Asked in AstronomyThe MoonElvis Presley
What causes a blue moon?
June 18, 2008 11:28PM
The term "blue moon" refers to the extra moon that falls in a month that has two full moons. Each 2.7 years there is a month that has two moons in it and this rare event (actually the first moon in this month) is referred to as a "blue moon." The phrase, "once in a blue moon" means something is rare.