Twilight in three depths: civil, nautical and astronomical
The Sun 6°, 12° and 18° below the horizon mark three different kinds of darkness — and the time between them ranges from 20 minutes to none at all.
Twilight is not one thing. It is graded by how far the Sun has sunk below the horizon, and the three familiar stages mark three different practical limits.
| Stage | Sun’s altitude | What it means |
|---|---|---|
| Civil twilight | 0° to −6° | Outdoor objects still visible without artificial light; brightest planets and stars appear |
| Nautical twilight | −6° to −12° | The horizon is no longer distinguishable at sea; navigation by stars becomes possible |
| Astronomical twilight | −12° to −18° | The sky’s own glow still hides faint stars and nebulae |
| Night | below −18° | No residual sky brightness from the Sun |
The names are not arbitrary. Civil twilight is the limit at which ordinary activity outdoors needs no lamp. Nautical twilight is the point at which the sea horizon — the reference for every celestial fix taken with a sextant — disappears. Astronomical twilight is the point at which the sky stops interfering with faint-object observation. Each was defined by a group of people who needed a number.
How long each stage lasts
It depends on the angle at which the Sun’s path crosses the horizon, which is a function of latitude and season.
At the equator the Sun drops almost vertically, so each 6° of depression takes about 24 minutes, and full darkness arrives about 70–75 minutes after sunset. At 50°N in December, the same descent is shallower and each stage stretches to around 40 minutes.
In midsummer at 50°N the Sun never reaches −18° at all: astronomical twilight does not end, and the sky keeps a permanent blue-gray glow. This is why summer stargazing in northern Europe is a matter of staying up until 1 am for a sky that is still not properly dark. In London in June the Sun never gets below about −15°, so the astronomical stage never finishes; at 60°N the Sun bottoms out near −6.5° and even civil twilight lasts all night.
North of the Arctic Circle, in the weeks around the June solstice, the Sun does not set at all — the sunrise table prints “does not set” for Longyearbyen rather than inventing a time.
The bright side of the ledger
Twilight is not only an obstacle. The two brightest planets and the brightest stars become visible during civil twilight, which is why Venus is so often noticed just after sunset — it is bright enough to punch through a sky that still has color in it.
The even illumination of civil twilight is also why it is the hour photographers care about: the sky is a large, soft light source and the contrast between it and the ground is low enough to record both. The “blue hour” is the tail of civil twilight, and its length is the same calculation as everything else on this page.
Where twilight is genuinely useful
- Finding a dark site. Astronomical twilight’s end is the earliest time a faint object is worth looking for; a site that reaches −18° an hour before your destination does is a better site for that hour.
- Planning a photograph. Civil twilight’s end is roughly when streetlights become the dominant light.
- Understanding why summer nights feel short. They are short for two independent reasons — the day is longer, and the twilight between day and night is longer too. At 50°N in June the second effect is the larger one.
The mechanism that decides all of this is the same one that decides day length: the angle the Sun’s path makes with the horizon. Twilight is the part of the day that the sunrise and sunset convention normally hides, because sunrise and sunset only mark one altitude and twilight is about the ones below it.
Twilight durations described here are for the defined solar depressions; the exact minutes at a given place and date come from the same solar algorithm as the site’s rise/set tables.
Frequently asked questions
- Why are there three kinds of twilight?
- Because different activities need different degrees of darkness. Civil twilight at 6° is the limit at which you can still see ordinary objects outdoors without artificial light. Nautical twilight at 12° is when the horizon is no longer visible at sea, so traditional navigation by stars requires it. Astronomical twilight at 18° is when the sky itself stops glowing enough to interfere with observations of faint objects.
- Why is twilight so much longer in summer?
- Because the Sun's path crosses the horizon at a shallow angle at high latitudes in summer, so it takes longer to sink the 18° needed for full darkness. In London in June the Sun never gets below about 11°, so astronomical twilight never ends — the sky never becomes fully dark. North of the Arctic Circle it never reaches civil twilight, and the sky stays lit all night.
- What is the blue hour?
- The part of civil twilight just after sunset or before sunrise, when the sky is deep blue and evenly lit — photographers like it because the light is soft and the streetlights are on without being dominant. It is not an hour; it is usually 20 to 40 minutes at mid-latitudes, and shorter near the equator.
- How long does twilight last?
- At the equator, about 24 minutes per stage, so 70 to 75 minutes from sunset to full darkness. At 50°N in midwinter it is close to 40 minutes per stage. At 50°N in midsummer there is no full darkness at all. The variation is enormous, and it is entirely predictable from latitude and date.
Sources
Each source is a primary or official reference, checked before publication. Links open the original.
- U.S. Naval Observatory, Astronomical Applications Department — the definitions of civil, nautical and astronomical twilight at 6°, 12° and 18° solar depression, and the twilight tables this site's figures are based on (aa.usno.navy.mil/data/RS_OneDay)
- NOAA Global Monitoring Laboratory, Solar Calculator — the solar position algorithm used to compute twilight times at any latitude and date (gml.noaa.gov/grad/solcalc/)
- Meeus, Astronomical Algorithms, 2nd edition, chapter 15 — the hour-angle relation for a given solar depression, which is what a twilight time is
- Royal Observatory Greenwich — the practical definitions of the three twilights and the summer nights at high latitude where astronomical twilight does not occur