Astronomical Twilight
noun - Astronomical Twilight begins when the sun is 12° below the horizon and ends when it is 18° below the horizon.
After sunset, the sky transitions through different phases of twilight including the Civil Twilight that begins before sunrise and ends after sunset when the sun is 6° below the horizon, the Nautical Twilight that begins when the sun is 6° below the horizon and ends when it is 12° below the horizon, and the Astronomical Twilight that begins when the sun is 12° below the horizon and ends when it is 18° below the horizon.
Atmospheric sundown
Atmospheric sundown refers to the period after the sun has set below the horizon, marked by the gradual darkening of the sky and the appearance of twilight. This phenomenon is a result of atmospheric refraction and scattering of sunlight.
Sunset
Sunset is astronomically defined as the moment when the upper limb of the sun's disk disappears below the horizon. However, due to atmospheric refraction, the sun appears to set a little later than it actually does.
