Expert: Tom Whiting - 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: Hi Binh,
Yes, this is a very common event.  It's not an optical illusion.
It's called a corona around the moon caused by conditions in the atmosphere itself.  (not just at full moon, but anytime you see it).

First, realize that anytime you see something different in the sky like the stars twinkling, a ring or cloud around the moon or sun, different colors of stars, moon, and sun, different shapes of the sun or moon, then it's always caused by atmospheric events,  and not the object itself.
If the Earth didn't have an atmosphere, we would see the
stars as sharp, bright pin-points all the way down to the horizon....and we would see the moon and sun just as they
always are on a very clear day, all the time.

So what you describe is caused by thin clouds (or perhaps high levels of moisture or dust in the air) between your eyes
and the moon. Sometimes even the core of the Jet Stream right overhead can cause swelling and changes in color of the various objects in the sky.
So I would guess you either had a high, very thin cloudy haze or very high moisture and/or dust in the air.
Also watch for a 22 degree halo around a bright moon or sun,
caused by high cirrostratus clouds refracting the light and forming what we call a "halo" around the bright object.

What you saw is technically called a "corona" around the moon, and it's actually quite common.  And not only at full
moon, but anytime there is the slightest high altitude haze,
moisture, or dust in the air above you.
Hope all this helps,
Clear skies,
Tom Whiting
Erie, PA   USA

---------- 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
Hi again Binh,
Yes, that is the standard 22 degree halo around the bright moon  (it also happens around the sun too).
And it is caused, like I said in my first answer to you, by the presence of high, thin cirrostratus clouds. You can
see the thin clouds in the photo that you sent me.
Those are the only clouds that can cause a halo because they
are made up of ice crystals that refract the light at a
22 degree angle.  (There is also a much rarer halo formed at
45 degrees, much larger but dimmer, and that's why it is more
rare). Like I said, any change from "normal conditions" are
always atmospheric conditions (except for an eclipse, of course, which this is not).
I can even identify that constellation above the moon in that picture you sent...that bright star at the top of the picture is the bright zero magnitude star Capella in the constellation Auriga. And that's the star Beta Tauri to the far right of the moon.
So yes, it's just the "normal" 22 degree Halo around the
bright moon.  The halo is also a good future weather indicator since cirrostratus clouds usually precede (come
before) a storm system, so you can expect rain and rain clouds within 24 hours of seeing a halo around the moon.
(Not always, but most of the time.)
Hope all this helps, now that we have a positive identification of this strictly atmospheric phenomena...
Clear skies,
Tom Whiting
Erie, PA  USA


FOLLOW UP:
Directly in the center is not a cloud... That's
the moon, with a more thicker cloud right over it, making
the moon very diffuse and hazy looking.
Clear skies,
Tom Whiting