Three altitude thresholds
Civil twilight uses a solar-center altitude of −6 degrees, nautical twilight −12 degrees, and astronomical twilight −18 degrees. Morning crossings mark the beginnings of these stages; evening crossings mark their endings. The twilight angles are geometric, without a refraction correction.
Twilight is not a fixed number of minutes
Latitude and season change the angle at which the Sun moves relative to the horizon. Twilight intervals can be short, long or absent. At high latitudes in summer, an evening may never reach astronomical darkness even though the Sun sets.
Use the correct boundary
Civil dusk is different from EENT, and EENT is different from astronomical dusk. For star observing, also check the Moon. A moonless astronomical-night window combines a Sun below −18 degrees with the Moon below the horizon; it still cannot account for clouds or artificial light.