What causes that bright ring to sometimes form around the moon at night?
That ring, usually called a halo or nimbus, or sometimes an icebow, can appear around the sun, moon, or other bright light source, and is a result of reflection and refraction of light in the atmosphere. Specifically, the light is usually interacting with ice crystals in cold cloud-tops. These halos were sometimes used in antiquity as rudimentary methods of weather forecasting.
