From an msn group (Just thought is interesting to share)
Blue Moon?
When you ask someone: "what is a blue moon?" the chances are ten to
one that you'll get the answer: "when you have two full moons in
one month". If we use this definition, then we have a blue moon
approximately every 2.7 years. Having two blue moons per year is
extremely rare, the full moon then skips a month. This happens only
three to four times per century. Two blue moons per year always
fall in January and March, as February with its 28 day is the only
month short enough to not contain a full moon. 1999 had two blue
moons.
Definitions
The second full moon in a month has just the same color as always.
It does not turn blue at all. So why do people call this a blue
moon? Can the moon really turn blue? There are several definitions
of 'blue moon'.
Two of them are currently still in use,
1. Four hundred years ago, if someone said, "he would argue the
moon was blue," it would be understood as, "he'd argue that black
is white."
2. Also long ago, 'blue moon' was synonym for 'absurd'. "when the
moon turns blue", would be similar to the expression "when pigs
fly". It was so absurd that it would never happen.
3. In astronomy, we have a blue moon when certain types of dust and
smoke from volcanoes or forest fires absorb red light. When this
happens the moon and the sun get a blue tint. This however is a
very rare phenomenon. It happened in 1883 when the Indonesian
volcano Krakatoa exploded, in 1927 when a late monsoon in India set
up conditions for a blue moon and in 1951 when huge forest fires in
Alberta threw smoke particles up into the sky.
4. The phrase "once in a blue moon" means, that an event is fairly
infrequent, but not quiet regular enough to pinpoint.
5. There are popular songs in which "blue moon" is a symbol of
sadness and loneliness.
6. In recent years, the less scientific, but more popular
definition of the blue moon has become widespread, namely that it
is the second full moon within one month. This new meaning has
mainly become popular through the Trivial Pursuit game in 1986.
Ever since, this definition has been the most widely used.
So which version is
correct?
Officially, the third definition, the astronomical one, is correct,
the sixth definition, the two full moons in one month variety, may
appeal to many people, but has no roots in folklore. But as this
popular definition is now so widely used, it doesn't make sense to
ignore it in favour of the astronomical description. When people
claim that the 'second full moon' definition is wrong, then they
are technically correct. Language is a dynamic medium however. So
when enough people use a false expression, it will in time become
correct. Therefore, at this moment, it is not more than logical to
state that a 'blue moon' is a rare astronomical phenomenon, but
also an occurence of two full moons within one month.
This article was based on:
"The rising of the bogus blue moon" - Terence Dickinson
"Folklore of the 'Blue Moon'" - Philip Hiscock