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Today, here in Brazil, I have observed (and is still observing) an interesting phenomenon.

The Moon is near to a big star in the sky, but this is normal. The interesting part is what's around them.

A huge circle in which the moon and the star is inside. Why does this circle appear? What is it?

(This isn't a cloud. It looks like more something like a light reflection of a lamp.)

Unfortunately, I hadn't a good camera to catch this. I would appreciate having an image posted for this.

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Not so normal, check some ephemerides table, I believe that the star was to be Jupiter. – arivero Dec 7 '11 at 2:14
up vote 12 down vote accepted

You might be describing a moon aura.

Moon Aura

It's caused by diffraction on tiny ice crystals in the atmosphere: When the moonlight hits the ice, you get interference effects that depend on the angle of the incident light. The idea is similar to powder diffraction in crystallography.

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Yes. It's was I'm looking for. – GarouDan Dec 7 '11 at 12:10
Thx about the explanation! – GarouDan Dec 7 '11 at 12:10
to expand a little, the crystals are flat but sit in the same orientation and diffract the light through 23 degrees (I think!) – Nic Dec 7 '11 at 12:30
Also known as "lunar halo"… . – jbatista Dec 10 '11 at 0:18
+1 for the great photo! :) – Count Zero Jan 6 '12 at 21:21

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