The Meaning of Blue Moon
The older meaning of blue moon to name an extra full moon, as was used in the Maine Farmer's Almanac, was the third full moon in a quarter of the year when there were four full moons. (Normally a quarter year has three full moons.) The division of the year into quarters for this purpose has the dividing line set between March 21 and March 22. This has to do with the rule for setting the date for the Christian Holy Day of Easter, which depends on the last full moon (as calculated by a somewhat inaccurate formula) on or before the Equinox on March 21 (which is also somewhat inaccurate).
In recent times, people have taken to calling a full moon a blue moon based on the Gregorian calendar. By this use of the term, a blue moon is the second of two full moons to occur in the same calendar month. Calendar blue moons occur infrequently, thus using the saying once in a blue moon to describe a rare event is still meaningful.
The next Blue Moon is May 31st, 2007, at 9:04 p.m. ( The regular May's Full Moon, the Flower Moon, is May 3rd, 2007, at 6:09 am.)
Dramoor : )
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