Look to the sky tonight and you'll see something that only happens once in a blue moon.

That event is a, er, blue moon.

Except it's not going to be blue, it's just going to be the, er, moon.

A blue moon is called when two full moons appear in the same calendar month – there was a full moon on July 2.

Tonight's blue moon will be the first since 2012 and the last until 2018.

Very occasionally the moon really can be coloured blue - but you normally need to be near a volcanic eruption to see that.

A blue moon is usually silvery grey, like normal.

Blue moons are due to the difference between calendar and lunar months. While a calendar month lasts from 28 to 31 days, a lunar month - the time interval between two full moons - is always 29.53 days long.