A blue moon occurs every three years when we experience two full moons in a single month.
Full moons typically occur every 29 days, according NASA’s National Space Science Data Center, but since most months are 30 to 31 days long, it is possible to have two full moons in a month every 2 1/2 to three years.
There was a full moon on July 2 and July 31 will see the second - two very average-colored full moons.
Blue-colored moons do exist, but they only occur “once in a blue moon.”
For a moon to take on a blueish hue, it is typically due to smoke or dust particles in the atmosphere such as during a volcanic eruption or forest fire, according to NASA Science.
In 1883, an Indonesian volcano named Krakatoa exploded and scientists likened the blast to a 100-megaton nuclear bomb. Plumes of smoke rose to the top of the Earth’s atmosphere and the moon turned blue.
Basically, if either a fire or an eruption produces ash or oily smoke containing a significant amount of 1-micron particles, the moon appears blue.
The most recent blue-hued moon was seen in Edinburgh, Scotland in September 1950, caused by ash from forest fires burning in Alberta, Canada.
The modern phrase associated with two full moons in a single month is attributed to a 1946 Sky & Telescope article entitled Once in a Blue Moon by James Hugh Pruett.
Before Pruett wrote his article, a “blue moon” referred to the third blue moon in a season, however, Pruett misinterpreted the information, according to Sky & Telescope, and the 1946 version of the magazine adopted his more modern definition.
A popular radio program, called StarDate, referenced the two full moons in a month definition on Jan. 31, 1980, and it experienced widespread adoption.
So, yes, the lunar experience on Friday is based on a misinterpretation of the original definition of a “blue moon,” neither of which have anything to do with the moon’s color.
Regardless, it’s a good excuse to get outside and view a somewhat-rare occurrence.
The last time we saw two full moons in month was in 2012 and we will not see the event again until 2018.
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Moon photo by local photographer Jimmy Sisk of Charleston, MO.
