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Have you ever noticed that there can sometimes be a ring around the Moon. In fact, if you look closely, it even looks a bit like a rainbow circling around the Moon. What’s going on with the Moon and the atmosphere to cause this Moon ring?
The ring around the Moon is caused by the refraction of Moonlight (which is really sunlight) through ice crystals suspended in the upper atmosphere. These ice crystals always have a hexagonal shape, so they always refract light at the same angle. Of course, the atmosphere is filled with crystals, all refracting moonlight off in different directions. But at any moment, a huge number happen to be in just the right position to be refracting light towards your eyes. You just aren’t in a position to see all the other refracted light. This is what causes the Moon ring.
We see a rainbow because the different colors are refracted at slightly different angles. This is exactly what happens with a rainbow. The moonlight is broken up into its separate colors because they all refract at different angles, and so you see the colors split up like a rainbow.
A moon ring or moon halo will always be roughly the same size – 22-degrees across the sky. The full moon is half a degree, so the ring around the Moon will encompass an area 44 times bigger than the Moon. This is because the crystals bend the light at a 22-degree angle from their original path.
This is the same principle of why we can sometimes see a ring around the Sun.
This site has a great description of why we see the 22-degree halo.
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?