Sunset is not the end of usable light: planning a walk-out by the clock

Sunset is the time the sun’s centre falls below the horizon. Light lingers after it through civil twilight, but a walk-out has to be planned against the clock, with the sunset time as the hard limit for open ground and an earlier limit under trees. The steps below set out the sunset figure, the twilight stages, the effect of a horizon obstruction, and a worked walk-out that subtracts a walking time and a margin.

What the sunset time assumes

NOAA’s calculation details state that, for sunrise and sunset calculations, they assume 0.833° of atmospheric refraction. Refraction bends the light of a low sun upwards, so the sun is still visible a little after its geometric position has dropped below the horizon. The sunset time is therefore the moment the sun’s centre sits 0.833° below a flat, unobstructed horizon.

In zenith terms, the sun is 90° from straight up at its geometric horizon. Adding the refraction allowance gives 90 + 0.833 = 90.833°, which is the value the Sun & daylight tool uses for sunset. The tool’s times are in UTC, so convert them to local time by adding the UTC offset for your place and date.

Civil, nautical and astronomical twilight

Twilight is the period after sunset while the sun is still below the horizon, and it is divided by how far the sun has sunk. The National Weather Service defines each stage by the depth of the sun’s geometric centre below the horizon. The table gives those depths and what the NWS says the light is like at each.

For a walk, civil twilight is the stage that describes the open sky. The horizon and the ground can still be discerned through civil twilight, but civil dusk comes after sunset, so it is a later time and a poor deadline. On a trail under trees the light goes before the open-sky figures suggest.

Terrain and cloud cut the light earlier

The sunset time assumes a clear, flat horizon. A ridge, a valley wall or a line of trees hides the sun before the calculated sunset. The Sun & daylight tool has a horizon input from 0° to 20°, which subtracts an obstruction angle from the sunset calculation. The value is a rough scenario, not a measured horizon, and you need to set it by looking at the skyline you will actually see. Only the skyline in the direction where the sun sets matters for the sunset time, so a high ridge on the far side of the valley does not change it.

For example, at 46.5° N, 8° E on 15 October 2026, the tool gives a sunset of 16:43 UTC with no obstruction. With a 5° horizon obstruction the sunset moves to 16:13 UTC, which is 30 minutes earlier. Cloud and forest canopy also darken the ground earlier than the sunset time suggests, and the tool cannot model either.

Planning a walk-out that ends at sunset

The walk-out time is the walking time from the trailhead to the car or camp, plus a margin. The example uses the solar tool for the sunset and Naismith’s rule through the trail time tool for the walk. A 5° horizon obstruction is set, because the ridge to the west is assumed to hide the sun early.

flat time   = 6 km ÷ 5 km/h = 1.2 h = 72 min
climb time  = 400 m ÷ 100 × 10 min = 40 min
moving time = 72 + 40 = 112 min (1 h 52 min)

latest start to reach sunset = 16:13 − 1 h 52 min = 14:21 UTC
with a 30 min margin          = 14:21 − 30 min = 13:51 UTC

The walking time for the same route comes from the trail time tool. With the default descent setting, Naismith’s rule adds no time for descent, so a steep return does not lengthen the estimate. Choose the gentle or steep descent correction only when the descents on the route match its slope range.

Planning a turn-back time from the sunset

A turn-back time is the latest point at which you start back for the planned finish. It is the sunset figure, less the walk-out time, less a margin for stops and lost route. The turnaround time guide gives the full method, and this page supplies the sunset input. The margin is your decision: a route with a steep descent, a river crossing or a navigation check needs more of it than a well-marked trail.

Walking time estimates from Naismith’s rule assume a steady pace and a fixed allowance for climbing, so they are a starting point, not a forecast for the day. The Naismith’s rule guide explains the rule itself and how it is applied to a route.

Carry a light for the last part of the walk

The sunset figure is a calculated result for an open horizon. A walk that finishes under trees, in a valley or in cloud can end in darkness well before the calculated time. A headlamp is worth carrying even on a short walk that is planned to end well before sunset, because the time of a delay is hard to predict from a map. Carry a headlamp, spare batteries and the other items in the ten essentials guide, whatever the planned finish time. Afternoon weather can also end a walk early, and the lightning safety guide covers how to respond to it on a hike.