Blue Moon alert

Connoisseurs of the full moon will be getting a double treat next month, which will be bookended by full moons on the 2nd and 31st.

Given that the lunar cycle unfolds over 29.5 days, it is unusual to get two full moons in a month, and that second one is commonly called a "blue moon." It happens once every two or three years.

Yet that definition of "blue moon" is quite modern, falling into common usage long after Rodgers and Hart wrote their famous song, "Blue Moon," and the Marcels covered it with a doo-wop version of the chorus.

The lyrics, by the way, stick with the evocative and poetical, offering no insights to the origins of the phrase, which may be lost to antiquity.

Writing in Sky & Telescope magazine, Canadian folklorist Philip Hiscock states that in the 16th Century, the term was used to denote something absurd, not unlike saying the moon was made of green cheese. Here is his essay.

Hiscock said in those days, a person who hold to an absurdity -- such as "an entity called the  Philadelphia Phillies will win something callled a World Series in 2012" -- would just as well argue that the moon was blue.

Green cheese aside, the moon actually has appeared blue to earthly observers on a couple of occasions, Hiscock points out.

After the eruption of the Krakatoa volcano on Aug. 27, 1883, in Indonesia, the ejected volcanic material was said to be so dense that it created an atmospheric veil that gave the moon a bluish appearance.

A similar illusion occurred in the skies over eastern North America in 1951 when the sky filled with Canadian forest-fire smoke, he said.

The actual astronomical defintiion of blue moon is a somewhat muddled saga as this Sky & Telescope article points out.

And it is only in the last 30 years has "blue moon" come to mean the second full moon of the month.

Regardless, we are always grateful for a full moon, and especially grateful for any month that gives us two.