Once in a blue moon!

For over 400 years the expression 'once in a blue moon' has been used to describe something that rarely happens.But where does this old saying come from? What is a blue moon?

The first recorded reference was in a proverb back in 1528

If they say the moon is blue
We must believe it is true

Literally of course, the moon never turns blue. There have been occasions when it has appeared to do so. Most notably in 1883 when Krakatoa , the Indonesian volcano exploded. The dust caused the sunsets to appear green and the moon a shade of blue for two years. And in north eastern America the moon turned blue in 1957 when smoke particles from forest fires in Canada caused a similar effect.

What does sometimes happen, is for a full moon to appear twice in one month. When this occurs, the term used to describe the event is ' blue moon'. It is is very rare and it only happens once in two and a half years - 41 times a century.

And it's about to happen! In London we will have a full moons - on June 1st and June 30th! In New York however it has already happened. May2nd and May 30th!