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Twilight
1. Twilight is the illumination that is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere when the Sun itself is not directly visible because it is below the horizon, so that the surface of the Earth is neither completely lit nor completely dark.
2. The further the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18 degrees below the horizon, twilight’s brightness is nearly zero. Evening twilight ends and night begins. When the Sun again reaches 18 degrees below the horizon, night ends and morning twilight begins.
3. Twilight is defined according to the solar elevation angle, which is the position of the geometric center of the sun relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight («first light»), and astronomical twilight (darkest).
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The scientists distinguish three subcategories of twilight: civil twilight, nautical twilight, and astronomical twilight.
Twilight is the illumination that is produced by sunlight scattering in the upper atmosphere and is defined according to the solar elevation angle.
The intensity of twilight depends upon the solar elevation angle: the further the Sun is below the horizon, the dimmer the twilight.
Twilight is the illumination that is produced by sunlight scattering, its intensity depending upon the position of the Sun concerning the horizon.