Illumination angle
From Infogalactic: the planetary knowledge core
In computer graphics and geography, the illumination angle of a surface with a light source (such as the Earth's surface and the Sun) is the angle between the inward surface normal and the direction of light.[1] It can also be equivalently described as the angle between the tangent plane of the surface and another plane at right angles to the light rays.[2] This means that the illumination angle of a certain point of the Earth's surface is zero if the Sun is precisely overhead and that it is 90 degrees at sunset and at sunrise.
