"Once in a Blue Moon." Do we mean something unusual, rare and special?
The full moon of August 20-21 is also a kind of blue moon--a seasonal blue moon. Depending on the definition, a blue moon can be the second full moon in a month, or the third of four full moons in a season.
According to Space.com., the confusion began with an article by James Hugh Pruett in Sky & Telescope Magazine in 1946, citing a 1943 piece in Sky & Telescope by James Lafleur, who defined the term blue moon according to the 1937 Maine Farmer’s Almanac. Pruett interpreted the almanac (seasonal) definition to mean the second full moon in a calendar month.
Astronomer Deborah Byrd popularized Pruett's calendar definition on her Stardate program in January, 1980. Since then, this definition of a blue moon as the second full moon in a month has become widely accepted. In fact, it is even used as the definition of blue moon in Trivial Pursuit.
This is the most recent meaning, explains folklorist Philip Hiscock in his article in Sky & Telescope. The term blue moon has been in use for over 400 years.
It was originally used to describe something undoubtedly absurd--like saying the moon is made of green cheese. It took on more romantic connotations in the 16th century, as a symbol of impossible love. It still retains some of that moody blue feeling in popular songs.
But the moon can actually look blue--it's quite rare, but not impossible.
The eruption of Krakatoa on August 27, 1883 produced enormous ash clouds that caused spectacular-colored sunsets, lavender suns, and cobalt-blue moons!
More recently, the eruptions of Mount St. Helens in 1980 and Mount Pinatubo in 1991 also produced ash particles that acted as a color filter, and caused bluish-tinted moons.
I think we can agree that a blue moon is something special, however you define it. And lonely moon viewers, don't feel so blue, it's not just a trivial pursuit.
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