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Blue Moon

blue moon:
1. something that happens very rarely;
2. the third Full Moon in a season which has four;
3. in poetry, can be used with a double meaning: sadness and rarity

When is the Moon "blue," in a calendrical sense?

  • According to modern folklore, a Blue Moon is the second full Moon in a calendar month.
    This type of Blue Moon can occur in any month but February, which is always shorter than the time between successive full Moons.
    1999 had two such Blue Moons (occurs only once every 19 years).

  • According to the original tradition (source: the Maine Farmer's Almanac, 1819 to 1962), a Blue Moon occurs when a season has four full Moons, rather than the usual three.
    This type of Blue Moon is found only in February, May, August, and November, one month before the next equinox or solstice.

Once in a Blue Moon .... what exactly is a Blue Moon?

Folklore of the "Blue Moon" by Philip Hiscock
Why is the original definition different than the modern one? from Sky & Telescope magazine

versions/updates: 19 Dec 2001, 13 Feb 2005