blue moon 2010

Technically, a “blue moon” is defined as the second full moon in a single calendar month. Because this occurs only once every two or three years, the phrase “once in a blue moon,” meaning a rare event, was coined. But 64 years ago tonight, on September 24, 1950, residents of eastern Canada and the United States witnessed an even rarer event: the rise of a full moon that was literally colored blue.

The cause of the “blue moon” was an environmental catastrophe. The day previously, a few long-smoldering low-level fires in a muskeg-covered area of Alberta suddenly blew up into a major fire, most likely due to high winds. Muskeg is a form of ground cover that’s basically synonymous with peat moss: a springy carpet of decomposed oily plants that accumulate over time in a place where trees don’t normally grow. The muskeg fires got out of control and began pumping huge clouds of smoke into the atmosphere. The unusual wind patterns of September 1950 carried the particulates east and south quite quickly, and as they hung in the lower atmosphere, shining through them the full moon appeared blue.

It is so rare for this to happen because the only kind of atmospheric particulates that can cause a “blue moon” are an extremely specific size and configuration. Only particles slightly wider than the wavelength of red light, which is 0.7 microns, can absorb that frequency; as it happens, the particulates from burning muskeg are exactly 1 micron in length. Hence, red and yellow wavelengths were absorbed as the moon reflected them to observers on the ground, and they saw only the blue ones. The clouds of muskeg particles crossed the Atlantic the next day, and on September 25 the residents of Britain witnessed their own “blue moon.”

This condition caught my eye because, as it turns out, one other thing can cause atmospheric particulates like this: volcanic eruptions. Volcanic effects on climate are the subject of my historical research. There are reports of “blue moons” being seen after the Krakatoa eruption of August 1883, and evidently a few after Pinatubo in 1991. I haven’t yet found a “blue moon” report from the Cold Decade (1810-1820), but that doesn’t mean there weren’t some.

As soon as you clicked on this article you knew I was going to include Creedence Clearwater Revival’s “Bad Moon Rising,” so here it is to send you on your way.

The photo of the moon is actually a real “blue moon” (meaning the second full moon in a month), photographed in November 2010. The colors are a result of the photography. The photo is by Wikimedia Commons user Astroval1 and is used under Creative Commons 3.0 (Attribution) license.