What's a Blue Moon?
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Recent decades have seen widespread popular embrace of the idea that when a calendar month contains two full Moons, the second one is called a "Blue Moon." The unusual pattern of lunar phases in early 1999 - two full Moons each in January and March, and none at all in February - has triggered a groundswell of public interest. Countless newspapers and radio and TV stations have run stories about Blue Moons.
In Sky & Telescope's March issue (page 52), folklorist Philip Hiscock traced the calendrical meaning of the term "Blue Moon" to the Maine Farmers' Almanac for 1937. But a page from that almanac, displayed in Hiscock's article, belies the second-full-Moon-in-a-month interpretation, as Donald Olson and Roger Sinnott pointed out in a companion article that called for further research.
With help from Margaret Vaverek (Southwest Texas State University) and several other librarians, we have now obtained more than 40 editions of the Maine Farmers' Almanac from the period 1819 to 1962. These refer to more than a dozen Blue Moons, and not one of them is the second full Moon in a month. What's going on here?