Azuliskye — Atmospheric optics and sky colour: why the | azuliskye.net
Sunsets turn red because the low sun's light crosses about 38 times more atmosphere than it does overhead, and molecular scattering removes the blue from the beam before it reaches you.
At the zenith, sunlight crosses one atmosphere-depth of air. Near the horizon it enters at a grazing angle and crosses about 38 times more. Every extra kilometre of that path scatters a little more blue out of the direct beam.
The removal is selective. Scattering scales with 1/λ⁴, so 450 nm blue is stripped about 5.5 times faster than 700 nm red. As the sun sinks, its disc moves through yellow and orange into a deep red built from the 620-700 nm band — the wavelengths that survive the journey.
On narrow screens, swipe or scroll the plate sideways.
The sky away from the sun keeps its own schedule. Within about ten minutes of sunset a grey-blue band rises along the eastern horizon with a pink rim above it, while the west still burns orange.
Then comes the blue hour: roughly 20-40 minutes when the sun sits 4-8° below the horizon and the whole sky settles into an even, deepening blue — the same scattered short-wave light as at midday, now reaching the ground only indirectly.
For the physics of that blue, see Why the Sky Is Blue; for the rare glows that can appear high above a sunset, see Halos, Nacreous Clouds and Aurora. The main page carries a 40-minute observation routine with the typical failure named at each step.
Further reading