Where it sits
Never closer than 22 degrees, because a hexagon always has 60-degree prisms.
Tell it apart
| Aspect | Sun dog | Sun pillar |
|---|---|---|
| Physics | Refraction | Reflection |
| Colour | Dispersed | White |
| Shape | Patch at 22° | Vertical shaft |
| Same crystals | Yes | Yes |
Both come from the same falling plate crystals, but by different physics. A sun dog is refraction — light bends THROUGH the crystal, so it splits into colours at 22 degrees. A sun pillar is reflection — light bounces OFF the flat faces, so it stays white and forms a vertical shaft.
What it doesn't mean
The 22-degree minimum is fixed by physics, not chance. It's the minimum deviation angle for light refracting through the 60-degree prism formed by alternating faces of a hexagonal ice crystal — the same geometry that sets the 22-degree halo.
They're dispersed into colours, and the order is diagnostic: red closest to the sun, grading to blue on the outer edge. If you can see that ordering, you're looking at refraction rather than reflection.
What it really means
There appear to be three suns. One real one, low and cold, and a bright patch either side of it — often faintly rainbow-edged, sitting at exactly the same height.
They are sun dogs, and the word exactly is doing real work. They never appear closer than 22 degrees to the sun. That floor is set by crystal geometry rather than by weather: with the sun on the horizon they sit at almost exactly 22 degrees, and as the sun climbs they slide further out, always staying level with it. A hexagonal ice crystal has side faces meeting at 60 degrees, forming a prism, and 22 degrees is the minimum deviation angle for light refracting through a 60-degree prism. The same number produces the 22-degree halo.
What makes sun dogs specifically appear beside the sun rather than all around it is that the crystals are flat plates, fluttering downward with their faces roughly horizontal — so they're all oriented the same way, and each acts as a tiny prism pointing your way.
The colour order is diagnostic and worth checking: red closest to the sun, blue on the outside. That ordering tells you you're seeing refraction — light bent through the crystal and split — rather than reflection.
Which is exactly what separates a sun dog from a sun pillar, the vertical white shaft you sometimes see instead. Same crystals, different physics: pillars are light bouncing off the flat faces, so nothing splits and nothing colours.
The formal name is parhelion — Greek for beside the sun. Where 'dog' came from, nobody actually knows.
Where it comes from
Greek — beside
Greek — sun
'Parhelion' means literally 'beside-the-sun'. The English 'sun dog' is of obscure origin — the 'dog' element has no agreed etymology, and explanations about the sun's dogs following it are folk etymology.
- technical naming
The formal term is parhelion, plural parhelia; the NWS glossary lists 'Sun Dog' as a pointer to it.
- mechanism
Water freezes into small flat six-sided ice crystals that flutter downward with their faces parallel to the ground, each acting as a tiny six-sided prism that bends sunlight by at least about 22 degrees.
In real life
- On a frosty morning under thin high cloud, two bright, rainbow-edged spots sit level with the low sun, one on either side.
- Look closely and the edge of each sun dog nearest the sun is red, shading to blue on the far side.
- 'Why are there three suns?' 'There aren't. The other two are sun dogs — parhelia — made by ice crystals in the cloud.'
Questions
Q01What is a sun dog technically called?
A parhelion — Greek for 'beside the sun'. The plural is parhelia, and it's the term you'll find in the NWS glossary, with 'sun dog' listed as a pointer to it.
Q02Why 22 degrees?
It's the minimum deviation angle for sunlight refracting through the 60-degree prism formed by the side faces of a hexagonal ice crystal. Because the crystal geometry is fixed, so is the floor — sun dogs are never closer than 22 degrees. They sit at 22 degrees when the sun is on the horizon and slide further out as it climbs, staying level with it.
Q03What weather produces them?
Thin cirrus cloud made of tiny hexagonal ice crystals, with the sun low in the sky. The crystals need to be flat plates fluttering down with their faces roughly horizontal, so each one acts as a tiny 60-degree prism.
Sources
- What Causes Halos, Sundogs and Sun Pillars? — NWS La Crosse.
- Sun Halo over Sweden — NASA Astronomy Picture of the Day.
- παρά (etymology) — Wiktionary.