Here's a Sun Dog I snapped one evening last week.
Sun Dogs are seen when the sun's light is refracted by flat hexagon ice crystals drifting in the clouds.
In favourable conditions, if the cloud formation is right and no nearby objects get in the way, we may see a pair of Sun Dogs, 22° left and right of the sun, but this is rarer than seeing just the one.
They fall into the class of phenomena you don't notice until something or someone makes you aware and then you realise they are quite common and see them frequently. Sun Dogs are more often visible than rainbows and I spot them often, usually looking West as the sun is setting in a bank of thin high cirrus clouds. You can see them to the East at sunrise too, of course, it's just that I see sunset more often.
These Sun Dogs are just a small part of a whole complex of arcs and circles that have sometimes been observed, although unless we are in the mountains or in Antarctica we are unlikely to witness them. My sketch below only includes the best known halo-related phenomena; at least 119 have been documented.
