astronomical twilight—See twilight.
atmosphere—The mass of air surrounding the Earth.
ASTRONOMICAL TWILIGHT
The time after nautical twilight has commenced and when the sky is dark enough, away from the sun's location, to allow astronomical work to proceed. It ends when the center of the sun is 18? below the horizon.
Astronomical twilight refers to the interval of incomplete darkness that occurs when the sun's center is approximately 18 degrees below the horizon. There is no discernible horizon glow left over the sun's azimuth.
3) Astronomical Twilight: The period of time before sunrise and sunset when the sun is not more than 18 degrees below the horizon.
Twister: In the United States, a colloquial terms for a tornado.
As is true of nautical and astronomical twilight, civil twilight's length varies greatly with latitude and time of year.
Nautical Twilight - The period of time before sunrise and after sunset when the sun is not more than 12° below the horizon.
Astronomical Twilight - The period of time before sunrise and sunset when the sun is not more than 18° below the horizon.
Astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. During dawn, the reverse order occurs until full daylight.
Astronomical Twilight: When the sun is no more than 18 Degs. below the horizon before sunrise and after sunset.
 See also: Twilight, Light, Degree, Weather, Air
  
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