Solar position is measured through the solar elevation angle, the vertical distance of the sun relative to the local horizon expressed in degrees. During the day, this angle is positive. At sunset and sunrise, it sits at zero. Once the sun descends, scientists track negative values, often termed solar depression.
Civil twilight occupies the initial band: 0° to -6°. During this window, abundant scattered light passes through the troposphere and stratosphere. Terrestrial features remain plainly distinct. You can identify building lines, tree canopies, and parked cars without squinting. Natural ambient light levels drop from roughly 400 lux at sunset to approximately 3.5 lux at civil dusk.
Nautical twilight takes over between -6° and -12°. The rate of ambient photon collapse accelerates dramatically here. High-altitude atmospheric layers continue scattering scattered red and blue wavelengths, but the ground receives insufficient direct illumination. Light levels plunge from around 3.5 lux down to a meager 0.01 lux. At that point, unassisted human vision fails to resolve surface details on open terrain.
