Moon on a stick
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.