Civil, Nautical and Astronomical Twilight Explained
Twilight is defined by how far the Sun is below the horizon: 6° for civil, 12° for nautical, 18° for astronomical. What each looks like and why its length varies.
Twilight is the time before sunrise and after sunset when the Sun is below the horizon but still lights the sky. It is divided into three stages by how far the centre of the Sun is below the horizon: civil twilight down to 6°, nautical twilight from 6° to 12°, and astronomical twilight from 12° to 18°. Below 18° it is night.
The definitions
All of these are geometric angles, measured to the centre of the Sun’s disc, as used by the U.S. Naval Observatory and most almanacs.
| Moment | Sun’s centre below the horizon |
|---|---|
| Sunrise / sunset | 0.833° (50 arcminutes) |
| Civil twilight begins / ends | 6° |
| Nautical twilight begins / ends | 12° |
| Astronomical twilight begins / ends | 18° |
In the evening the sequence runs downwards: sunset, then civil twilight until the Sun reaches 6°, nautical twilight until 12°, astronomical twilight until 18°, then night. In the morning it runs in reverse, so “dawn” without a qualifier usually means the start of civil twilight.
Sunrise and sunset do not use 0°. The figure of 0.833° comes from two corrections added together: the Sun’s apparent radius, about 16 arcminutes, because sunrise is the moment the Sun’s upper edge appears, and atmospheric refraction, about 34 arcminutes at the horizon, which bends sunlight over the edge of the Earth so you see the Sun before it is geometrically up. Refraction varies with temperature and pressure, so real sunrise times can differ from published ones by a minute or so, more at high latitudes.
What each stage looks like
These descriptions assume clear weather, a flat horizon and no artificial light. Cloud, haze and terrain change things considerably.
Civil twilight (0.833° to 6°). There is enough natural light for most ordinary outdoor activities without lamps. The brightest planets and stars appear. The horizon is sharp. In the United States, the Federal Aviation Administration’s definition of “night” for pilots runs from the end of evening civil twilight to the beginning of morning civil twilight.
Nautical twilight (6° to 12°). The sky is noticeably dark and artificial light is needed for most tasks, but the horizon is still visible at sea. That is the origin of the name: navigators could measure the altitude of stars above a visible horizon with a sextant. Most of the stars visible to the naked eye are out.
Astronomical twilight (12° to 18°). To a casual observer, especially in a city, this looks like full night. But faint objects such as dim galaxies and nebulae are still washed out by scattered sunlight. After the end of astronomical twilight, scattered light from the Sun is weaker than light from starlight and other natural sources, which is what astronomers want.
How long twilight lasts
Twilight length depends on how steeply the Sun’s path crosses the horizon. The Sun always moves about 15° across the sky per hour, but only near the equator does it descend nearly straight down. At higher latitudes it sinks at a shallow slant, so it takes longer to drop the same number of degrees.
Civil twilight (sunset to the end of evening civil twilight) on the two 2026 solstices, calculated for each city’s coordinates and shown in local time:
| City (latitude) | 21 June 2026 | 21 December 2026 |
|---|---|---|
| Singapore (1.4°N) | sunset 19:13, ends 19:36 (23 min) | sunset 19:05, ends 19:27 (22 min) |
| London (51.5°N) | sunset 21:22, ends 22:10 (48 min) | sunset 15:54, ends 16:34 (40 min) |
| Reykjavík (64.1°N) | sunset 00:05 on 22 June, never ends; runs until sunrise at 02:56 | sunset 15:30, ends 16:49 (79 min) |
Morning civil twilight is the mirror image and lasts the same length on these dates.
A few patterns stand out:
- Near the equator twilight is short and almost constant. Singapore’s civil twilight is 21–23 minutes all year; it is shortest at the equinoxes.
- At mid-latitudes twilight is longest at midsummer. London’s civil twilight is 48 minutes in late June, 40 minutes in late December, and only about 33 minutes near the equinoxes.
- Far north it can run all night. At midsummer the Sun’s lowest point at Reykjavík is only about 2.4° below the horizon, so civil twilight lasts from sunset to sunrise.
White nights and the missing darkness
Around the June solstice the midnight Sun is at its shallowest below the northern horizon. Simple geometry (ignoring refraction) gives the latitudes above which each stage of twilight never ends:
| Latitude (approx., around 21 June) | What happens at midnight |
|---|---|
| Above 48.5°N | No astronomical night: the Sun stays above −18° |
| Above 54.5°N | No nautical darkness: the Sun stays above −12° |
| Above 60.5°N | Civil twilight all night: so-called white nights |
| Above 66.5°N | Midnight sun: the Sun does not set |
The same bands apply in the Southern Hemisphere around the December solstice, though few people live that far south.
London (51.5°N) is in the first band. On 21 June 2026 the evening sequence there runs sunset 21:22, end of civil twilight 22:10, end of nautical twilight 23:25, and then nautical twilight begins again in the morning at 02:41. Astronomical twilight never ends. Six months later, on 21 December, the full sequence fits comfortably into the evening:
| London, 2026 | 21 June | 21 December |
|---|---|---|
| Astronomical twilight begins | does not occur | 06:00 |
| Nautical twilight begins | 02:41 | 06:41 |
| Civil twilight begins | 03:56 | 07:24 |
| Sunrise | 04:44 | 08:05 |
| Sunset | 21:22 | 15:54 |
| Civil twilight ends | 22:10 | 16:34 |
| Nautical twilight ends | 23:25 | 17:18 |
| Astronomical twilight ends | does not occur | 17:58 |
(Times are local: BST in June, GMT in December. See how daylight saving time works for why the clock shifts.)
At the poles everything is stretched to its limit. The Sun sets once a year, around the March equinox at the South Pole and the September equinox at the North Pole, and then takes roughly two weeks to sink 6° below the horizon. Civil, nautical and astronomical twilight each last for weeks.
Golden hour and blue hour
These are photographers’ terms, not astronomical ones, and their definitions vary.
- Golden hour is usually described as the period when the Sun is low, within roughly 6° of the horizon, giving warm, soft, directional light. It is not a fixed hour: near the equator the Sun covers the last 6° above the horizon in under half an hour, while in high-latitude summers it can last for several hours.
- Blue hour is the deep-blue sky after sunset or before sunrise, usually placed around the end of civil twilight and the start of nautical twilight, with the Sun a few degrees below the horizon. Like golden hour, it is shorter near the equator and longer at high latitudes.
Because both depend on the Sun’s angle rather than the clock, they shift through the year with sunrise and sunset. The sunrise and sunset pages give the underlying times for any city; the change in day length across the seasons is covered in equinoxes and solstices.
Quick reference
- Sunset: Sun’s centre 0.833° below the horizon (radius plus refraction).
- Civil twilight: down to 6°. Enough light to see without lamps.
- Nautical twilight: down to 12°. Horizon still visible at sea.
- Astronomical twilight: down to 18°. Below that the sky is as dark as it gets.
- Twilight is shortest at the equator and longest at high latitudes in summer, where it can last all night.
Sources
- U.S. Naval Observatory: Rise, Set, and Twilight Definitions
- National Weather Service: Twilight Types
- NOAA Global Monitoring Laboratory: Solar Calculator
Dates, offsets and astronomical times quoted here are computed with the same code that powers the site's tools — see data sources and methods. Found an error? Tell us.