Canyon aura

Kris and I talked about the "canyon aura" Sonia asked about in the group sitting at dinner last night. She asked why the canyon top was brighter at night: the sky at the edge of the canyon appears lighter than the night sky looking away from the edge. Was the cause reflected sunlight, moonlight, city lights?

Charly piped up and asked, "Oh! Do you mean the canyon aura?" He named the phenomenom, but couldn't explain it (which, admittedly, frustrated me a bit, because it stopped the conversation, without explaining the phenomenom).

I told Kris I though the aura was an artifact of the way the eyeball works. Unfortuantely, I didn't explain my thoughts well, so we talked for a long while about it. Essentially, I tried to describe the phenomenom where, if you stare at an object in big contrast to its surrounding, then look away, you see an retinal imprint of the object.

Looking at the canyon rim, which is solid dark, next to the slightly bright sky, your eye doesn't focus on one point. The slight movement, with a slight retinal imprint, gives the impression of an aura along the canyon edge.

I don't recall which of rods or cones (update: rods for light, cones for color), the the light sensitive ones may have a loner lag time, which makes the aura more pronounced. Actually, when its this dark, color is irrelevant, so the lag time is also irrelevant.

Kris asked if it was the same phenomenom as when a blue circle in a dark backgroun is seen as a darker blue than the same blue in a white (or lighter than the previous circle's) background. I said not quite, but we'd do the experiment later at home.

I do, think, however that I eventually explained it well enough.

Andy slept through the whole conversation.