Daylight
The gap between sunset and genuine darkness is anywhere from twenty minutes to over an hour depending on where and when you are standing. Planning around sunset alone wastes light or runs you out of it.
Sunset is a precise astronomical event: the moment the upper limb of the solar disc drops below the horizon, accounting for atmospheric refraction. It is also, for anyone outdoors, the wrong number to plan against.
After sunset the sun continues to illuminate the atmosphere above you. Scattered light reaches the ground for a period that can be considerably longer than people expect — or considerably shorter.
Civil twilight runs from sunset until the sun reaches 6 degrees below the horizon. The horizon remains clearly defined, the brightest stars and planets appear, and terrestrial objects are still distinguishable without artificial light. Ordinary outdoor activity remains practical throughout. This is your real usable-light boundary.
Nautical twilight runs from 6 to 12 degrees below. The name comes from marine navigation: both the horizon and navigational stars remain visible, permitting sextant sightings. On land, outlines are visible but detail is not. Terrain reading becomes unreliable.
Astronomical twilight runs from 12 to 18 degrees below. Sky glow from scattered sunlight still interferes with faint astronomical observation but is imperceptible to the eye. Below 18 degrees is astronomical night.
Twilight length depends on how steeply the sun descends relative to the horizon, and that angle is set by latitude and date.
Near the equator the sun's path is close to perpendicular to the horizon. It drops 6 degrees quickly, and civil twilight can be over in twenty minutes. Tropical darkness genuinely does arrive abruptly.
At higher latitudes the sun descends at a shallow angle, taking much longer to cover the same six degrees. At 50 degrees north in midsummer, civil twilight can run well over an hour. Above roughly 60 degrees, the sun may not reach 6 degrees below the horizon at all around the solstice, producing the continuous dusk of white nights.
| Latitude | Civil twilight, equinox | Civil twilight, June solstice |
|---|---|---|
| 10°N | ~22 min | ~23 min |
| 30°N | ~24 min | ~28 min |
| 45°N | ~28 min | ~40 min |
| 55°N | ~34 min | ~65 min |
| 62°N | ~40 min | All night |
Season matters at any given latitude too. Twilight is shortest near the equinoxes and longest near the summer solstice for mid- and high-latitude locations, because the sun's declination changes the geometry of its path across the horizon.
Every figure above assumes a flat horizon. Real ground rarely provides one.
In a valley, on the east side of a ridge, or in dense forest, functional darkness arrives well before the calculated civil dusk — sometimes an hour or more before, if a substantial ridgeline sits to your west. The sun disappears behind terrain long before it reaches the true horizon, and direct light stops immediately.
The reverse also holds. On an exposed summit or a west-facing slope, you keep direct sun after valley floors have gone into shadow, and the effective usable window extends.
Cloud cover cuts twilight further, since the mechanism is scattered light and heavy overcast blocks it. A dull evening under thick cloud can go dark noticeably earlier than the tables predict.
Dark adaptation is slower than most people account for. Cone-mediated adaptation completes in five to ten minutes. Rod-mediated adaptation — the part that actually delivers useful low-light vision — takes twenty to thirty minutes, and a single glance at a bright white light resets it substantially.
Two practical implications. Moving continuously from bright conditions into twilight means your vision is adapting behind the light level for the whole transition. And if you use a headlamp in twilight and then switch it off, you have surrendered adaptation that will take half an hour to rebuild. Red light preserves rod adaptation far better than white, which is why headlamps offer a red mode.
A reasonable planning method: take the end of civil twilight as your hard finish, subtract your outing duration, then subtract a buffer sized to the consequence of being wrong. Thirty minutes on familiar level ground. Sixty to ninety on technical terrain, in unfamiliar country, or with a group whose slowest member sets the pace.
Note also that pace degrades in low light. Descending rocky ground in nautical twilight is meaningfully slower than the same ground at midday, so the final section of a route often takes longer than the plan assumed.
The daylight window calculator returns civil dusk alongside sunset and computes the latest departure time for a given outing length and buffer, which is the number you actually need.