[Find out when is the next blue moon.]

Thursday, May 31 brings us 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, that the second full Moon is called the "Blue Moon."

The full Moon that night will likely look no different than 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

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 will turn full on May 31 at 9:04 p.m. Eastern Daylight Time (6:04 p.m. Pacific Daylight Time).

But for those living in Europe, Africa, Asia and Australia, that same full Moon occurs after midnight, on the calendar date of June 1. So in these regions of world, this will not be 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.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.