What is a "blue moon"?

I'd always thought it was the second full moon in a month. (Once in a while a second full moon will occur in a calendar month.)

But I recently read another view. If an astronomical season has four full moons, the third is called the "blue moon". (I presume that "astronomical season" means the period from one equinox or solstice to the next.)

What's you're understanding of "blue moon". Is there any history of interest behind this term? And if it means the "third full moon in an astronomical season of four," then is there a special term for the fourth?

"Blue Moon" is a tavern in Seattle!

Wikipedia, the Word Detective, and the National Maritime Museum all have articles about "blue moon". But I think this is the best one. Philip Hiscock, a folklorist with the Memorial University of Newfoundland, has found six meanings to "blue moon"!

Tinman

Tinman's prime link has this interesting line: "all the months but February are longer than the moon's synodical cycle." The word is a new one to me.

synodical:
1 of or relating to a synod : SYNODAL
2 relating to conjunction; especially relating to the period between two successive conjunctions of the same celestial bodies (as the moon and the sun)

OK, so what's "conjunction" in this sense?
conjunction: 3a: the apparent meeting or passing of two or more celestial bodies in the same degree of the zodiac

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Originally posted by shufitz:
What is a "blue moon"?

I'd always thought it was the second full moon in a month. (Once in a while a second full moon will occur in a calendar month.)

But I recently read another view. If an astronomical season has four full moons, the third is called the "blue moon".

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I just found this article that explains how one definition morphed into the other.

That article didn't give the date of the Farmer's Almanac that talked about "blue moon," but this one does (August 1937).

Tinman

This message has been edited. Last edited by: tinman,

This is fascinating! Thanks for the links!

The term 'blue moon' refers to the fact that on very rare occassions the moon can appear blue. This usually happens after volcanic eruptions, such as Mount Pinatubo in 1991, El Chichon in 1982 and , of course, Krakatao in 1883. It is the dust in the atmosphere that causes the moon to appear a different colour and it is the fact that it is so unusual that the phrase 'once in a blue moon' has come to signify a very rare event.

I found "blue moon" in the OED Online:

quote:

blue moon (colloq.), a rarely recurring period

1821 P. EGAN Real Life in London I. xiv. 249 ‘How's Harry and Ben?—haven't seen you this *blue moon.’ [Footnote] Blue Moon This is usually intended to imply a long time.
1869 E. YATES Wrecked in Port xxii. 242 That indefinite period known as a ‘blue moon’.
1876 M. E. BRADDON J. Haggard's Dau. xxiv. 246 A fruit pasty once in a blue moon.

Wow...that took awhile.

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Originally posted by Kalleh:
Wow...that took awhile.

Yes, it did. I ran across it when I was looking for "blue laws."