Most people are familiar with the term "Once in a blue moon", meaning "very infrequently". Variations on this phrase have been in use for some 400 years, and certainly a visibly blue moon is a rare event. They can be caused by smoke or dust particles in the atmosphere, such as happened after forest fires in Sweden in 1950 and Canada in 1951 and, notably, after the eruption of Krakatoa in 1883 which caused blue moons for nearly two years.
However, the best
known physical definition (and the one we're most interested in from a
magical point of view) is the one which states that the second full moon
in a month is called a blue moon. Due to the Moon's cycle being 29½
days, there are occasionally - as in July 2004 - two Full Moons in one
month this only happens on average every 2.7 years and NOT every year
as some Pagan books and internet sites with Moon Information will have
you believe.
The next Blue Moon will be in June 30th 2007,
the next one after that will be on December
31st 2009, what a brilliant start to 2010 that will be
:) Incidentally don't expect the Moon to actually look blue, it may if
the atmospheric conditions are conjusive, but it could just look like
any other moon.
In 1999 there was a blue moon in January and one on March and February
had no moon at all, the next time that occurs will be in 2018.
