Quick answer
Twilight is the time when the Sun is below the horizon but still lights the sky. It has three stages defined by the Sun's angle below the horizon: civil twilight (0° to 6°), nautical twilight (6° to 12°) and astronomical twilight (12° to 18°). Below 18°, the sky is fully dark.
Why twilight happens
After sunset, the Sun is hidden below your horizon but still shines on the atmosphere high above you. That sunlit air scatters light back down, so the sky stays bright for a while and fades gradually. The deeper the Sun sinks, the less sunlit air remains overhead.
Civil twilight: Sun 0° to 6° below the horizon
In clear weather there is usually enough natural light for most outdoor activities without artificial lighting. The brightest planets and stars begin to appear. Evening civil twilight begins as the Sun reaches the horizon and ends at civil dusk; morning civil twilight begins at civil dawn and ends as the Sun reaches the horizon. Dayclipse uses the geometric horizon (0°) as the upper edge, so in the evening civil twilight starts a moment before the standard sunset instant, and in the morning it ends a moment after sunrise.
Nautical twilight: 6° to 12° below
The name comes from navigation at sea: during nautical twilight the horizon is still faintly visible while many bright stars are out, which historically allowed sailors to take star sightings. On land, it is too dark for most outdoor activities without light.
Astronomical twilight: 12° to 18° below
To the eye, the sky can look almost completely dark, but a faint glow remains — enough to wash out the dimmest objects astronomers want to observe. Once the Sun is more than 18° below the horizon, sunlight no longer contributes to the sky's brightness: this is astronomical darkness, the true night.
Summary table
| Phase | Sun altitude | Typical experience (clear sky) |
|---|---|---|
| Civil twilight | 0° to −6° | Bright enough outdoors; first planets visible |
| Nautical twilight | −6° to −12° | Horizon fading; many stars visible |
| Astronomical twilight | −12° to −18° | Nearly dark; faint sky glow remains |
| Astronomical darkness | below −18° | No sunlight in the sky |
How long does twilight last?
Each stage covers 6° of Sun altitude, so its length depends on how steeply the Sun descends. Near the equator, civil twilight lasts only around 20–25 minutes. At mid-latitudes, such as New York or Madrid, it is closer to half an hour, and it grows longer toward the poles and around the solstices.
Places where it never gets fully dark
Around the June solstice, the Sun at local midnight sits about 66.6° minus your latitude below the horizon (ignoring refraction). That simple rule explains some striking effects in the Northern Hemisphere:
- North of about 48.6° (for example London or Vancouver), the Sun never reaches 18° below the horizon for a period around midsummer — there is no astronomical darkness.
- North of about 54.6° (for example Edinburgh), nautical twilight can last all night.
- North of about 60.6° (for example Anchorage or Reykjavik), civil twilight can last all night — the famous “white nights.”
The same happens at matching southern latitudes around the December solstice.
Related reading: golden hour vs blue hour, which overlap the brightest part of twilight.