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The Question

(Submitted January 02, 1997)

Last night I looked at the Moon. The Moon was full and there was a huge ring around it. I have seen this type of thing before and I don't know what it is. I call it a Moon ring. I was wondering if you could tell me what it is called and how it happens. I think it has something to do with clouds. I am not a scientist but I do have an interest in astronomy. Please write me back.

The Answer

While our expertise is in high-energy astrophysics, we can point you in the right direction for the answer to your question. You are correct in thinking that it is an atmospheric effect. The ring around the Moon is caused by the reflection of Moonlight (which of course is reflected sunlight) from ice crystals in the upper atmosphere. The shape of the ice crystals results in a focusing of the light into a ring. Since the ice crystals typically have the same shape, namely a hexagonal shape, the Moon ring is always the same size.

Here is the title and author of a good reference book to physical phenomenon seen in our atmosphere (geared for HS senior or college freshman level):

Minnaert, M., The Nature of Light and Color in the Open Air, Dover Publications, New York, 1954.

Sincerely,
Andy Ptak and Michael Arida
for the Ask an Astrophysicist team

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