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"What's a Blue Moon?"  by Donald W. Olson, Richard T. Fienberg, and Roger W. Sinnott, Sky & Telescope. Article explaining that originally a blue moon meant the 3rd full moon in a season of 4 full moons, and how the "2nd in a month" error began.

"Once in a Blue Moon" at infoplease.com

"Blue moon" at World Wide Words

Blue moon calculator by David Harper

"On Blue Moons" by Kevin Clarke

"It's a big night for a 'blue moon'"  at nbcnews.com

"What does blue moon really mean?" blog entry on lunar calendars and computing

"Blue moon definition based on a mistake by Tony Rice

"Blue Moon" (list of blue moons this century) slide show by Tofique Fatehi