The duration of a sunset—the time it takes for the entire solar disk to sink out of sight—is surprisingly variable across the globe. Many people assume a sunset takes a fixed amount of time, yet the actual process can range from as little as two minutes to five minutes or more. This visual change is governed by Earth’s geometry and atmosphere.
Defining the Sunset Event
In astronomy, sunset is precisely defined as the moment when the upper edge, or limb, of the Sun touches and disappears below the horizon. The duration of the sunset event is the time from when the sun first touches the horizon to when its last sliver vanishes. This duration is influenced by the sun’s apparent size and the speed at which it drops relative to the observer.
The sun appears to be on the horizon even when it is geometrically below it due to atmospheric refraction. Earth’s atmosphere bends the light rays, causing the solar disk to appear higher than its true position, effectively lengthening the day. The true measure of the sunset’s duration is the time required for the sun’s 0.5-degree-wide disk to travel its full diameter below the horizon line.
The Role of Latitude
The observer’s latitude is the primary factor determining the speed of a sunset, as it dictates the angle at which the sun approaches the horizon. Near the equator, the sun’s path is almost perpendicular to the horizon line, resulting in the fastest possible sunset. In these tropical regions, the solar disk can disappear completely in approximately two minutes.
As an observer moves toward the poles, the sun’s trajectory becomes increasingly oblique, or slanted, relative to the horizon. This shallow angle means the sun must travel a longer distance along the horizon to fully set, slowing the process considerably. For instance, at a mid-latitude location like London, the sunset can take three to four minutes. The sunset is longest during the winter months when the sun is lowest in the sky and its path is shallowest.
The Phases of Twilight
Once the solar disk has fully disappeared, twilight begins. Twilight is the illumination of the lower atmosphere by sunlight that is already below the horizon. This period is divided into three distinct phases based on the sun’s angle of depression beneath the horizon.
Civil Twilight
The first phase is Civil Twilight, which lasts until the sun is six degrees below the horizon. During this time, there is enough natural light for ordinary outdoor activities, and the horizon remains clearly visible.
Nautical Twilight
Following this is Nautical Twilight, defined by the sun being between six and twelve degrees below the horizon. This stage marks the point where the horizon becomes indistinguishable, historically relevant for sailors navigating by the stars.
Astronomical Twilight
The final stage is Astronomical Twilight, which concludes when the sun reaches eighteen degrees below the horizon. At this point, the sky is considered truly dark, and faint light scattered from the sun’s position no longer interferes with astronomical observations. The total duration of twilight is much longer at higher latitudes, sometimes lasting for hours in polar regions, compared to the tropics where the entire period can be over in less than an hour.
The Atmospheric Optics of Sunset
The colors that accompany sunset are a product of Rayleigh scattering, which explains how sunlight interacts with gas molecules in the atmosphere. Sunlight is composed of a spectrum of colors, with blue and violet having shorter wavelengths than red and orange. During the day, the shorter blue wavelengths are scattered most effectively in all directions by nitrogen and oxygen molecules, making the sky appear blue.
As the sun descends toward the horizon, its light must travel through a much greater thickness of Earth’s atmosphere to reach the observer. This extended path causes nearly all the shorter-wavelength blue light to be scattered away and removed from the direct line of sight. The remaining light is dominated by the longer wavelengths—yellow, orange, and red—which penetrate the atmosphere more directly to create the vibrant hues we observe. The presence of atmospheric particulates, such as dust, smoke, or volcanic ash, can also influence the display, as these larger particles scatter light differently and can intensify the reds and oranges.