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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
Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html
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