Sunset can outlast the Sun. Even when the entire solar disk has slipped below the geometric horizon, Earth's atmosphere can bend its rays toward an observer, leaving a visible sliver where an airless sky would show none. The view cheats geometry. What appears to be the Sun's position is partly the work of intervening air.
Air deserves the credit. Because atmospheric density generally increases toward the ground, its refractive index also increases, causing sunlight passing through those layers to curve toward denser air. The effect is measurable. Near the horizon, atmospheric refraction typically raises an object's apparent position by about 34 arcminutes, slightly more than the Sun's apparent diameter of roughly half a degree. Appearance outruns physical position. An observer interprets the arriving rays as straight lines, tracing them backward to a point higher in the sky than the Sun actually occupies.
Precision needs a caveat. Since temperature, pressure and the arrangement of atmospheric layers affect refraction, that familiar numerical correction is an approximation rather than a promise for every sunset. Shape gives another clue. The solar disk often looks flattened because its lower edge, which lies nearer the horizon, is lifted more than its upper edge. Color tells another story. While refraction shifts the image, scattering along the long atmospheric path removes more blue light from the direct beam, helping reds and oranges dominate. The horizon keeps its secret. Above it hangs the last light of something already gone.
