Sunset is formally defined as the moment the upper edge of the Sun’s disk disappears below the horizon. Following sunset is a period of gradual transition known as twilight, caused by the atmospheric scattering of sunlight. This process progressively diminishes as the Sun drops further below the horizon. The duration of this transition changes based on location and season, leading to three distinct, scientifically defined stages of waning light.
The Practical Darkness of Civil Twilight
The first phase immediately following sunset is known as Civil Twilight, which lasts until the geometric center of the Sun is six degrees below the horizon. This period represents the general public’s most common understanding of “dusk,” as there is still a significant amount of ambient light. During this time, the horizon remains clearly defined, and terrestrial objects are easily distinguishable without the need for artificial illumination.
This phase has practical and legal implications because the illumination is sufficient for most outdoor activities to continue unimpeded. Many jurisdictions use the end of civil twilight as the standard for when vehicle headlights must be turned on. The brightest stars and planets, such as Venus and Jupiter, begin to become visible in the sky during the latter half of this period.
In locations closer to the equator, the duration of civil twilight is shorter, sometimes lasting only about 24 minutes. Conversely, at higher latitudes, this stage stretches out considerably longer. The end of this six-degree drop marks the point at which artificial lighting becomes a necessity for comfortable vision outdoors.
Defining True Darkness Nautical and Astronomical Twilight
The transition to genuine darkness is categorized by two subsequent phases, defined by the Sun’s increasing depression angle below the horizon. Nautical Twilight begins when the Sun is six degrees below the horizon and lasts until it reaches 12 degrees of depression. During this stage, the sky darkens significantly, and while the general outline of the landscape may still be perceptible, the horizon line itself becomes very difficult to distinguish.
This phase earned its name because, historically, mariners relied on the ability to see the horizon to take accurate celestial navigation readings with a sextant. Once the Sun drops past the 12-degree mark, the sea and sky merge into an indistinct dark mass, making traditional navigation methods impossible. By the end of nautical twilight, many more stars are visible, but residual sunlight still prevents the faintest celestial objects from being observed.
The final stage is Astronomical Twilight, which spans the time from the Sun being 12 degrees below the horizon until it reaches a depth of 18 degrees. This phase represents the final fading of all scattered sunlight from the atmosphere.
The 18-degree mark is the scientific threshold for night, as the scattered solar light is weaker than the natural light emitted by celestial sources. For astronomers, this is the point when the sky is dark enough to observe the most distant galaxies and nebulae. In most mid-latitude locations, the entire process, from sunset through all three twilight stages to true darkness, typically takes between 70 and 100 minutes.
Why Twilight Duration Changes
The length of time required to cycle through the three twilight stages varies dramatically depending on two main geographical and orbital factors. The primary factor is the observer’s latitude, which determines the angle at which the Sun appears to set. Closer to the equator, the Sun’s path drops sharply, nearly perpendicular to the horizon.
This steep angle means the Sun covers the 18 degrees of twilight depression very quickly, resulting in a rapid transition to darkness. Conversely, at higher latitudes, the Sun sets at a much shallower, more oblique angle relative to the horizon. This shallow angle means the Sun takes a longer time to move through those 18 degrees, extending the twilight period significantly.
The time of year also influences the duration of twilight due to the Earth’s axial tilt. Twilight is shortest around the spring and autumn equinoxes, when the Sun is directly over the equator and its setting path is at its steepest. The longest twilight periods occur around the summer and winter solstices. Near the poles, this effect is so pronounced that during the summer solstice, the Sun may never drop below the 18-degree threshold, meaning true darkness never occurs, a phenomenon known as the “midnight sun.”