Blue Moon Rises: What Does It Mean?

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A full moon rises over Kodachrome Basin State Park in Utah.
CORBIS

When the moon rises Tuesday night (Aug. 20), it brings us the August full moon and in addition, it will also technically be a "Blue Moon."

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"But wait a minute," you may ask. "Isn't a Blue Moon defined as the second full moon that occurs during a calendar month? Tuesday’s full moon will be the only full moon of August 2013. So how can we call it a 'Blue' moon?"

Yet it still is a Blue Moon, but only if we follow a now somewhat obscure rule of astronomy. In fact, the current "two full moon in one month" rule has superseded the rule that would allow us to call Tuesday’s full moon "blue."

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Confused yet? Well, as the late Paul Harvey used to say, here now, is the rest of the story:

A Blue Moon: The Almanac Moon Rule

Back in the July 1943 issue of "Sky & Telescope magazine, in a question and answer column written by Lawrence J. Lafleur, there was a reference made to the term "Blue Moon." Lafleur cited the unusual term from a copy of the 1937 edition of the now-defunct Maine Farmers’ Almanac (NOT to be confused with The Farmers' Almanac which is still published in Lewiston, Maine).

On the Maine Farmers' almanac page for August 1937, the calendar definition of the Blue Moon explained that occasionally "one of the four seasons would contain four full moons instead of the usual three."

"There are seven Blue Moons in a Lunar Cycle of nineteen years," continued the Almanac, ending on the comment: "In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon.'"

Unfortunate Astronomical Oversight

While LaFleur quoted the Almanac's account, he made one important omission: He never specified any date for the Blue Moon. And 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.

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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 is a Blue Moon rule" that is so popular today?

Pruett's Mistake

Once again, we must turn to the pages of Sky & Telescope. This time to page 3 of the March 1946 issue.

In that issue, author James Hugh Pruett wrote the article "Once in a Blue Moon" in which he made a reference to the term "Blue Moon" and referenced LaFleur's S&T article from July 1943. But because Pruett had no specific dates 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 wrong. For the Blue Moon date of Aug. 21 was most definitely not the second full moon that month!

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