Rare 'blue moon' on the horizon tonight

'Once in a blue moon' is not just a phrase that's used for occasional events.

It is also a unusual astronomical phenomenon that occurs when a full moon occurs twice in a calendar month.

The lunar event will take place tonight and will be the first blue moon to occur since 2012.

This image shows an image of the last blue moon, which occurred on August 31, 2012.

 Blue moons are not as rare as other celestial events, but there won’t be another for three years, until January 31, 2018.

Anyone hoping to see a dazzling sapphire orb crossing the sky will be disappointed, however, because a blue moon is usually a silvery grey.

Tonight's lunar phase fits the calendar definition of ‘blue’ because there was another full moon on July 2.

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.