The moment the Sun’s upper edge dips below the horizon marks sunset, but darkness is not instantaneous. The transition from day to night is a gradual, predictable process known as twilight. This drawn-out fading of light occurs because the Earth’s atmosphere efficiently scatters sunlight, even after the Sun is no longer visible from the surface. The duration of this period is governed by precise astronomical measurements of the Sun’s position relative to the horizon.
Understanding the Three Stages of Twilight
Scientists define the darkening process by tracking the Sun’s angle below the horizon, dividing the period into three distinct phases. The first phase is Civil Twilight, which begins immediately after sunset and continues until the Sun is six degrees below the horizon. During this time, there is enough ambient light for most outdoor activities without artificial lighting, and the brightest planets and stars become visible.
As the Sun drops further, the light transitions into Nautical Twilight, spanning the time the Sun is between six and twelve degrees below the horizon. This phase is named for when sailors traditionally needed to see the horizon and the stars simultaneously for celestial navigation. While outlines of terrestrial objects are still discernible, the natural light is too faint for detailed outdoor tasks without a supplementary source.
The final stage is Astronomical Twilight, which lasts until the Sun reaches eighteen degrees below the horizon. At this point, the last traces of scattered sunlight have disappeared from the sky. Once the Sun is fully eighteen degrees below the horizon, true darkness has arrived, allowing the faintest celestial objects to be observed.
Standard Duration for True Darkness
The total time required to pass through these three stages depends on how quickly the Sun’s position changes. For locations in the mid-latitudes, such as the central United States or Europe, each twilight stage lasts for about 30 to 40 minutes. This means Civil Twilight concludes approximately half an hour after sunset.
Nautical Twilight adds another half hour, bringing the total time since sunset to roughly 60 to 80 minutes. Astronomical Twilight also takes about 30 to 40 minutes to complete. This results in the total elapsed time from sunset until the sky reaches its darkest state being around 90 to 120 minutes.
The practical answer is that it takes approximately one and a half to two hours after sunset to get truly dark. This duration is a generalized average, and the precise timing is subject to the geographical location of the observer.
How Location and Season Change the Timing
The duration of twilight is not uniform across the globe, with latitude being the most significant variable. Near the Equator, the Sun’s path appears almost perpendicular to the horizon as it sets, causing it to drop the necessary degrees quickly. This near-vertical trajectory means that twilight is compressed, with Civil Twilight lasting as little as 24 minutes.
In contrast, at higher latitudes, the Sun sets at a much shallower, more oblique angle relative to the horizon. It takes a considerably longer time to descend through the required six-degree increments for each twilight stage. This effect is most pronounced near the poles, where the Sun may never drop the full eighteen degrees during summer, resulting in a “white night” where the sky never achieves true darkness.
Seasonal changes also affect twilight duration in temperate zones due to the Earth’s axial tilt. Twilight tends to be slightly longer in the summer and shorter in the winter at mid-latitudes. Local atmospheric conditions, such as heavy cloud cover or pollution, can make the sky feel darker sooner, but they do not change the precise astronomical duration of the twilight stages.