Seasons and sky
Astronomical Dawn
Direct answer
Astronomical Dawn is the morning instant when the Sun center rises through 18 degrees below the horizon.
Astronomical Dawn begins astronomical twilight at a -18-degree solar altitude.
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At a glance
- Found in
- Seasons & Earth Almanac
- Related
- Nautical Dusk, Astronomical Dusk, Astronomical Season, Equinox
Astronomical Dawn begins 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 Dawn
The eighteen-degree standard provided observatories a repeatable boundary for residual solar illumination.
Why Astronomical Dawn matters
Astronomical Dawn marks the conventional end of full natural night for sensitive sky observation.
Astronomical Dawn in practice: an example
Faint-object imaging often ends near Astronomical Dawn, before any visible sunrise glow for casual observers.
Where readings of Astronomical Dawn go wrong
Light pollution, airglow, Moon, and high-latitude summer can make practical darkness differ from the calculated boundary.
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 Dawn?
Astronomical Dawn is the morning instant when the Sun center rises through 18 degrees below the horizon.
What do people get wrong about Astronomical Dawn?
Light pollution, airglow, Moon, and high-latitude summer can make practical darkness differ from the calculated boundary.
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