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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The Online Etymology Dictionary gives the following origin:
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.
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:
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).
Usually, blue moon appears in the phrase, once in a blue moon. An alternative is once in a while.