Moon halos are caused by tiny ice crystals that have gathered twenty thousand feet above the ground, as thin, wispy clouds. These clouds are so thin, you might not notice them at night, if it weren't for their effect on the moonlight. Incoming light rays from the moon are bent--or "diffracted"--by these ice crystals at an angle of twenty-two degrees. This means that in addition to the direct moonlight, you will also see diffracted moonlight in a circle twenty-two degrees away from the moon. This is about the distance of your fist, held at arm's length. Like a rainbow, this halo can even be slightly colored; red on the inside, and blue on the outside.