Blue moons are rare, that is why their name has become the root of a very used expression when we want to refer to something that happens far and in-between.

But what exactly is a blue moon? Is it a moon that is blue? In reality, the name has nothing to do with the moon's color. It is unclear how the name 'blue moon' came about, but calendar makers of he past used a little blue moon symbol to mark the odd full moon whenever it happened.

A blue moon occurs when there are 13 full moons in one year, instead of the usual 12. The moon cycle takes about 29.53 days to complete; almost a month. That is where the word month comes from and that's why there's one full moon once a month, at least on most years, anyway.

Because the lunar cycle does not reach a full 30 days, the full moon happens about a half a day earlier each month. So over time, this small difference eventually pushes the date of the full moon until there can be two full moons in one month.

When this happens, the second full moon in that month is called the blue moon. So, indeed, this comes about quite rarely, only about seven times in 19 years, which is equal to one blue moon every 2.7 years more or less.

Luck has it, that the next blue moon will happen this month, December 2009. The first full moon occurred on December the 2nd, so 29.53 days later, on December 31st and on New Year's Eve, there will be none other than a blue moon!