Science & history · The Sun and Earth's sky
Civil, nautical and astronomical twilight
Twilight is not one thing. Astronomers mark three stages by how far the Sun's centre is below the horizon.
The three boundaries
Civil twilight ends in the evening when the Sun's centre is geometrically 6 degrees below the horizon; nautical at 12°; astronomical at 18°. After civil twilight, artificial light is normally needed outdoors. Nautical twilight is when sailors could take reliable star sights for navigation.
Astronomical night is geometry, not a forecast
Below 18°, scattered sunlight is less than starlight and other natural light. That says nothing about clouds, moonlight or streetlights. Near midsummer at high latitudes the Sun never gets that low, and astronomical night does not happen at all.
Sources
- US Naval Observatory Rise, Set, and Twilight Definitions (opens in a new tab) for sunrise/sunset and the three twilights; refraction and accuracy caveats · checked 2 October 2026