What exactly is a Blue Moon, anyway?

Tonight you'll have a chance to witness a rare "Blue Moon" with your very own eyes. But what does that mean, exactly? You've probably heard the term "once in a Blue Moon" used to refer to rare events — but just how rare is a Blue Moon, really? And what's so special about them in the first place?

Top image by Sabrina Campagna

Here's what you need to know when it comes to Blue Moons. One: they aren't blue (or rather, no more or less blue than any other full moon). And two: there's some disagreement over the definition of what a "Blue Moon" really is. For instance: if you're a purist, tonight's Blue Moon is the real deal – not like last year's, which got by thanks to a bygone technicality. But we'll get to that in a little bit.

Here's the thing. Generally speaking, seasons and calendar months tend to match up pretty well, chronologically, with the phases of the moon. The year is divided up into four seasons, with three full moons per season; or, alternatively, twelve months, with one full moon per month.

But nature has a knack for falling out of sync with humanity's artificially imposed timekeeping methods. As a result, sometimes one of the seasons will cram in four full moons instead of three, giving rise to 13 full moons in a year instead of the usual 12. This happens, on average, about once every 2.7 years.

Traditionally, it is the third full moon in a season with four that is referred to as a "Blue" moon. That's what tonight's moon is: the third full-faced orb of a summer season with four full moons, in a year with 13 total.

But by this traditional definition, there technically was no Blue Moon on August 31, 2012. Last year's summer season — i.e. the time between the summer solstice (June 21) and the autumnal equinox (September 22) contained the standard three full moons, as did the rest of 2012's seasons. So why did everyone call August 31 2012's moon "Blue"?

Because in 1946, the definition of Blue Moon was accidentally expanded to include instances where two full moons occur in the span of a single calendar month. In cases such as these, the second full Moon of the month (not the third full Moon of a four full moon season) is referred to as a Blue Moon. The newer definition makes it possible for a Blue Moon to occur even during a year with a normal seasonal distribution of full Moons.

Interestingly, the revised definition also makes it possible for two Blue Moons to occur in a single calendar year. The last time this happened was in 1999, when there were two full moons in January, two full moons in March, and no full moons whatsoever in February. It also gives rise to weird geography- and timezone-dependent technicalities; last year, for instance, time zones east of UTC+09 experienced two full Moons not in the month of August, but September (once on September 1, and again on September 30th), which means that their Blue Moon will actually occur at the end of September, rather than the end of August. See how this gets confusing?

So which definition is better? That depends on who you ask. Personally, I like EarthSky's stance on the issue:

In recent years, a controversy has raged — mainly among purists — about which Blue Moon definition is better. The idea of a Blue Moon as the third of four in a season may be older than the idea of a Blue Moon as the second full moon in a month. Is it better? Is one definition right and the other wrong? After all, this is folklore. So the folk get to decide, and, in the 21st century, both sorts of full moons have been called Blue.

As if things weren't convoluted enough, tonight's full moon actually goes by a number of other names, too – all of them deeply rooted in tradition. It's also the Full Sturgeon Moon, the Green Corn Moon, the Grain Moon and the Full Red Moon.

A version of this post appeared last year, when we explained the Ts and Cs of last year's twice-in-a-month Blue Moon