The interval of incomplete darkness between sunrise (or sunset) and the moment when the true position of the sun is 18 degrees below the horizon, at which there is no discernible horizon glow over the sun's azimuth.
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 âˆ’12Â° 0 âˆ’18Â°. During a clear evening's astronomical twilight, horizontal illuminance due to scattered sunlight decreases from âˆ¼0.008 lux to âˆ¼6 Ã— 10âˆ’4 lux. At 0 = âˆ’18Â°, 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.