Astronomical twilight

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astronomical twilight

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.

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