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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 times there is only a Dog on one side if the clouds are very spotty and the misalignment can not be determined. > > Larry Bogan > > Steve Shaw wrote: >> Interestingly, the one image of the same thing in Wikipedia (search for 'Moon dogs') shows the same discrepancy, with the two dogs each elevated by perhaps 10-15 degrees above an imaginary horizontal line drawn through the moon. The short associated text there indicates that the explanation for Moon Dogs is exactly analogous to that for Sun Dogs (refracting ice crystals 22 degrees out), but this view has to be suspect. >> >> The longer article on Sun Dogs stresses that these always lie in the same horizontal plane as the sun. The ice crystals supposedly sometimes sink placidly and line up vertically in the atmosphere, whereupon two sun dogs may appear. >> >> As the line-up in the two optical phenomena is somewhat different, suggesting a common explanation for both can't be exactly correct -- we're missing something. Perhaps the form of the ice crystals differs when these are formed in sunlight (warmer) than when it is colder up there, at night in moonlight? >> Steve >> >> >> Quoting David & Alison Webster <dwebster@glinx.com>: >> >>> Hi Again, July 25, 2010 >>> For some reason this has not been posted yet so will try again with cc. DW >>> ----- Original Message ----- >>> From: David & Alison Webster >>> To: naturens@chebucto.ns.ca >>> Sent: Sunday, July 25, 2010 9:53 AM >>> Subject: Re: [NatureNS] Pic of Moondogs over Scots Bay >>> >>> >>> Hi Paul & All, July 25, 2010 >>> Why the dogs are slightly above moon center is an interesting question. I will take a qualitative stab at attempting to rationalize this and perhaps someone versed in optics and spherical geometry can generate numbers. >>> >>> The dogs lie on and are, in effect, unusually bright areas in a 22o halo around the moon. One may consequently conclude that the dogs arise by convergence of refraction from numerous points. >>> >>> If one assumes that the Cirrus clouds containing the ice crystals line in a plane that by projection would form a concentric spherical shell and with the light source (sun or moon) at a suitably low angle, this shell of ice crystals will act as a lens in the sense that numerous points in a localized zone of this curved shell will lie at 22o between the observer and the moon. >>> >>> This convergence of refracted rays will be greatest where substantial light is refracted from the curved shell that lies above the moon/observer path, thus leading to a dog slightly above this level. >>> >>> Yt, Dave Webster, Kentville >>> ----- Original Message ----- >>> From: Paul S. Boyer >>> To: naturens@chebucto.ns.ca >>> Sent: Saturday, July 24, 2010 7:23 PM >>> Subject: Re: [NatureNS] Pic of Moondogs over Scots Bay >>> >>> >>> Congratulations, Ian. This is a very interesting shot. >>> >>> >>> I think that next time you could try a quick sequence of shot at different exposures. Then combine the picture using HR (high dynami