Why can we sometimes see a ring around the moon?
High up in some parts of the atmosphere there are tiny ice crystals. When the conditions are right, these ice crystals form a ring around the light coming from the moon. In these special conditions, the moon’s light can be redirected by millions of ice crystals. The light rays travel in a straight line from the moon, but then they bend as they pass through a crystal and come out in another direction. Other light rays are reflected from flat surfaces on the crystals.
If you are standing in the right place, light that passes through the ice crystals will look like a bright thin circle with the moon in the middle.