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May
30, 2007: At 9:04 pm Eastern Daylight Time on May
31st, the full moon over North America will turn blue.
Not
really. But it will be the second full moon of May and, according
to folklore, that makes it a Blue Moon.
Above:
The first full moon of May 2007, photographed May 2nd by Tony
Wilder of Wisconsin. May's second full Moon on May 31st will
probably look as gray as this one, although according to folklore,
it is "blue."
If
you told a person in Shakespeare's day that something happens
"once in a Blue Moon" they would attach no astronomical
meaning to the statement. Blue moon simply meant rare or absurd,
like making a date for "the Twelfth of Never."
But
"meaning is a slippery substance," writes
Philip Hiscock of the Dept. of Folklore, Memorial University
of Newfoundland. "The phrase 'Blue Moon' has been around
a long time, well over 400 years, and during that time its
meaning has shifted."
 The
modern definition sprang up in the 1940s. In those days the
Maine Farmer's Almanac offered a definition of Blue Moon so
convoluted even professional astronomers struggled to understand
it. It involved factors such as ecclesiastical dates of Easter
and Lent, tropical years, and the timing of seasons according
to the dynamical mean sun. Aiming to explain blue moons to
the layman, Sky & Telescope published an article in 1946
entitled "Once in a Blue Moon." The author James
Hugh Pruett (1886-1955) cited the 1937 Maine almanac and opined
that the "second [full moon] in a month, so I interpret
it, is called Blue Moon."
This
was not correct, but at least it could be understood. And
thus the modern Blue Moon was born. A detailed account of
the story may be found here.
Surveying
the last four centuries of literature and folklore, "I
have counted six different meanings which have been carried
by the term," recounts Hiscock. In song, for instance,
Blue Moons are a symbol of loneliness; when love conquers
all, the Blue Moon turns gold. (See old Elvis records for
more information.) "This makes discussion of the term
a little complicated," he says.
One
complication is that the Moon can turn genuinely blue, as
shown in this photo taken by Tom King of Watauga, Texas:
Above:
A blue Moon photographed in October 2003 by Tom King of Watauga,
Texas. "Here is a
picture from a different night provided as a sanity check
to assure you that my moon shots do not always have this blue
hue," says King.
"I
had never paid any real attention to the term 'Blue Moon'
until one October evening in 2003," he recalls. "I
had my telescope set up in the backyard and the moon began
rising in the east with a strange blue tint I had not seen
before."
The
cause of the blue was probably tiny droplets of water in the
air. "The
air was damp and heavy with moisture," notes King. When
water droplets are about 1 micron (one millionth of a meter)
in diameter, they strongly scatter red and green light while
allowing other colors to pass. A white moonbeam passing through
such a misty cloud turns blue.
Clouds
of ice crystals, fine-grained sand, volcanic ash or smoke
from forest fires can have the same effect. "The key,"
notes atmospheric optics expert Les Cowley, "is that
the airborne particles should all be of very similar size,
a micron or so in diameter." Only then do they scatter
the correct wavelengths of moonlight and act as a blue filter.
There
are other reasons for blue Moons, he notes. "Our eyes
have automatic 'white balances' just like digital cameras.
Go outdoors from a cozy cabin lit by an oil lamp (yellow light)
and the Moon will appear blue until your eyes adjust."
What
kind of Blue Moon will you see this week? There's only one
way to find out!
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Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
| More
to the story... |
What
about Europe? Because of time zones, this
week's full Moon occurs over Europe on June 1st rather
than May 31st. In Europe, therefore, it is the month
of June which has two full Moons, and a Blue Moon on
June 30th. In North America, May has two full Moons
and a Blue moon on May 31st.
A
note of irony:
Blue moons that really exist are wildly unpredictable.
Who can say when a volcano will spew perfectly-proportioned
ash into the stratosphere? Or when a Texas mist of micron-sized
water droplets will waft in front of the rising moon?
The
absurd blue moons of folklore, on the other hand, no
longer take anyone by surprise. Full moons are separated
by 29.5 day intervals--the moon's synodic
period. By counting out synodic intervals, and checking
to see when a pair of full moons falls into a single
30- or 31-day month, we can easily and precisely figure
out when the next blue moon is due: Dec. 31, 2009. Mark
your calendar.
"This
new blue moon has a kind of technical meaning which
most of the earlier ones lacked," notes Hiscock.
"Perhaps as a result it will last a whole lot longer."
Folklore
of the Blue Moon -- by Philip Hiscock of the Dept.
of Folklore, Memorial University of Newfoundland
Blue
Moons and Lavender Suns -- (Alaska Science Forum)
What's
a Blue Moon? According to the editors of Sky &
Telescope, the definition of "blue moon" as
the second full Moon in a month is a mistake.
NASA's
future: The
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