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Bo 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 > dynamic range) software. [See: > http://en.wikipedia.org/wiki/High_dynamic_range_imaging] > > > I was puzzled to note that the two “dogs” do not lie in a straight > line with the moon. I can't explain that, but is is the same a the > configuration shown in the Wikipedia article on moon dogs > [http://en.wikipedia.org/Moon_dog]. > > > —Paul > > > On Jul 24, 2010, at 6:42 PM, Ian McKay wrote: > > > I managed to capture something I had only seen once before - > Moondogs. Much like Sundogs but caused by the light of the moon. The > corona was huge. I had to use a pretty big aperture to catch a > little of what my eye was seeing. Not great but it comes close. > http://www.amimckay.com/graphics/moondogs-scotsbay.jpg > > Ian McKay > Scots Bay > > No virus found in this outgoing message. > Checked by AVG - www.avg.com > Version: 9.0.851 / Virus Database: 271.1.1/3025 - Release Date: > 07/24/10 03:36:00 >
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