On Aug. 31 the much-heralded blue moon appeared, 30 days after the month's first full moon. Our phrase "once in a blue moon" is meant to evoke the rarity of this phenomenon rather than the big rock's actual color. Blue moons make an appearance about every 2.7 years: Our last look at one came back in December 2009, on New Year's Eve. We won't see another lunar double feature like this until 2015.
- full moon