Originally Posted By: JCS
the reference is not so much a 'long period of time', but rather rarity.


I'm with you there, JCS. The phrase "once in a blue moon" does seem to indicate the period of time from one blue moon to the next which would lead to the shift in meaning. In present usage it refers to the second full moon in a month, but originally it was the third full moon in a quarter that had four full moons. In this case a quarter is defined as the period from an equinox to the following solstice or from a solstice to the following equinox.