Once In A Blue Moon
Seasons of Earth and Sky: blue moons and a cold wave.
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ILLUSTRATION BY ANN NEUMANN
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SEASONS OF EARTH
AND SKY
RELATED CONTENT
How to watch the winter patterns of the moon 8
planets .
By Fred Schaaf
The crescent moon from top to bottom: Feb. 21, Feb.
20, Feb. 19, Feb. 18, Feb. 17. The planets at dusk, Feb.
23,after sunset: Saturn top center, then
Venus,Jupiter and Mercury. On Feb. 18, Venus and Jupiter
will be farther apart but lie not far to the lower right of
the moon, with Jupiterhigher than Venus.
This February won't provide us with a
full moon, though the sky will compensate us in March with
the rare (and long-fabled) "blue moon."
The time between
one full moon and the next averages (with very slight
variation) about 29-1/2 days. So there can never be two
truly full moons in February not even in leap year, when
February is still a half day too short. our longer months,
on the other hand, can have two full moon s in fact, this
happens once every few years. When it does, the second full
moon of the month is called a "blue moon." However, I
suspect that this definition of a blue moon came into being
in recent years or decades. The very much older phrase
"once in a blue moon" probably originated with an event
much more rare than a two full-moon month: initially, it
probably referred to those rare instances when the moon
literally looks blue.
What causes the moon and sometimes
the sun to look blue (or green or bronze) is a pall
containing particles of just the right size to
preferentially scatter the blue wavelength of moonlight or
sunlight. Such a cloud may be produced by widespread forest
fres or by vast volcanic eruptions. The only really
widespread and striking case of blue moon (and sun) in the
U.S. during this century was the one in September 1950,
which was visible across New York, New England, and part of
the Mid-Atlantic states. It was caused by Canadian forest
fires.
If you want rare, however, the pattern of a
two-full-moons-January, no-full-moon February, and
two-full-moons March is pretty rare. Instances of it can
never occur less than 19 years apart, but it doesn't always
occur that often. This year's instance is only the fourth
of the 20th century, and the last time it happened was in
1961. It won't happen again until 19 years from now, in
2018.
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