Once in a blue moon...

A PASSENGER jet preparing to land in Cape Town, South Africa, beneath a 'blue moon' in partial eclipse on Thursday.

For the second time in nearly two decades, December ended with earth illuminated by a 'blue moon', the name given to the second full moon appearing in a single month. The moon was also at its maximum on Dec 2.

The double full moon phenomenon happens on average every 2.7 years, with the most recent in May 2007, and the next set for August 2012.

Why 'blue moon'? There seems to be no particular reason. One theory says the moon can take on a blue tint if there has been a recent volcanic eruption. The term is also a misnomer that can be traced back to an editorial blooper 65 years ago in the Sky and Telescope magazine.

But the original meaning of 'blue moon' was the third full moon in a season with four instead of the usual three.