This is what a blue moon looks like. Maybe.

(Picture of yesterday’s full Moon from Astronomy Picture of the Day 1/9/12.)

Blue moons are popularly held to be rare events but, astronomically, they occur reasonably regularly.

A blue moon is nowadays often referred to as the second full Moon in a calendar month.

The link above (to Sky and Telescope) has the original definition as being the fourth full Moon in a season (winter, spring, summer or autumn) with seasons based on the tropical rather than calendar year.

According to that definition yesterday’s full Moon wasn’t blue but there will be one in August next year. See the graph below (taken from the Sky and Telescope article, where they take the blame for the confusion):-

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This happened today and being the second full moon to occur in a calendar month is not as infrequent an occurrence as the phrase might lead people to believe.