A composite, digitally edited photo of a full moon in 2007. Dino Abatzidis/Flickr

Sometimes it’s not just a figure of speech.

The term “blue moon” describes a second full moon occurring in one month and, of course, an unusually long span of time.

But if conditions are right, this Friday’s blue moon—the first since August 2012 and the last until January 2018—might subtly look blue in parts of the world. That’s because tiny particles from an unusually active fire season are flooding the atmosphere, and potentially scattering red light to give the moon the impression of a floating blueberry.

Tony Phillips from the stellar site Spaceweather explains more:

A truly-blue Moon usually requires a volcanic eruption. Back in 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded with the force of a 100-megaton nuclear bomb. Plumes of ash rose to the very top of Earth's atmosphere, and the Moon became an azure-colored disk….

Forest fires can do the same trick. A famous example is the giant muskeg fire of Sept. 1953 in Alberta, Canada. Clouds of smoke containing micron-sized oil droplets produced lavender suns and blue Moons all the way from North America to England. At this time of year, summer wildfires often produce smoke with an abundance of micron-sized particles—just the right size to turn the Moon truly blue.

In North America, the odds of catching an indigo moon are greatest where wildfires are roaring—the Western contiguous states, Western Canada, and Alaska. Like a blue moon itself, photos of this lunar phenomenon are hard to come by, but Spaceweather has collected a few examples—peep this partially eclipsed moon with a blue halo—and witness accounts, including a fantastic one from rural Pennsylvania during heavy wildfires in 1950:

"It was a cloudy day. Early in the afternoon, the sun disappeared and it became as dark as midnight. Lamps were lit and lanterns brought out. I went outside to walk around and check the livestock, and found that the chickens had all gone to roost, all the wild birds had gone to sleep, and the farm animals had gone into their normal nighttime sleeping places in barns and coops. We could see that the streetlights had come on in the nearby town. We had no Television at the time, but heard over the radio that there was a forest fire in Canada producing so much smoke that it had blacked out the sun.

"People were very frightened, and some thought the world was coming to an end. Others thought the Russians had done 'something' as the Cold War was in full flower then. Some thought the dreaded nuclear holocaust had come, but most people I knew thought it was a secret government smoke-screen experiment related to Cold War defense. The true believers in conspiracy never would accept the forest fire 'cover-up' story as they called it. Some of my classmates to this day still stick to the 'government cover-up' tale. Whatever one's thoughts at the time, it was a very frightening day. As the day waned, the smoke thinned a bit and the sun could be seen through the blackness as a faint blue orb, but it never did get light outside. After nightfall the moon which was full that night, was blue. I read later that blue moons were seen as far away as Europe on Sept. 26, 1950.

"There was a pervasive smoky smell in the air that didn't smell like wood smoke, but like peat smoke, a smell similar to smoldering wet hay. I remember being ill for several days afterward with a cold-like respiratory disorder including runny nose, stuffy head and irritated eyes."

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