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>>> I think that next time you could try a quick sequence of shot at different exposures. Then combine the picture using HR (high dynami
Hi Paul & All, July 28, 2010
I still have more questions than answers and even the idea that the
altitude of moon dogs is usually greater than moon altitude remains
conjectural; something to watch for.
However, the angle 22o keeps coming up so any concept of halo or dog
generation must be consistent with an angle of reflection/refraction of 22o.
This seems to make at least qualitative sense if flat crystals are assumed;
i.e. two flat parallel surfaces (assuming reflection increases as the angle
of incidence increases).
PLATES
With a plate-like ice crystal [index of refraction 1.31] vertical, about
parallel to the eye-moon axis and 22o from this axis, the angle of incidence
would be 68o (implying substantial reflection) and the angle of refraction
would be 45o (implying substantial and perhaps repetitive internal
reflection) with about half of the light that entered the crystal exiting at
22o to supplement the reflected light.
At angles greater than 22o from the axis there would be less reflection
from the air/ice interface and an angle of refraction of less than 45o
leading to less internal reflection (i.e. more escape of light away from the
observer).
At angles less than 22o from the axis, more of the incident light would
be reflected but area of crystal intercepted by light will decrease as
sin(90-angle of incidence)
and e.g. moving in from 22o to 15o will decrease intercepted crystal area
from 0.37 to 0.25. And the angle of reflection would exceed 45o leading to
more internal reflection and less escape of light from the crystal.
RODS
With rods, presumably hexagonal in cross-section, I can't see how 22o
fits unless a crystal alignment parallel to the eye-moon axis is assumed as
well as a hexagonal face normal to the moon-crystal-eye plane.
On the macro scale I think the following framework should be valid. With
the 22o angle represented by a cone at the observer's eye with apex of 44o
and axis pointing at the moon, the zone of additional light will be
somewhere along the intersection of a hemispheric shell and the cone (the
observer is in the middle of a flat disk and under the hemisphere).
This zone of intersection will be a nearly circular halo when the moon
is relatively high in the sky and a roughly football-shaped when low [i.e.
truncated on the lower side and squashed down on top].
On an unrelated matter, in the days before weather forecasts, the number
of stars inside of this 'ring-around-the-moon' was used to estimate the
number of days before a storm... no stars; storm soon, 3 stars; storm in 3
days etc.
Yt, Dave Webster, Kentville
----- Original Message -----
From: "Paul S. Boyer" <psboyer@eastlink.ca>
To: <naturens@chebucto.ns.ca>
Sent: Tuesday, July 27, 2010 9:49 AM
Subject: Re: [NatureNS] Pic of Moondogs over Scots Bay
>I don't think that the difference in temperature (day to night) is an
>important factor here, because the temperature where the proper form of ice
>crystal occurs would be the same in any case, in order to get the crystals
>of the proper form. The crystals responsible (and most effective) for many
>of these phenomena are elongate prisms, They are small (but up to a mm
>long) rod-like things, and so they fall very slowly because they are small
>and not as dense as a raindrop because of air inclusions. As they fall, the
>prisms line up (at least statistically) in a roughly vertical position,
>like bombs falling out of an airplane (if you think of the old WWII
>documentaries).
>
> Critical must be the orientation of the crystals relative to the plane
> defined by the light source (sun or moon), the spot in the sky you are
> viewing, and your eye. The best, strongest effect would then be when
> these little prisms would be aligned perpendicular to that plane.
>
> Ordinarily, one would expect the dogs to form off to the sides of the
> light source (sun or moon) because the prisms are falling down toward the
> center of the Earth, and therefore works best for a plane perpendicular to
> the downward-direction. This is, indeed, what we would expect to see if
> looking parallel to the plane containing all those little refracting
> ice-crystals, aligned vertically as they fall.
>
> It is possible that patchiness of the clouds contributes to this, but I
> rather doubt it. For one thing, these crystals usually form in
> cirrostratus clouds, which form a more or less continuous veil, under the
> conditions that give us the circle (or circles) around the moon. There is
> no strong convection in such circumstances, and thus little patchiness.
>
> On the Internet I find all the solar dogs are in line with the sun, yet
> the few moon-dog pictures show a displacement of one or both of the dogs
> above the angular elevation of the moon. So, I am still puzzled, and I
> don't know the answer definitively.
>
> As an hypothesis, I might suggest that we are dealing with a matter of
> perspective here. Almost all the sun-dog pictures I have seen have the
> sun quite low to the horizon; therefore, we are in such cases looking at
> the layer containing the ice crystals almost edge-on.
>
> The reason we seldom see sun-dogs high in the sky, is probably that the
> sun is just too bright then.
>
> The moon dogs, however, show up nicely even when the moon is higher in the
> sky. Assuming that the layer with the ice crystal prisms is horizontal
> (or, more accurately, shell-like around a segment of the earth — though
> this difference may not be important), then we have a more complex
> geometry. The crystals are much closer to us than the light source, which
> can be considered at infinity in this case. We are looking slighting
> upward and obliquely at the underside of the refracting layer. So it may
> be that the crystals most effectively refracting the light from the moon
> are those which appear, by perspective to us, somewhat higher in angular
> elevation that the illuminating body.
>
> So, my tentative guess is that this is matter of perspective. A simple
> experiment with a lightbulb and an optical prism might in this case be
> very “illuminating.”
>
>
> On Jul 26, 2010, at 9:06 AM, Larry Bogan wrote:
>
>> The misalignment of the Moon Dogs is due to the non-uniformity of the
>> cloud cover.
>>
>> The Sun/Moon Dogs are created by the ice crystals in the clouds and are
>> usually arcs of some length (up and down). In most cases the clouds are
>> not uniform and they aren't in this photo. It just so happens that on
>> the right side there is no clouds on the lower part so it looks as if the
>> Moon Dog is above the line between the Moon and Dogs. Many