Expert: Philip Stahl - 8/30/2007

Question
QUESTION: Hi , today , the full-moon day , I am seeing a strange phenomenon ,
the moon is full and cloud is surrounding the moon , forming a "ring cloud" , like there is something from the moon that penetrate the cloud to form a PERFECT circle .

Scale is that , if the circle that the cloud forms is a dish , then , the moon would be the cap of bottle in the centre .

I really saw this before , it only lasts for few hours , in full moon day , I am viewing it in VietNam ,SaiGon city south VietNam .
there are a few things I need to clarify , please,
1.I know what I am seeing , I am sure its not optical illusion
2.I checked and actually , there are many people who confirm it for me , certainly , who are in same location to me.

Please, sir ,I am eager to know why , please email me if you need some more infomation.
Thank you very much , hope hearing from you soon !
Binh.


ANSWER: Hello,

You do not state the date or time of the observation, but given we are within 48 hrs. of a major lunar eclipse - my bet is it is that which you saw. (It should have been visible from Viet Nam, as it was from Australia and many points in the Pacific).

The perfect "circle" you saw would have been the Earth or the shape of the Earth's circular shadow edge as it crossed the Moon. In all likelihood it also would have appeared ochre or reddish in color.

If my guess is not correct, and you know about the eclipse, then it is likely you saw a Moon "halo" formed by ice crystals in the high atmosphere. This sometimes occurs and the conditions must be ideal for it.

Hope this helps.

---------- FOLLOW-UP ----------

QUESTION: Hi, thank you for your help , but there was not a lunar eclipse and there is a picture like what I saw , except there was no cloud in the middle of the ring .
http://www.astropix.com/IMAGES/SHOW_DIG/009.JPG
( this pic shows there is cloud in the middle of the ring )
I am sure it was not optical phenomenon , just plainly , cloud crowded perfectly around the moon . The sky , cloud I think was low , seemed not very far from me , there was no wind .

Do you have any ideas ?

Answer
Hello,

I have to stand by what I said previously, that the effect is due to clouds - composed of ice crystals-  giving rise to a halo or ring.

Indeed, the photo you provided (link) is very similar to one appearing in Plate ix in the book 'The Nature of Light and Color in the Open Air' by M. Minnaert. The explanation, again, is the one I gave earlier.

Hope this info satisfies your curiosity!