Is this a true blue moon?

As it turned out, in 1937, it occurred on Aug. 21. That was the third full moon in the summer of 1937, a summer season that would see a total of four full moons.

Names were assigned to each moon in a season: For example, the first moon of summer was called the early summer moon, the second was the midsummer moon, and the last was called the late summer moon.

But when a particular season has four moons, the third was apparently called a blue moon so that the fourth and final one can continue to be called the late moon.

So where did we get the "two full moons in a month rule" that is so popular today?

A moon mistake

Once again, we must turn to the pages of Sky & Telescope.

This time, on page 3 of the March 1946 issue, James Hugh Pruett wrote an article, "Once in a Blue Moon," in which he made a reference to the term "blue moon" and referenced LaFleur's article from 1943.

But because Pruett had no specific full moon date for 1937 to fall back on, his interpretation of the ruling given by the Maine Farmers' Almanac was highly subjective. Pruett ultimately came to this conclusion:

"Seven times in 19 years there were – and still are – 13 full 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 Moon."

How unfortunate that Pruett did not have a copy of that 1937 almanac at hand, or else he would have almost certainly noticed that his "two full moons in a single month assumption" would have been totally wrong.

For the blue moon date of Aug. 21 was most definitely not the second full moon that month!

IN PICTURES: Full Moons

Blue moon myth runs wild

Pruett's 1946 explanation was, of course, the wrong interpretation and it might have been completely forgotten were it not for Deborah Byrd who used it on her popular radio program, "StarDate" on Jan. 31, 1980.

We could almost say that in the aftermath of her radio show, the incorrect blue moon rule "went viral" — or at least the '80s equivalent of it.

Over the next decade, this new blue moon definition started appearing in diverse places, such as the World Almanac for Kids and the board game Trivial Pursuit.

I must confess here, that even I was involved in helping to perpetuate the new version of the blue moon phenomenon. Nearly 30 years ago, in the Dec. 1, 1982 edition of The New York Times, I made reference to it in that newspaper's "New York Day by Day" column.

And by 1988, the new definition started receiving international press coverage.

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