It's an effect that's easiest to see on a wet winter night, with a streetlight shining through a tangle of bare tree twigs: the surfaces that most directly reflect the streetlight to the observer form a circle around it, a halo of streaks. Each streak is itself more or less straight, but they're arranged in a circle, a sort of crown of thorns. It moves as you do, tracking with the light.
You don't usually see it with the sun, I think because the sun is just too bright: if you're looking that directly towards it you're too dazzled to see anything else.
The week of the fall equinox, though, the rising sun lines up with the east-west streets, and if you happen to be walking east on a tree-lined street at exactly sunrise, and the trees are wet from the recent rains, you can see the sun's version of it: a brilliant circle of golden fire. A doorway into a world of unbearable light.
You can't look at it for long, of course, and when you turn away and close your eyes, the negative image turns with you, in bruise purple and dark green. Within seconds, what you saw is replaced by what you wish you had seen; with fragments of Dante, with words for light. The golden apples of the sun. Mithraic altars built by homesick legionaries in godforsaken, rainswept Britain; Byzantine mosaics in candlelight. What did you really see? What door did you fail to open?
It's gauche, profane, even to talk about it. Is it better to leave it be, and maybe forget it; or to talk about it, and certainly distort it? Forgetting it seems like the larger disaster.