Civil, Nautical and Astronomical Twilight Explained
Three lines drawn at 6, 12 and 18 degrees below the horizon. The definitions are everywhere; the durations are not.
Twilight is divided into three bands, each defined by how far the centre of the sun sits below the horizon. The definitions are easy to find. What is harder to find, and more useful, is how long each band actually lasts where you live — and the fact that above certain latitudes some of them never finish.
The three bands
Civil twilight runs from sunset to −6 degrees. The horizon stays clearly visible, the brightest stars and planets appear, and you can still function outdoors without a light. This is the band that answers "when does it get dark" for ordinary purposes, and most legal definitions of lighting-up time sit at or near its end.
Nautical twilight runs from −6 to −12 degrees. The name is literal: this is the band in which a navigator can still make out the sea horizon while stars are visible, which is exactly the condition a sextant sight requires. Below −12 the horizon becomes too indistinct to use.
Astronomical twilight runs from −12 to −18 degrees. At the bottom of it the sun stops contributing measurable light to the sky. Below −18 degrees is what astronomers mean by night: the sky is as dark as it is going to get, and faint objects become reachable.
| City | Latitude | Civil | Nautical | Astronomical |
|---|---|---|---|---|
| Longyearbyen | 78.2°N | never completes | never completes | never completes |
| Tromsø | 69.6°N | 83 min | never completes | never completes |
| Reykjavík | 64.1°N | 57 min | 85 min | never completes |
| Stockholm | 59.3°N | 46 min | 61 min | 86 min |
| London | 51.5°N | 36 min | 44 min | 50 min |
| New York City | 40.7°N | 28 min | 34 min | 36 min |
| Cairo | 30.1°N | 25 min | 29 min | 30 min |
| Mumbai | 19.1°N | 22 min | 26 min | 27 min |
| Bogotá | 4.6°N | 21 min | 25 min | 25 min |
| Nairobi | 1.3°S | 21 min | 24 min | 24 min |
| Rio de Janeiro | 22.9°S | 23 min | 26 min | 26 min |
| Sydney | 33.9°S | 25 min | 29 min | 29 min |
| Wellington | 41.3°S | 28 min | 32 min | 32 min |
| Ushuaia | 54.8°S | 37 min | 42 min | 42 min |
What the table shows
Three things worth noticing.
The bands are not equal. Civil twilight is a 6-degree band, the other two are 6 degrees each, and yet the durations differ, because the sun's descent is not linear in the way that matters. Near the horizon refraction and the geometry of the sun's path both compress the early part.
They scale together with latitude. Nairobi's three bands are roughly 21, 22 and 23 minutes. London's are roughly 36, 40 and 44. The ratio between them stays similar; the whole sequence stretches.
They stop finishing. Read up the table and at some point entries turn into "never completes". This is not a rounding problem or a gap in the data. Above a certain latitude, at certain times of year, the sun genuinely does not get that far below the horizon.
The white nights problem
Astronomical twilight is the first to go. Above roughly 48 degrees of latitude — which includes Paris, Vienna, Vancouver and most of the United Kingdom — there is a period around midsummer when the sun never reaches −18 degrees. There is no astronomical night at all, for weeks.
This is why deep-sky astronomy in northern Europe has a season. From late May to mid-July there is nowhere in Britain where the sky gets properly dark.
Push further north and nautical twilight goes the same way, then civil. When civil twilight stops completing, you have arrived at the phenomenon people call white nights: the sun sets, but the sky never leaves dusk. Saint Petersburg is famous for it; Stockholm and Reykjavík have it too.
The thresholds are a consequence of simple geometry. The sun's lowest point in a day sits at an altitude of roughly (latitude + declination − 90) degrees. Set that below −18 and solve, and you get the latitude above which astronomical night disappears for a given date. Around the June solstice, with declination at +23.4°, it works out at about 48.5 degrees.
Why the sun's own size matters
Sunset itself is not defined at 0 degrees. It is defined at −0.833 degrees, and the reason is worth knowing because it is where a common software error creeps in.
Two effects combine. The sun is not a point: its disc has an apparent radius of about 16 arcminutes, and sunset conventionally means the moment the upper limb disappears, not the centre. And the atmosphere bends light, lifting the sun's apparent position by roughly 34 arcminutes when it is near the horizon. Add them and you get 50 arcminutes, which is 0.833 degrees.
So at the moment you see the sun's last sliver vanish, its centre is already more than half a degree below the true horizon. The twilight bands are all measured from that centre.
This matters because some widely used implementations subtract the solar radius again, on top of an altitude that already accounts for it, and end up about ninety seconds out. We measured that against the US Naval Observatory.
The southern hemisphere
Everything reverses cleanly. Sydney's longest twilights are in December, Wellington's in December, Ushuaia's white-nights season is around the December solstice. The table above is computed for today, so depending when you read it, the southern rows may be the long ones.
Ushuaia at 54.8°S loses astronomical night for several weeks around Christmas, the exact mirror of Britain in June.
Looking it up
Every city page on this site lists all six twilight boundaries for today — three in the morning, three in the evening — and the extended monthly table gives them for every day of the month. Where a band never completes, the page says so rather than printing a time.
Where the definitions came from
None of the three thresholds is a law of nature. Each was chosen for a job.
Nautical twilight is the oldest and the most concrete. Celestial navigation requires two things at once: a star to shoot, and a visible sea horizon to measure its altitude against. Too early and the stars are not out. Too late and the horizon has gone. The band between −6 and −12 degrees is the window where both exist, and it is the reason navigators worked in two short bursts a day rather than whenever they felt like it.
Civil twilight was defined for the law. Lighting-up times for vehicles, the hours during which outdoor work can proceed, the point at which a sporting fixture must be abandoned — all of these needed a defensible line, and −6 degrees is close to where ordinary outdoor tasks stop being possible without artificial light.
Astronomical twilight was defined by exclusion: it ends at the point where the sun stops adding measurable brightness to the sky. Below −18 degrees, any remaining glow is airglow and light pollution rather than the sun.
Using the bands
Civil is the one to plan around for anything practical. Walking out, driving without lights, finishing outdoor work, getting off a hill. The end of civil twilight is the honest answer to "when does it get dark".
Nautical is the photographer's band. Blue hour occupies the first third of it, and the rest is where long exposures of cities work best: the sky still has structure and gradient, and artificial lighting is dominant but not yet blown out against a black background.
Astronomical is only relevant if you are trying to see something faint. For the Milky Way, a comet, or a deep-sky object, you want the sun below −18 degrees, and in much of the northern hemisphere that is a seasonal opportunity rather than a nightly one.
One consequence worth stating: on a summer night in Britain, the difference between "dark" in the everyday sense and "dark" in the astronomical sense is the difference between about forty minutes after sunset and never.