Sunset is defined as the instant the upper edge of the solar disk drops below the horizon. True darkness is delayed because the Earth’s atmosphere continues to scatter the sun’s light long after the visible disk is gone. This scattered light, known as twilight, slowly diminishes as the sun sinks deeper beneath the horizon, creating a gradual transition to night. This dimming period is segmented into three distinct phases based on the sun’s angular position relative to the observer.
The Three Stages of Twilight
The transition from day to night is divided into three official stages. The first phase, Civil Twilight, begins immediately after sunset and lasts until the sun reaches six degrees below the horizon. During this period, there is enough ambient light for ground-level objects to be clearly visible. Most normal outdoor activities can be conducted without artificial illumination, and while the brightest stars may appear, the horizon remains sharply defined.
Following Civil Twilight, the sky enters Nautical Twilight, which continues until the sun is twelve degrees below the horizon. The ambient light has faded significantly, making the horizon difficult or impossible to discern. This historically complicated navigation for sailors. While the outlines of terrestrial objects are largely lost, many bright stars become easily visible.
The final stage is Astronomical Twilight, which lasts until the sun is eighteen degrees below the horizon. This period filters the last remnants of sunlight from the upper atmosphere, and the sky is considered truly dark for observational purposes. Once the sun drops past the eighteen-degree mark, the atmosphere is no longer illuminated, allowing astronomers to view the faintest celestial objects, such as distant nebulae and galaxies.
How Long Does Each Phase Last?
The duration of the entire twilight period, from sunset to true darkness, varies widely, but in temperate latitudes, it spans between 90 and 120 minutes. Each of the three six-degree segments requires a specific amount of time for the sun to traverse. Civil Twilight often lasts approximately 20 to 30 minutes, providing the initial buffer of light after the sun dips out of sight.
Nautical Twilight lasts an additional 25 to 35 minutes, during which the sky deepens to a dark blue. Astronomical Twilight adds another 30 to 40 minutes to the total time. For an observer in the mid-latitudes, the sky achieves the blackness of true night roughly an hour and a half to two hours after the official sunset time.
Why Location Matters
The most significant factor determining the length of twilight is the observer’s latitude, which dictates 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 the necessary eighteen degrees very quickly. Due to this steep angle, the entire twilight period is compressed, with civil twilight sometimes lasting as little as 23 to 24 minutes.
Conversely, at higher latitudes, the sun sets at a much shallower, more oblique angle. This geometry means the sun takes much longer to cover the required eighteen degrees of vertical drop. In summer at latitudes above 50 degrees, the sun may not sink far enough below the horizon to exit the nautical or even the civil twilight phase before it begins to rise again. This results in prolonged summer twilights, sometimes called “white nights,” where the sky never achieves true darkness.
The Influence of Seasons and Cloud Cover
The duration of twilight exhibits seasonal variation, even at the same latitude, due to the Earth’s axial tilt. During the summer months in the mid-latitudes, the sun’s trajectory at sunset creates a shallower angle with the horizon, causing twilight to be longer. In contrast, the sun’s path is steeper around the winter solstice, leading to a faster transition through the twilight phases and a shorter duration overall.
While the astronomical definitions of twilight remain fixed regardless of weather, cloud cover drastically influences the perception of darkness. On clear nights with minimal artificial illumination, the sky is at its darkest because all light sources, including starlight and natural airglow, are visible. In heavily light-polluted environments, an overcast sky can reflect ground light back downward, making the sky appear brighter and delaying the subjective feeling of true darkness.