"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.
Less typical are the halos that may be produced by different angles in the crystals. They can create halos with an angle of 46 degrees."