MM> MM> Err ... my OED sees `blue moon' as `very rare',  and fitting a
MM> MM> lunar cycle inside a calendar month is indeed a rare event...
MM> MM> justifying the common usage of the term, one would think.

MM> JB> it happens about once every year. - 13 lunar cycles into 12 calendar
MM> JB> months...

MM>Mmmm ... I haven't tried to verify it, but the local paper claimed
MM>t'other day that Auckland's previous Blue Moon was in September,
MM>2002;  I wonder if there are any `records' associated with this -
MM>shortest, longest or `year with most' ?

Actually, it's not all that rare, but it only happens about every three
years or so.

Here's some Sky and Telescope articles about the topic.  I've also
included the link to the first article because the two connected
articles have some illustrations that I could not include.

http://skyandtelescope.com/news/article_1310_1.asp

Is the July 31st Full Moon Really "Blue"?
By the Editors of Sky & Telescope

July 27, 2004
On Saturday evening, July 31st, a full Moon will rise for the second time
this month (the first time was on July 2nd). Many people call the second
full Moon in a calendar month a "blue Moon" and use the
expression "once
in a blue Moon" to mean something that occurs only rarely. While the
latter meaning can be traced back centuries, the former definition is
much newer - and it's wrong!

It is rare to have two full Moons in a single month. The reason is
simple: the average time between full Moons is 29.5 days. Thus
February, with at most 29 days, can never accommodate two full Moons. To
squeeze a pair into a month with 30 days, the firs t must occur on the
1st of the month. Months with 31 days, including July, can have two full
Moons only if the first one occurs by the 2nd of the month, as happens
in July 2004. The last time a calendar month included two full Moons was
November 2001. No t until May 2007 (in North American time zones) or June
2007 (Europe) will it happen again.

If you want to tell friends that Saturday's full Moon is a blue Moon, go
right ahead. Countless newspapers, radio and TV stations, and Web sites
will certainly do so. But be aware that, technically, every one of these
reports will be in error! According to Canadian folklorist Philip
Hiscock, the term "blue Moon" has been around for more than 400 years,
but its modern calendrical meaning has become widespread only in the
last 25. And as discovered five years ago, it can be traced to a mistake
published in Sky & Telescope in the 1940s!

Sky & Telescope admitted to its "blue Moon blooper," an error that had
crept onto the magazine's pages 53 years earlier, in its May 1999
issue. Hiscock and Texas astronomer Donald W. Olson helped the magazine's
editors figure out how the 1946 mistake was made, and how the erroneous
meaning of blue Moon (as the second full Moon in a month) eventually
spread around the world. Before 1946, a blue Moon always meant
something else.

S&T writer James Hugh Pruett (1886-1955) made an incorrect assumption in
1946 about how the term had been used in the Maine Farmers' Almanac,
where it consistently referred to the third full Moon in a three-month
season containing four. (By this definition there is no blue Moon in
July 2004, and the next one happens in August 2005.)

There's no turning back now. The concepts of a blue Moon as the second
full Moon in a month and the third full Moon in a season containing
four are listed as definitions 1a and 1b, respectively, in the American
Heritage Dictionary (Houghton Mifflin Co. , 4th edition, 2000).


What's a Blue Moon?
The trendy definition of "blue Moon" as the second full Moon in a
month is a mistake.
By Donald W. Olson, Richard Tresch Fienberg, and Roger W. Sinnott

Recent decades have seen widespread popular embrace of the idea that when
a calendar month contains two full Moons (as does July 2004), the
second one is called a "Blue Moon." The unusual pattern of lunar phases
in early 1999 - two full Moons each in January and March, and none at
all in February - triggered a groundswell of public interest. Countless
newspapers and radio and TV stations ran stories about Blue Moons.

In an article "Once in a Blue Moon", folklorist Philip Hiscock traced the
calendrical meaning of the term "Blue Moon" to the Maine Farmers'
Almanac for 1937. But a page from that almanac belies the
second-full-Moon-in-a-month interpretation.

With help from Margaret Vaverek (Southwest Texas State University) and
several other librarians, we have now obtained more than 40 editions of
the Maine Farmers' Almanac from the period 1819 to 1962. These refer to
more than a dozen Blue Moons, and not one of them is the second full
Moon in a month. What's going on here?

Several clues point to a strong connection between the almanac's Blue
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