The second full moon of the month, "Blue
Jewel Samad  /  AFP-Getty Images file
When the full moon appears twice in a month, the second one is called blue moon. This one is seen in the sky over Islamabad, Pakistan.
updated 5/31/2007 10:45:35 PM ET 2007-06-01T02:45:35

Thursday night brought the second of two full moons for North Americans this month. Some almanacs and calendars assert that when two full moons occur within a calendar month, the second full moon is called the "blue moon."

For most people, Thursday's full moon looked much the same as any other full moon. But the moon can change color in certain conditions.

After forest fires or volcanic eruptions, the moon can appear to take on a bluish or even lavender hue. Soot and ash particles, deposited high in the Earth's atmosphere can sometimes make the moon appear bluish. Smoke from widespread forest fire activity in western Canada created a blue moon across eastern North America in late September 1950. In the aftermath of the massive eruption of Mount Pinatubo in the Philippines in June 1991 there were reports of blue moons (and even blue suns) worldwide.

Origin of the term
The phrase "Once in a blue moon" was first noted in 1824 and refers to occurrences that are uncommon, though not truly rare. Yet, to have two full moons in the same month is not as uncommon as one might think. In fact, it occurs, on average, about every 32 months. And in the year 1999, it occurred twice in a span of just three months!

For the longest time no one seemed to have a clue as to where the "blue moon rule" originated. Many years ago in the pages of Natural History magazine, I speculated that the rule might have evolved out of the fact that the word "belewe" came from the Old English, meaning, "to betray." "Perhaps," I suggested, "the second full moon is 'belewe' because it betrays the usual perception of one full moon per month."

But as innovative as my explanation was, it turned out to be completely wrong.

More mistakes
Slideshow: Month in Space: January 2014 It was not until the year 1999 that the origin of the calendrical term "blue moon" was at long last discovered. It was during the time frame from 1932 through 1957 that the Maine Farmers' Almanac suggested that if one of the four seasons (winter, spring, summer or fall) contained four full moons instead of the usual three, that the third full moon should be called a "Blue moon."

But thanks to a couple of misinterpretations of this arcane rule, first by a writer in a 1946 issue of Sky & Telescope magazine, and much later, in 1980 in a syndicated radio program, it now appears that the second full moon in a month is the one that's now popularly accepted as the definition of a "blue moon."

This time around, the moon officially turned full on May 31 at 9:04 p.m. ET (6:04 p.m. PT).

But for those living in Europe, Africa, Asia and Australia, the moon turned full after midnight, on the calendar date of June 1. So in these regions of world, this was not the second of two full moons in May, but the first of two full moons in June. So, if (for example) you live London, you'll have to wait until June 30 to declare that the moon is "officially" blue.

This report was updated by

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