The twilight stage during which the sun's unrefracted center is at elevation angles −12° > h0 > −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 h0 = −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.