First time I have ever witnessed these but they are apparently common

According to the net ........
The ring around the Moon is caused by the refraction of Moonlight (which of course is reflected sunlight)
from ice crystals in the upper atmosphere. The shape of the ice crystals results in a focusing of the light
into a ring. Since the ice crystals typically have the same shape, namely a hexagonal shape, the Moon
ring is almost always the same size.


McGarvaPhotography