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.
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