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Was just wondering how this phrase came into being? Was it inspired from some natural or astronomical observation? or is it the result of poetic imagination?

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Good question. I thought about this too before, but didn't dwell on it very long... – Noldorin Jan 15 '11 at 20:59

The Online Etymology Dictionary gives the following origin:

blue moon
1821 as a specific term in the sense "very rarely," perhaps suggesting something that, in fact, never happens (cf. at the Greek calends); suggested earliest in this couplet from 1528:

Yf they say the mone is blewe,
We must beleve that it is true.

Though this might refer to calendrical calculations by the Church, so that the general sense of the term and the specific one (commonly misinterpreted as "second full moon in a calendar month," but actually a quarterly calculation) are difficult to disentangle. In either case, the sense of blue is obscure. Literal blue moons do occasionally occur under extreme atmospheric conditions.

So as you suggest, its origin could be either poetic or astronomical. Perhaps the latter inspired the former? A lot older than I would have guessed, in any case!

As for alternatives, the most obvious one is simply "[very] rarely". A more interesting one is "black swan", which can be used to describe a very infrequent event.

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Black swan (at least originally) means 'something believed impossible but proved true'. The phrase was commonly used in early philosophy books as an example of contingent impossibility (as a matter of fact, all swans are white, but there is no necessity for it), until the discovery of Australia which does in fact contain black swans. – TimLymington Feb 28 '12 at 11:24

A Blue moon is the second full moon in the same month.

As a full moon appears every 28 days the chances of a full moon happening twice in the same month are very rare.

This brings us to the common expression:

Once in a blue moon.

Meaning it happens very rarely.

Though Dr Hiscock in one of his books noted this is a modern interpretation of the term "Blue Moon"; Historically it has probably been related to the moon changing colour because of atmospheric conditions (which was even more rare).

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I have never heard of this meaning before. Please support your claim with evidence. – Noldorin Jan 16 '11 at 22:08
This meaning is disputed, but is not unusual: en.wikipedia.org/wiki/… – Dour High Arch Jan 16 '11 at 22:23
Looks like it comes from an incorrect interpretation of an earlier text. But I stand by what I have written (even if this is the modern interpretation). The phrase still means very rarely. – Loki Astari Jan 16 '11 at 23:27
I don't know if this is the real origin but it is certainly one of the meanings around. – neil May 13 '11 at 9:25

Usually, blue moon appears in the phrase, once in a blue moon. An alternative is once in a while.

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@ Jimi Oke -- 'once in a blue moon' means something more like 'very rarely': 'once in a while' doesn't carry the same sense of a very rare event. – AAT Jan 18 '11 at 23:38

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