California, US (BBN)-Be on the lookout for a blue moon this Friday evening -- just don't expect the moon itself to actually appear blue.
In recent decades, the phrase "blue moon" seems to have acquired a dual meaning. The phrase has commonly been used to refer to the second appearance of a full moon within a calendar month, reports the Discovery.
According to NASA, this variety of blue moon occurs roughly every two-and-a-half years.
Our last full moon appeared on July 1 -- thus, Friday's full moon qualifies as a blue moon.
This calendar-derived blue moon originated through a confusing string of editorial errors in Sky & Telescope magazine dating back to the 1940s.
In a more literal sense, the phrase has also been used to refer to a moon that appears to have a blueish tinge, a rarer phenomenon.
When a major volcanic eruption or large forest fire spews enough ash into the atmosphere, the resulting cloud can scatter red light under certain conditions, leaving the moon with a distinct blue appearance.
After the 1883 eruption of Krakatoa, blue moons were reportedly a common sight for years.
More recently, the 1991 eruption of Mount Pinatubo spurred blue moon sightings.
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