Another common sight in daytime skies is the halo, sometimes called "a ring around the Sun." Haloes are caused by the refraction of sunlight through hexagonal ice crystals in cirrus clouds.
As the sunlight passes through one of the six sides of the tumbling crystal, it is bent through a 22 degree angle, producing a halo with a radius of 22 degrees. Occasionally, when sunlight passes through the ends of hexagonal prisms, or through cubic ice crystals, it is bent through a greater angle, producing a second, fainter, 46 degree halo.
When the Sun is low in the sky, and when high cirrus clouds are present, look for two bright spots of sunlight on either side of the Sun. These spots, which will be tinged red on the inside and blue on the outside, are commonly called sun dogs or mock suns. The technical name is parhelia.
Sun dogs are produced by sunlight refracting through hexagonal ice crystals that are oriented with their bases parallel to the horizon. Sun dogs sometimes touch the outside edge of a 22 degree halo.
The safest way to observe sun dogs and haloes is to block your view of the Sun with a nearby structure such as a tree limb, chimney, or flagpole.
Yet another ice-related phenomenon, sun pillars, can he seen around sunrise or sunset. When low-angle sunlight reflects office crystals in cirrus clouds, a spike—or pillar—of light may be projected up from the just-set or predawn Sun.
At night, moonlight can create moon haloes, moon dogs or mock moons, and moon pillars, but these effects fainter and less common than the Sun phenomena.