As shown in the above figure, the color of sky after sunset from apex to center is blue, orange, blue. With only the sun as the light source, there shouldn’t be blue from the apex. Because, as shown in the following figure, sunlight diminishes as photon’s traveling distance increases (e.g. with the green-blue sun, the traveling distance increase from a to b), which changes from blue to red. My question is, why’s there still blue color from apex? Are there other light sources other than the sun?