Generally, there are 12 full moons each year. Each of the four seasons has three months, with each month containing a full moon. Occasionally, one of the seasons has a fourth full moon.
The third full moon of a season with four full moons is called a blue moon. Half of the moon is illuminated at all times by the sun. As the angle of Earth, the moon and sun change, different portions of the illuminated side are visible. During the full moon, the moon is on the opposite side of Earth, and the entire sunlit portion of the moon can be seen.