Jay Respler <jrespler@superlink.net>

Colloquially the term "blue moon" is used to mean "a very long time."
In fact, there have been at least seven different uses of the term
"blue moon" in the past several hundred years.

The alt.usage.english FAQ discusses these different meanings of the
term "blue moon."  The two definitions most relevant to astronomy are
the following:

1. Under certain conditions of atmospheric haze, the moon may actually
look blue.  A notable example occurred after the explosion of the
volcano Krakatoa.  The appropriate conditions are extremely rare.

2. The second full moon in a calendar month.  Since the synodic month
is 29.53 days, this kind of blue moon occurs roughly once out of 60
30-day months and once out of 21 31-day months or about once in 2.5
years on average.  It can occur in January and the following March if
there is no full moon at all in February.  There are some indications
that some calendars used to put the first moon in the month in red,
the second in blue, hence the origin of the term.

 Philip Hiscock, writing in the 1999 March issue of Sky & Telescope,
expands upon the history of this definition.  This definition of "blue
moon" is of fairly recent vintage and came into widespread use in the
late 1980s as a result of the board game Trivial Pursuit.  He was able
to trace its origin to an (incorrect) entry in the 1937 edition of the
_Maine Farmer's Almanac_.

The alt.usage.english FAQ is available from
<URL:
ftp://rtfm.mit.edu/
pub/usenet-by-group/alt.usage.english/alt.usage.english_FAQ>
or
<URL:
http://www.cis.ohio-state.edu/
hypertext/faq/usenet/alt-usage-english-faq/faq.html>.