Sky science
Astronomical Twilight
Direct answer
Astronomical Twilight is the interval when the Sun is between 12 and 18 degrees below the horizon; beyond it, the sky is conventionally considered fully dark for astronomy.
Astronomical Twilight occurs while the Sun is 12 to 18 degrees below the horizon.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Astronomy, Skywatching, Celestial Sphere, Ecliptic
Astronomical Twilight occurs while the Sun is 12 to 18 degrees below the horizon.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Astronomical Twilight
Civil, nautical, and astronomical twilight conventions divide the gradual transition between daylight and night by solar depression angle.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Astronomical Twilight matters
The term gives observers a more useful darkness threshold than sunset alone, especially for faint stars, galaxies, and meteor watching.
Astronomical Twilight in practice: an example
A deep-sky session may begin after evening astronomical twilight ends rather than immediately at sunset.
Where readings of Astronomical Twilight go wrong
It is a mistake to see twilight boundaries as abrupt physical switches; they are practical conventions applied to a gradual change.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Rise, Set, and Twilight Definitions - Definitions used by USNO for sunrise, sunset, moonrise, moonset, and twilight.
- U.S. Naval Observatory: Astronomical Information Center - Official index for time, calendars, seasons, coordinates, and observable sky phenomena.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
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Common questions
What is astronomical twilight?
Astronomical Twilight is the interval when the Sun is between 12 and 18 degrees below the horizon; beyond it, the sky is conventionally considered fully dark for astronomy.
What do people get wrong about astronomical twilight?
It is a mistake to see twilight boundaries as abrupt physical switches; they are practical conventions applied to a gradual change.
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