nineteenthly
Because of ice crystals in the atmosphere acting as prisms.
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The ring, or halo effect, is caused by the refraction of sunlight through ice crystals in the upper atmosphere, around 3-6 miles above the Earth. Usually the ring is seen along with cirrus clouds, the thin, wispy clouds seen at high altitude.
Chuck Norris
When a leprechaun hides their pot of gold on the moon, it causes a rainbow ring around the moon.
daniel g
Humidity in the air like fine water drops. That refracts the light like a prism or true rainbow.
Nyx
Ice crystals in Earth's atmosphere.
http://www.snowcrystals.com/halos/halos.html