What Causes the Ring Around Moon?
We can sometimes see a big ring of light around the moon. A ring around the moon is caused when the moon shines on thin clouds made of tiny ice crystals floating high in the sky. The ice crystals bend and reflect the moonlight to make a halo of light.
As light passes through these hexagon-shaped ice crystals, it is bent at a 22 degree angle, creating a halo 22 degrees in radius (or 44 degrees in diameter). A double halo, sometimes with spokes, may be seen on rare occasions when light reflects off water or ice.
The prism effect of light passing through these six-sided ice crystals separates the light into its various colors, resulting in a halo tinged with very pale rainbow colors with red on the inside and blue on the outside. The phenomenon of a lunar halo is similar to a rainbow produced by sunlight and rain falling between your eye and the sun.
Sometimes the ring is small. A small ring around the moon is caused by the reflection of moonlight off tiny water droplets in the air. When you see a ring around the moon, it often means that a storm is coming. Rings occasionally appear around the sun, too—and for exactly the same reasons.