Here is a photograph of a particularly bright sundog or
mock Sun. This optical phenomenon is a relative of the rainbow.
Rainbows, with their
familiar size (42°) and orientation (with the Sun behind the viewer)
result when light passes through and is reflected from droplets of water.
Sundogs and many other
optical phenomena result in a similar way, but from ice crystals in the
upper atmosphere rather than from liquid water droplets.
Atmospheric crystals of water ice have a well-defined, hexagonal form, and tend to align themselves in the way that maximises air resistance as they fall. As a result of of this well-defined shape and preferred alignment, they can produce a variety of halos, both Sun- and zenith-centred, as well as various bright patches and arc-shaped celestial brightenings. Which of these are seen on any particular occasion reflects the mix of crystal sizes present: long, thin, pencil-shaped ones will have their long axis parallel to the ground. Much shorter ones will look like hexagonal plates, and will tend to have their hexagonal cross-sectional planes parallel to the ground. So alignments may vary, within a restricted set of possibilities, and in consequence the range of positions on the sky from which brightenings will be seen will also vary.
Not infrequently, particularly in still but cool conditions, a halo results of about 22° angular radius, centred on the Sun (22° is about the span from of the tips of your outstretched pinky and thumb, held out at arms' length - small-handed people tend to have shorter arms to make this near enough right for everybody). When this is bright it can be seen to be coloured, with the inner and outer edges reddish and bluish, respectively. Sometimes the two patches on either side of this halo, at the same altitude as the Sun, will be seen to be brighter than the rest of the halo. Sometimes these patches, the sundogs will be be bright although none of the rest of the halo is clearly visible. Depending on clouds and general sky conditions, one or both sundogs may be clear to see.
In this picture, taken about 7.00 pm on 12 June 2001, the Sun was to the right, out of the picture. I used a 50 mm lens at f11, Kodak Gold Ultra film (ASA 400) and a 1/500 second exposure. You can see part of the 22° halo, particularly below the sundog, and a bright 'spike' extending across the sky from it, away from the Sunward direction. This spike is often seen in bright sundogs, but I at least have seldom seen such a lengthy one as was visible here, perhaps 15° in length.
With
an excellent image-viewing program I enhanced the brightness
and contrast, producing the image at left (click on it for a bigger version). In this close-up view
the colours are highly exaggerated, but this artificial process brings out more clearly the 22°
halo, particularly below the sundog, and the anti-sunward spike.
Also visible on this particular day, but lost forever as I came to the end of the film, was the upper tangent arc. This looks like a segment of another circular celestial brightening that just touches the 22° halo at its highest point above the horizon. All of these various components, and indeed others may be seen in various combinations depending on how atmospheric conditions have allowed ice crystals to grow. Many more details of these delightful topics are in two excellent books: Rainbows Halos and Glories, by Robert Greenler (Cambridge University Press, 1980), and Color and Light in Nature by D K Lynch and W Livingston (Cambridge University Press, 2000), and in a rather impressive website devoted to Atmospheric Optics.
