue. There are only two recorded instances in recent times: the first was in 1883, after the eruption of Krakatoa, when volcanic dust provided just the right ingredients, and more recently, in 1950, when the cause was extensive forest fires in Canada.
What John really wants to know, however, is how often the other "blue moon" occurs, and it is not difficult to calculate. Let us take, for example, a period of 1,000 years. There are 365.242 days each year, so a millennium has 365,242 days. We also know that each lunation - the period from one new moon to the next - is 29.5306 days, so our 1,000 year test period contains, dividing one into the other, 2,368 full moons.
Now we also know that 1,000 years has 12,000 months. It follows that in 1,000 years there must be 368 "blue moons - extra moons that must be fitted in as "seconds" into particular months. And if this occurs 368 times every 1,000 years, it must happen, on average, once every 2.7 years. Sure enough, the last time we had two full moons in the same month was about 2.7 years ago, in September, 1993, and the time before was in December, 1990. I expect the next will be in the spring of 1999.