The lingering illumination after the sun disappears below the horizon is known as twilight. Sunset is defined as the instant the sun’s upper edge completely vanishes below the horizon line. Even after the solar disc has vanished, the atmosphere continues to scatter sunlight from the sun’s position just beneath the horizon, which creates the remaining light.
The Three Defined Stages of Twilight
The period between sunset and true night is categorized into three distinct phases, defined by the sun’s geometric position below the horizon. The first and brightest phase is Civil Twilight, which begins at sunset and lasts until the sun’s center is six degrees below the horizon. This period is the most relevant to remaining light, as there is typically enough illumination for most ordinary outdoor activities without artificial light sources. In mid-latitudes, Civil Twilight generally persists for about 30 minutes, though this duration varies depending on location and time of year.
The next stage is Nautical Twilight, which begins when the sun is six degrees below the horizon and ends when it reaches twelve degrees below. This phase is characterized by a significant decrease in light, though a faint atmospheric glow still exists. Sailors historically relied on this period because the horizon remained visible enough to take readings against the backdrop of newly visible stars for celestial navigation.
The darkest phase is Astronomical Twilight, which lasts from twelve degrees until the sun reaches eighteen degrees below the horizon. Once the sun drops past this 18-degree mark, the sky is considered fully dark, and the faintest sunlight is no longer scattered by the atmosphere. Each of the three twilight stages represents a six-degree drop in the sun’s altitude, and in mid-latitudes, each stage typically lasts 30 to 40 minutes.
Real-World Visibility During Twilight
During Civil Twilight, terrestrial objects are clearly discernible, and the horizon remains sharp. This phase is often colloquially called “dusk.” The brightest planets and stars become visible, but the overall illumination is high enough that most people can continue activities like driving or walking without activating artificial lights. The sky also displays the most vibrant post-sunset colors due to the scattering of light at low angles.
As the sun progresses into Nautical Twilight, the environment darkens substantially, though outlines of objects can still be distinguished against the fading skyglow. Artificial illumination becomes necessary for detailed outdoor work, but the horizon remains visible enough for mariners to use as a reference point. The shift to Astronomical Twilight makes the sky virtually indistinguishable from full night, especially for observers in areas with light pollution.
When the sun sinks beyond the 18-degree threshold, the atmosphere no longer scatters meaningful sunlight toward the observer. At this point, the sky is dark enough for professional astronomers to begin observing the faintest celestial objects, such as distant galaxies and nebulas. The Milky Way becomes visible in non-light-polluted areas, and the outline of the horizon completely disappears.
Understanding Why Twilight Duration Changes
The length of twilight is not fixed and changes depending on the observer’s latitude and the time of year. The primary factor influencing this duration is the angle at which the sun descends below the horizon. Near the equator, the sun’s path is nearly perpendicular to the horizon, meaning it drops below the six-degree mark of Civil Twilight quickly, often in as little as 20 to 25 minutes.
At higher latitudes, the sun’s path is much more oblique, or shallow, relative to the horizon line. This shallow angle means the sun must travel a longer, diagonal distance to sink the full six degrees required to end Civil Twilight, or the full eighteen degrees to end Astronomical Twilight. For example, at 45 degrees latitude, Civil Twilight can last closer to 35 minutes, a noticeable increase from the equatorial duration.
This geometric effect is amplified near the poles, where the sun may skim just below the horizon for an extended period, leading to extremely long twilight phases. During summer in far northern or southern regions, the sun may never sink past the 18-degree mark, or even the six-degree mark. This results in the phenomenon known as “white nights,” where true darkness is never achieved. The Earth’s axial tilt causes the sun to set at its shallowest angle during the summer, resulting in the longest twilight duration of the year for any given latitude.