Although full moons occur roughly every 29.5 days, blue moons are much more rare. According to Space.com, a blue moon happens on average once every 2.7 years. A blue moon has nothing to do with the color of this celestial orb, but rather it refers to when there are four full moons in a season of three months instead of three full moons. The fourth moon is referred to as the blue moon. Occasionally, two full moons appear during the same month. This will happen this month. Jan. 1 featured the full Wolf moon. At the end of the month, on Jan. 31, sky-gazers can view the second full moon, or the blue moon.