A bright "blue moon" will grace skies this Friday—the last one until July 2015. The last time sky-watchers were treated to a blue moon was New Year's Eve 2009.
If the skies are clear, revelers will get an eyeful of the second full moon of the month—commonly called a blue moon. A blue moon isn't actually blue—as commonly defined, the name reflects the relative rarity of two full moons in a month and is linked to the saying "once in a blue moon."
"There is nothing scientific about it, and it has no astronomical significance," Mark Hammergren, a staff astronomer at the Adler Planetarium in Chicago, Illinois, said in 2009.
"But I believe it does give us some insight into history and makes us think of how our calendar system has derived from motions of objects in the sky."
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Blue Moon Error
The popular definition of a blue moon isn't the only one—and it's one that's based on an editorial error, astronomers contend.
The widespread definition of the second full moon in a month stems from errors made in an astronomy magazine, when a writer misinterpreted how the term was used in the Maine Farmer's Almanac.
Later studies of almanacs published from 1819 to 1962 revealed that the term "blue moon" actually refers to the "extra" full moon that can occur in a year due to differences between the calendar year and the astronomical year.
(Related: "Leap Year—How the World Makes Up for Lost Time.")
Most years on average have 12 full moons, with 1 appearing each month.
That's because the lunar month—the time it takes the moon to cycle through its phases—corresponds closely to the calendar month.
But the calendar year is actually based on the solar cycle, or the time it takes Earth to make one trip around the sun. This means a year is not evenly divisible by lunar months, so every three years or so there are 13 full moons.
The farmer's almanac further divided the year into four seasons, with each season lasting three months. When a given season saw four full moons, the almanac dubbed the third moon as a blue moon.
Ultimately, a blue moon as defined by the calendar isn't that rare, added Hammergren. The term's significance instead lies in the way it links people to the motions of the cosmos.
"Just being able to recognize that we can have a full moon twice in a month and have [folklore] attached really highlights the fact that humans have been astronomers their entire existence," he said.
"True" Blue Moon
Before the editorial error, the term "blue moon" more often referred to the rare instances when the moon actually seemed to turn blue, as can happen under certain atmospheric conditions.
"After a forest fire or volcanic eruption, there may be enough particulate matter in the air so that the moon can take on a bluish tinge," Hammergren said.
For instance, a "true" blue moon occurred in 1950 after a large forest fire in Canada blew smoke across most of the Northern Hemisphere.
Another appeared in 1980 after the last major eruption of Mount St. Helens, which sent tons of ash into the upper atmosphere.
"There are plenty of wildfires burning in the hot, dry U.S.A. this month," according to NASA Science News. "If any of them produce smoke with an extra dose of micron-sized particles, the full moon might really turn blue."