What angle below the horizon defines true darkness after astronomical twilight has ended?
Answer
More than 18 degrees below the horizon
True darkness, which is crucial for maximizing the visibility of stars and deep-sky objects, is achieved only after the final stage of twilight concludes. Astronomers categorize twilight into civil, nautical, and finally, astronomical. The benchmark for achieving this complete absence of solar illumination interfering with observation is when the Sun has descended past 18 degrees below the horizon. This waiting period means that the first hour or two after sunset are inherently less productive because the sky remains too bright due to lingering sunlight scattered in the upper atmosphere.
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