Miscellaneous Thoughts and Observations: Blue Moon?

My brother is the astronomer in the family, not me. I do, however, appreciate the consistent and appropriate use of words. An astronomical phenomenon today illustrates popular misuse or misunderstanding of terms. Despite what many in the media would have us believe, today is not a "Blue Moon". Yes, it is the second full moon in a month of the Gregorian calendar. No, that does not make it a Blue Moon. Rather than being based on the Gregorian calendar, the proper use of the term Blue Moon relies on the tropical year - measured from one winter solstice to the next. Most tropical years contain twelve full moons; three each in winter, spring, summer, and fall. Each of the twelve full moons has a name, e.g., Harvest Moon in fall. Sometimes there are thirteen full moons in a tropical year, resulting in a season having four. When there are four full moons in a season, the third is called a Blue Moon. The fourth full moon of the season retains the name assigned to the last moon. Since the Winter solstice occurred 21 December 2009 A.D., today's full moon is the first of the tropical year and is called Old Moon, Moon After Yule, or Ice Moon, among other names.

The misconception of what constitutes a Blue Moon likely originated in a 1946 Sky and Telescope article by amateur astronomer James Hugh Pruett, "Once in a Blue Moon". The question about what is a Blue Moon had previously been addressed in the "Question and Answer" section of the July 1943 Sky and Telescope and referenced the 1937 issue of Maine Farmers' Almanac as its source. The answer given, however, failed to distinguish between Gregorian and tropical years. Mr. Pruett apparently relied on the 1943 Q&A, missed the distinction and reached an incorrect conclusion the Blue Moon was the second full moon in a Gregorian calendar month. Sky and Telescope and other otherwise reputable sources then used Pruett's article as a reference and perpetuated the misconception. Sky and Telescope subsequently recognized the error and acknowledge today is technically not a Blue Moon. Despite the correction from the source, most media references to today's full moon will call it a Blue Moon. If the so-called healthcare reform debate were not adequate to prove you should not trust everything you read or hear in the media, the use of the term Blue Moon provides further evidence.