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?


4 Answers 4


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.

  • 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. Feb 28, 2012 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).

  • I have never heard of this meaning before. Please support your claim with evidence.
    – Noldorin
    Jan 16, 2011 at 22:08
  • This meaning is disputed, but is not unusual: en.wikipedia.org/wiki/… Jan 16, 2011 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. Jan 16, 2011 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, 2011 at 9:25
  • 1
    I think it's much more likely that the "two moons in the same month" meaning was inspired by the "once in a blue moon" idiom. Aug 30, 2023 at 16:54

In answer to the request for a synonymous expression, here is a quaint regionalism:

Preston Guild is a fair which is held every twenty years in, surprise surprise, Preston in Lancashire (there are many Prestons in England but the Lancashire one is the biggest by several orders of magnitude). In my lifetime, and I’ll be 67 in a few weeks, there have only been three Preston Guilds: in 1972, 1992 and 2012. I caught a little bit of the 2012 one just to be able to say I had.

So, “once every Preston Guild” is a Lancashire expression meaning “very seldom”, certainly more rarely than “once in a blue moon”. You probably won’t hear it anywhere else.

[R Claire Mitchell; Quora]


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

  • @ 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, 2011 at 23:38
  • @ Jimi Oke, this is true, however it does't even try to answer the question, which is "how this phrase came into being" and not "what is the meaning of the phrase". Aug 30, 2023 at 8:36

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