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Blue Moon

A blue moon is seen from Skopje, Maceodonia, on Aug. 31, 2012.


If skies are clear, tonight and tomorrow you will see a blue moon, also known as ‘full sturgeon moon,’ ‘green corn moon’ and ‘grain moon.’ Here’s an explainer.

When you look up to the skies Tuesday and Wednesday nights, if the skies are clear, you will be looking at a rare, lovely sight — a blue moon.

The nights of Aug. 20- 21 will be a blue moon. Mostly known as the second (or third) full moon in a calendar month, it turns out there are more than a few kinds of blue moons out there. The full moon of Aug. 20-21, 2013, will be considered a seasonal blue moon — the first in nearly three years. Normally, a season has three months, and hence three full moons, but the eccentricities of our calendar mean that once every three years, a season has an extra full moon. The last seasonal blue moon occurred on Nov. 21, 2010, and won’t happen again until May 21, 2016.

But there’s another definition of blue moon: sometimes the calendar falls so that two full moons occur within a single month; the second full moon in a month is the blue moon. If we go by that rule, the next blue moon will occur in 2015, when full moons will occur on July 2 and 23.

The term blue moon may have its origins in the Old English “belewe,” which means “to betray,” writes the Farmers’ Almanac. Farmers took the cycles of the moon very seriously when it came to planting and harvesting their crops. Religious holidays were also built around them, and so an extra full moon would seem like the sky was up to some mischief.

But will the moon on Aug. 20 actually be blue? Not unless you’re near a volcanic eruption, or a certain kind of wildfire — when the moon looks blue, it’s because something has thrown particles of fine-grained smoke and dust into the atmosphere so that only blue light is filtered through it.

People reported blue moons in 1980 after the eruptions of Mount St. Helens in 1980, the El Chichón volcano in Mexico in 1983 and Mount Pinatubo in 1991. Historical records suggest that  the eruption of Krakatoa in 1883 caused the moon to appear blue for nearly two years.

According to Space.com, the history of the blue moon is full of a bit of misunderstanding.  

“In 1946, James Hugh Pruett (1886-1955), an amateur astronomer, was writing in Sky & Telescope magazine. Pruett ‘made an incorrect assumption about how the term had been used in the Maine Farmers’ Almanac — which consistently used ‘blue moon’ to mean to the third full moon in a season that contained four of them (rather than the usual three),’ Sky & Telescope editors later explained. The error had been repeated in a syndicated radio program in 1980,” according to Space.com.

The Old Farmer’s Almanac calls the August blue moon, “a full sturgeon moon.” According the almanac, “Some Native American tribes knew that the sturgeon of the Great Lakes and Lake Champlain were most readily caught during this full moon. Others called it the Green Corn Moon or the Grain Moon.”

Call it what you will, but definitely enjoy the skies tonight and tomorrow.



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