Blue moon

[siggy]

Why do we say 'once in a blue moon'?

[Jenny]

The moon sometimes appears blue because of freak atmospheric conditions (typically caused by volcanic dust or atmospheric pollution) but only on very rare occasions. The popular use of the phrase to describe the second full moon in a single calendar month is a little misleading. Since there are 13 full moons in a year and only 12 months, it averages out to one a year - fairly rare but not as uncommon as is usually implied by the phrase.

[sgs]

Click here for an Answerbank feature on blue moons: article 1737

[princejohn]

A blue moon is when a full moon occurs twice in the same month. The second occurence is the blue moon