For Astronomical twilight we have terms and definitions in 2 topics. The topics are Planetary Science and Weather.

Astronomical Twilight (Planetary Science)

The time when the Sun is 12-18 degrees below the horizon. When the Sun moves from 19 to 18 degrees below the horizon, astronomers can begin to detect an increase in the brightness of the sky, although to the naked eye the sky still looks dark.

Astronomical Twilight (Weather)

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.
Related term: twilight

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