the time of morning or evening when the sun is 18° below the horizon. (Solar zenith angle is 108°, solar elevation angle is -18°.) See also civil twilight and nautical 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. Related term: twilight

The twilight stage during which the sun's unrefracted center is at elevation angles −12° 0 −18°. During a clear evening's astronomical twilight, horizontal illuminance due to scattered sunlight decreases from ∼0.008 lux to ∼6 × 10−4 lux. At 0 = −18°, 1) no horizon glow is visible at the sun's azimuth (the bright segment's upper boundary is at the observer's astronomical horizon), 2) sixth-magnitude stars can be seen near the zenith, and 3) scattered sunlight's residual illuminance is less than that from starlight and airglow. Compare civil twilight.

The period between sunset and the time when the sun has dropped 18 degrees below the horizon. Astronomical Unit The mean (average) distance of the Earth to the Sun is termed 1 Astronomical Unit (1 AU). It is a convenient way of describing distances within our Solar System.