A reminder for those who missed it: A blue moon will grace the skies tonight, although Space.com has a primer for those who might be mistakenly expecting a change in the moon's color. While dust particles in the atmosphere can in fact make the moon at times appear blueish, the term "blue moon," which has a somewhat confusing history, generally refers to the second full moon in a calendar month. And they occur more frequently than the phrase "once in a blue moon" might have you believe: every 2.7 years. The last one happened to fall on New Year's Eve 2009. (More blue moon stories.)