Many are aware of the expression ‘once in a blue moon’, that is, a rare occassion. There are two interpretations of a ‘blue moon’.

The first states there must be two full moons in the same calendar month and the second stipulates four full moons in the same quarter.

Such a rare occassion will happen next year when there will be two months with a blue moon in the first quarter.  January will have a full moon on the 2nd and the 31st. There will be no full moon in February but there will once again be a full moon on March 2 and 31.

The phrase has nothing to do with the actual colour of the moon, although a literal ‘blue moon’ (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions, say, if volcanic eruptions or fires emit particles in the atmosphere of just the right size to preferentially scatter red light.

It has been suggested that the term ‘blue moon’ arose from folk etymology, the ‘blue’ replacing the no-longer-understood belewe, to betray. The original meaning would then have been ‘betrayer moon’.