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Today’s a Blue Moon … Or is it?

I thought there was only one kind of Blue Moon … the second moon in the rare month with two full moons. Like this month. The first full moon was on August 1, the second s TODAY, August 31, 2012.

Boy, was I ever wrong. In researching this post, I learned that there’s a an older, and very different, definition of a Blue Moon. Not only that, there’s a raging controversy over which Blue Moon is the real deal. Seriously.

The older definition of a Blue Moon comes from The Old Farmer’s Almanac (of the 19th and early 20th centuries), which listed the full moons in each quarter of the year, usually three per season. When an extra (fourth) full moon occurred in a season, the third full moon was called a Blue Moon. The next Blue Moon by this “seasonal” definition won’t happen until August 21, 2013.

The “calendar” definition of a Blue Moon (like today’s) came about by accident, when James Hugh Pruett misinterpreted the 1937 Maine Farmers’ Almanac in his March 1946 Sky and Telescope article “Once in a Blue Moon”, saying:

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.

Who knew?

Is Older Better?

As you can imagine, there’s a huge controversy over which Blue Moon definition is “true”. Some think that the older “seasonal” definition is the real Blue Moon, because it relates to the earth’s orbit around the sun. Others side with the newer “calendar” definition.

Personally, I think arguing over which definition is better is just silly. We should just give the rarer “seasonal” Blue Moon a different name, maybe something like … a True Blue Moon … after the a colloquial expression used long before the “seasonal” and “calendar” Blue Moon definitions even existed.

According to the Oxford English Dictionary, the first reference to a blue moon comes from a proverb recorded in 1528:

If they say the moon is blue,
We must believe that it is true.

Which Blue Moon camp are you in?

  • Are you a purist, a true Seasonal Blue Mooner?
  • Do you roll with the modern day Calendar Blue Mooners?


  • Are you with me in thinking outside the solar system, giving one of them a new name?