A full moon occurs every 29.5 days. It is the result of a 180-degree difference between the elliptical longitudes of the sun and the moon. In other words, the sun and the moon are opposite each other, with the Earth in the middle.
A full moon is created when the rays of the sun light up the surface of the moon. It is visible to observers on Earth because the Earth is in the middle of the sun and moon at that time.
If two full moons occur in one month, it is called a blue moon. Blue moons occur every 2.7 years.