Twilight and dark sky: civil, nautical and astronomical
The sky is fully dark only when the Sun is more than 18° below the horizon. Here is what happens before that.
After sunset, the sky does not go dark at once. Sunlight is still scattered by the upper atmosphere and lights the sky for a while. This is twilight, and it is divided into three stages, each defined by how far the Sun’s centre is below the horizon.
The three twilights
The US Naval Observatory (USNO) defines:
- Civil twilight, Sun 0° to 6° below the horizon: there is enough light for most outdoor activities without lamps. The brightest planets and stars appear.
- Nautical twilight, 6° to 12°: the horizon is still visible at sea, so navigators could measure star positions. The main stars of the constellations are visible.
- Astronomical twilight, 12° to 18°: to the eye the sky looks almost dark, but there is still some scattered sunlight, enough to hide the faintest objects such as distant galaxies.
When the Sun is more than 18° below the horizon, scattered sunlight is weaker than starlight and other natural sources. That is astronomical night, the darkest the sky gets. In the morning the sequence runs the other way.
When sunset happens
By convention, sunrise and sunset are when the upper edge of the Sun touches the horizon. Because the atmosphere bends light near the horizon (about 34 arcminutes on average) and the Sun has a radius of about 16 arcminutes, the USNO computes sunset when the Sun’s centre is 50 arcminutes below the geometric horizon. Even so, real rise and set times can differ by a minute or more because the atmosphere’s bending varies, and more at high latitudes.
How long twilight lasts
How long each stage lasts depends on how steeply the Sun sinks below the horizon. Near the equator the Sun goes down almost vertically and twilight is short. At higher latitudes it sets at a shallow angle and twilight lasts much longer, longest around the summer solstice.
Summer nights without darkness
Around the June solstice the Sun’s declination is +23.44°. At latitude φ, the lowest it gets at midnight is 90° − φ − 23.44° below the horizon. For it to reach 18° below, φ must be less than 90° − 23.44° − 18° ≈ 48.5° N. North of that, for some weeks around the solstice, astronomical twilight lasts all night: London, Berlin, Amsterdam and Paris have no astronomical night for part of the summer. Farther north, beyond about 54.6° N (where the Sun no longer reaches 12° below), even nautical twilight lasts all night. The same holds for the southern hemisphere around the December solstice.
The tonight page shows every twilight time for your place, how long the dark period lasts, and when no astronomical night occurs.
Darkness and your eyes
A dark sky also needs dark-adapted eyes. The American Meteor Society advises giving your eyes at least 20 to 30 minutes and avoiding bright lights and phone screens. Full adaptation takes longer. A dim red light is the least disruptive if you need to see.
A bright Moon and artificial light can brighten the sky much more than twilight. For the darkest sky, pick a night when the Moon is below the horizon (see Moon phases) and a place away from city lights.
Sources
- US Naval Observatory: Rise, set and twilight definitions
- American Meteor Society: FAQ (dark adaptation)
- Twilight times: Astronomy Engine, checked against NASA JPL Horizons. See methodology.
Frequently asked questions
When is it dark enough to see the stars?
The brightest stars and planets appear during civil twilight. The sky is fully dark, with no remaining sunlight, after astronomical twilight ends, when the Sun is 18° below the horizon.
Why is there no astronomical night in summer where I live?
Around the June solstice the Sun does not get 18° below the horizon anywhere north of about 48.5° N. From there the sky stays in twilight all night. The same happens in the south around the December solstice.