If anything unusual happens to you Friday, July 31, 2015, you'll be right to say it was "once in a blue moon." The July 31 appearance of the month's second full moon will be the first such occurrence in the Americas since August 2012. Every month has a full moon, but because the lunar cycle and the calendar year aren't perfectly synched, about every three years we wind up with two in the same calendar month. But Earth's satellite will most likely not appear blue at all. Typically, when a moon does take on a bluish hue, it is because of smoke or dust particles in the atmosphere,such as during a cataclysmic volcanic eruption. The U.S. Naval Observatory in Washington states that when a season has four full moons,the third one is called a blue moon. This lunar event will not be seen again until January 2018.
