The phrase “once in a blue moon” traditionally refers to an impossible event or an event that rarely happens. The term “ blue moon” has its roots in the Old English word belewe or “betrayer” because an extra full moon before Lent was viewed as a “betrayer moon.” Scholars believe that belewe eventually morphed into the word blue. In the mid-twentieth century, the Farmer’s Almanac by Sky and Telescope magazine mistakenly defined a blue moon as a second full moon in a calendar month. The moon can actually appear blue if there are particles in the air that are larger than red light wavelength (.7 micron), which can occur during volcanic eruptions or forest fires.