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O.k.! I said I'd be back regarding exactly what a Blue Moon is....and so I am. are indeed correct that the discrepancy about the "Blue Moon" originated in an article in the magazine known as "Sky and Telescope". You are also correct
that the issue was a failure (on the part of the writer) to be clear about what he meant. The author of the article used the word "year" in the process of explaining the
Blue Moon but what he failed to make known was that he was actually referring to "Tropical Year" and not the Calendar Year. The Calendar Year as we all know
begins on January 1st and ends on December 31st. The Tropical Year is the time period from one Winter Solstice to the next. Many who read the article
interpreted the "year" to mean the Calendar Year. To further complicate the matter 3 years later the discrepancy was perpetuated (again in Sky and Telescope) when
another article which was written by James Hugh Pruett (1886­1955), included the following words (which are a direct quote): "But seven time in 19 years - there
were - and still are - 13 moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue

The bottom line to all of this is that while technically speaking, the defining of a "Blue Moon" as the second Full Moon in a month is incorrect, it has become an
acceptable" definition. The following is a direct quote from 'Sky and Telescope':

"With two decades of popular usage behind it, the second-full-Moon-in-a-month (mis)interpretation is like a genie that can't be forced back into its bottle. But that's
not necessarily a bad thing. Rather than argue over whether to celebrate the dawn of the new millennium on January 1st in 2000 or 2001, those with the sunniest
outlooks will celebrate twice. Why not treat Blue Moons the same way, marking both the second full Moon in a calendar month and the third full Moon in a season
with four? "Even if the calendrical meaning is new," says Federer, "I don't see any harm in it. It's something fun to talk about, and it helps attract people to

I'm providing a link for anyone wanting to check the source itself:

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