Tuesday, September 16, 2008
Ring around the moon (Lunar halo) and moondogs. This is not lens flare.
This is a photo of the moon taken in front of my house Sunday night. I noticed the ring around the moon while I was walking the dogs and it was impressive enough that I called the Teleospouse on my cell phone to tell her to go outside and look at it. When I got home I took this photo. This is a wide angle shot; the ring occupied a big chunk of sky (44 degrees across). I had to go half-way down the block to get enough open sky to get most of the ring without trees. I overexposed the moon to bring out the ring around it and the moondogs to the left and right.
Rings around the Sun or the Moon are caused by ice crystals high in the atmosphere. Ice crystals are hexagonal and the strongest internal reflections bend the incoming light rays approximately 22 degrees. If they are randomly oriented this gives you a ring around the Sun or Moon. There will be some ice crystals above the moon bending the light 22 degrees down and some below bending the light up. Left, right, a bit above or below, everywhere on the ring 22 degrees out from the Sun or Moon there will be some crystals oriented to bend the light into your eyes. Rings are easy to explain.
The dogs are harder to explain. They are visible in the photo as bright spots slightly above the moon just outside the 22 degree ring. They are always to the left and right of the Sun or Moon -- never above and below or at any other orientation. I am told that they are cause by pencil-shaped ice crystals that are falling and line up vertically so they only refract light to the left and right.
If you look between the Moon and the dogs you can see a faint arc that runs through the Moon and the dogs. That is, the dogs are actually part of another ring that runs through the moon all the way around the sky with its center directly overhead. That ring, parallel to the horizon, is called the Parhelic Circle if it runs through the Sun. I dunno what to call it with the Moon -- Parselenial Circle, maybe? I think the Par-whatever Circles are caused by reflection off the shiny sides of the vertically-aligned pencils of ice.
On exactly one occasion I have seen a full Parhelic Circle on a sunny winter day twenty years ago. I also remember a second full ring around the Sun outside the 22 degree ring with very bright sundogs appearing where this ring crossed the Parhelic Circle. Here's a drawing of the sky as I remember it.
"Z" marks the zenith -- the point immediately overhead.
The dotted green line shows the part of the rings/circles/etc. that are visible in my photo of the moondogs above. I think that the outer ring that I remember is caused by the same pencil-shaped ice crystals that cause the Parhelic Circle only not vertically aligned. The discussions I have seen of ice halos don't mention this outer circle but I remember it vividly from that twenty-year-ago winter day and it does explain the position of the sun/moon-dogs outside the 22 degree ring (which my moondog photo does show.)
The Parselenial Circle is interesting because it looks curved but, thanks to the brain-hurting properties of non-euclidean geometry, it is also parallel at all points to the horizon which appears straight. This means that the moondogs which appear higher than the Moon aren't really. If I had turned my camera to place one of the dogs in the center the Moon would have appeared higher than the dog. Here is an explanatory diagram which I offer to deepen the mystery.
For more information about ice crystal halos, see Les Crowley's Atmospheric Optics Site or Steve's Atmospheric Phenomena