Blue moons happen due to the mismatch between the lunar calendar and solar calendars.

This means in one solar year with 12 full moons, there are around 11 extra days. Every two or three years, these days accumulate to result in two full moons in one calendar month.

Although the exact reason for naming them blue moons is obscure, the convention is part of an old English tradition of giving each full moon in the year a name.

The first moon of the year was the Old Moon, followed by the Wold Moon, Lenten Moon, and so on.

In recorded history, the first known reference to a blue moon comes from a proverb recorded in 1528:

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

It was later that the phrase 'once in a blue moon' was coined to describe an event that was unlikely to happen.

Although blue moons happen every few years, rare atmospheric conditions can cause the moon to actually appear blue,

Particles of ash as emitted by the Krakatoa volcano in the 19th century can scatter the light reflected off the moon -- depending on the size of the particles, the light can take on a variety of hues.

This year's blue moon does have the distinction of being a rarity -- these bonus full moons only coincide with the New Year once every two decades.

Such a festive occasion deserves an appropriate drink -- how about some Blue Moon, a delicious Belgian-style wheat beer brewed in the USA?