THE first thing you'll notice about a blue moon is its colour - it's not blue.

It's not that the moon can't be blue. Clouds of ash from volcanic eruptions can give our closest celestial body a blue tint, just as smoke from bushfires can make it look red or orange.

However, regardless of what common sense might suggest, the moon tinted blue by volcanic ash isn't a blue moon, it's just the regular moon that happens to look a bit blue at the moment. 

If that seems a little logic-bending, it's not really any more so than an actual blue moon. It's not even a real astronomical phenomenon. Rather, like New Year's Day, it's more about our calendar than the moon itself.

What we call a blue moon these days happens when there are four full moons instead of three within the three months of a season - and just in case that started to make any kind of sense, it's the second full moon, not the fourth, that gets the fancy title.

To find a reason (and I use that word loosely) for that, you have to go  back more than a century and across the Pacific to the state of Maine in the United States, where the Maine Farmers' Almanac started the practise by naming the third of four full moons in a season a "blue moon".

According to Wikipedia, the transition from the third to second full moon is generally believed to have happened about 70 years ago, resulting from an error in a magazine article that was picked up and repeated on a popular radio program before it could be corrected.

Presumably the almanac's publishers chose the term because it was already in the language as referring to something that happened rarely. The term "blue moon" is believed to date back to the 16th Century, when it was used to describe something impossible or ridiculous, in much the way we would today refer to flying pigs.

In a 2012 article published by US newscaster NBC, Newfoundland Memorial University professor Phillip Hiscock is quoted saying the term morphed over the centuries from "a kind of metaphor for absurdity or impossibility" to meaning a long time and from there to something that happened rarely.

So, the blue moon is at least rare in terms of how it fits within our calendar - the next one isn't due until May 22 next year.

And while, barring a little Photoshop work or a handy volcanic eruption, it may be utterly indistinguishable from any other full moon, it does at least provide us with a good excuse to display some very cool photos.