Seasons and sky
Astronomical Dusk
Direct answer
Astronomical Dusk is the evening instant when the Sun center descends to 18 degrees below the horizon.
Astronomical Dusk ends astronomical twilight at a -18-degree solar altitude.
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At a glance
- Found in
- Seasons & Earth Almanac
- Related
- Astronomical Dawn, Inferior Mirage, Astronomical Season, Equinox
Astronomical Dusk ends astronomical twilight at a -18-degree solar altitude.
A Sun-Earth orientation for seasonal, atmospheric, tidal, and terrestrial terms.
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The story behind Astronomical Dusk
Observatory schedules adopted the standard as a practical approximation for when scattered sunlight becomes negligible.
Why Astronomical Dusk matters
Astronomical Dusk marks the conventional beginning of full natural night for many astronomical observations.
Astronomical Dusk in practice: an example
At high summer latitudes Astronomical Dusk may not occur, leaving no formally defined astronomical night.
Where readings of Astronomical Dusk go wrong
The usual misreading is to assume the sky will be dark at the threshold when Moon, aurora, smoke, or artificial light is present.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Rise, Set, and Twilight Definitions - Official definitions used for sunrise, sunset, refraction, and twilight calculations.
- NOAA Global Monitoring Laboratory: Solar Calculator Glossary - NOAA definitions for solar geometry, atmospheric refraction, and civil, nautical, and astronomical twilight.
- Source
- The Great Almanac Dictionary
- Category
- Seasons & Earth Almanac
- Last updated
- 2026-09-26
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Common questions
What is Astronomical Dusk?
Astronomical Dusk is the evening instant when the Sun center descends to 18 degrees below the horizon.
What do people get wrong about Astronomical Dusk?
The usual misreading is to assume the sky will be dark at the threshold when Moon, aurora, smoke, or artificial light is present.
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