Photographers were out in force last night to capture a rare blue moon. No, blue moons aren't actually blue (although there have been moons that were blue, most recently in 1950); rather, the term refers to the second full moon in a month, USA Today reports. But wait, you exclaim! Last night was August's first full moon, so something must be amiss, right? International Science Times clears things up: The first (and, in its telling, "most valid") definition of a blue moon is the third of four full moons that occur in a single season. A season would typically have only three, and this extra moon comes about every two to three years. The next will be in 2015.