Curious Questions: What actually is a Blue Moon? And is it really blue?

It's a phrase which gets bandied around all the time, but what is a 'Blue Moon' ? Does it actually look any bluer than a normal moon? And do they really only come about as rarely as the phrase implies?

On Saturday May 18, a Blue Moon will be witnessed around the world. Not that you’d know it to see it. Sadly, for those of a romantic disposition, the astronomical definition of a Blue Moon  is absolutely nothing to do with its colour.

In other words, Blue Moons are no bluer than any other moon — and sometimes they’re even less so since, on occasion, the moon can appear blue. The moon apparently appeared blue for some time in the wake of Krakatoa’s explosion in 1883, for example, while smaller eruptions (and even forest fires) have been known to cause the moon to appear blue due to due to particles in the night sky playing with the moonlight. But these are blue moons, not Blue Moons.

Full moon over Resurrection Bay, Seward, Alaska. Only a volcanic eruption, atmospheric disruption or a digital corruption can make this happen.

According to the esteemed experts at NASA, a Blue Moon is not a natural phenomenon but rather an artificial one: it’s purely down to the arbitrary way in which we organise and think about our calendar. And so arbitrary is the definition, in fact, that there are actually two separate and contradictory ways to determine what a Blue Moon actually is: seasonal and monthly.

A monthly Blue Moon is the name for the second full moon within a calendar month; that is to say one of those occasions when the full moon appears in the first two or three days of the month, to that the second can be seen on the 30th or 31st of the same month. Thus, February can never have a Full Moon.

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A seasonal Blue Moon is similar, but instead refers to those rare occasions — apparently once every two and a half years — in which a single season witnesses four full moons, instead of the usual three. Confusingly, it’s not the fourth of those four which gets the ‘Blue’ tag, but instead the third.

That’s the Blue Moon which takes place this Saturday. The first full moon of Spring 2019 arrived on 20 March (the first day of Spring this year), followed by subsequent full moons on April 19, May 18 and June 17 — thus, May 18 is the Blue Moon.

Confused? You’re not alone. The existence of the monthly Blue Moon, according to, dates to a magazine article of 1946 which got similarly muddled, and wrongly defined it as the second Blue Moon in a month. That piece (in the Maine Farmer’s Almanac, of all places) appears to have been picked up and repeated again and again, to the point that it became so widely accepted that it even became a Trivial Pursuit question. Thus it is that we now have two entirely separate definitions of Blue Moon.

Even more confusingly, the phrase ‘once in a Blue Moon’ — which dates back to the 16th century — seems to apply to neither version. It is instead exactly what it sounds: a fanciful way of suggesting that something is so rare that you might as well forget about it ever happening, a somewhat softened version of the proverbial flying pigs.

So there you have it: Blue Moons are something of a linguistic and astronomical mess.

Still, there are lots of other lovely things to see in the night sky at this time of year as this handy Forest Holidays calendar itemises. If even that doesn’t soften the blow then there’s always Photoshop to put the world to rights:

And to think they used to say that the camera never lies…