some phenomenon?

[chitrangda]

Yesterday as I was walking during night, though it was cloudy,I saw that a perfect circle of clear sky formed around the moon.I was wondering that if that is some physical phenomenon or just coincidence?

[iNow]

Was it a moondog, like this?

 

 

[chitrangda]

the picture is not very clear though,but something like this.

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[iNow]

Well, regardless if it was a moondog or some other phenomenon, it's caused by the way the light from the moon refracts off ice crystals in the upper atmosphere.

 

Here's a link or two to explore. I'm not able to describe it in too much more depth than I already have. Maybe someone else can step in.

 

 

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970102b.html

http://amos.indiana.edu/library/scripts/moonring.html

http://home.hiwaay.net/~krcool/Astro/moon/moonring/

[Martin]

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970102b.html

http://amos.indiana.edu/library/scripts/moonring.html

http://home.hiwaay.net/~krcool/Astro/moon/moonring/

 

Those are helpful links. Wikipedia is not too bad on this either:

http://en.wikipedia.org/wiki/22°_halo

 

I guess lots of us have played with 60 degree (equilateral triangle) prisms made of glass. The angle such a thing bends a lightray depends on the transparent material (how much it slows down light passing thru it).

 

If you have a 60 degree prism made of clear ICE then it bends the lightray at least 22 degrees. Because water ice is not as slowy-downy as plastic or glass, a glass prism with the same geometry would bend a lightray more than 22 degrees, at least 37 degrees I think.

 

Chitranga, there is some neat simple physics involved here that if you don't know you could continue asking questions about. Different people might answer. I didn't tell you how a prism bends a lightray. But if you already understand that then no need to ask further.

Edited April 13, 2009 by Martin

[samtheflash82]

I've seen that before. I always thought it was something to do with my glasses. Whaddya know...

[Martin]

here is a neat little calculator

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/prism.html#c2

 

put in 60 degrees for sigma

 

put in 1.3 for the index for ice

 

put in 1.0 for air (negligible slowy-down-ness)

 

click on the little delta in the "active formula" and it will tell you delta.

 

It says delta should be 21 degrees, close enough to 22.

 

If you can't make this calculator work, ask for help. I checked that it works.