Azerbaijan National Academy of Sciences

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. 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.