The moment the sun visibly disappears below the horizon marks the technical definition of “sunset,” yet a period of lingering light, known as twilight, always follows. This transition period continues until the sky reaches a state of true darkness, or night. Twilight is a natural phenomenon caused by the Earth’s atmosphere acting as a massive diffuser for the sun’s rays.
Why Light Lingers After Sunset
Light continues to illuminate the sky after sunset because of atmospheric scattering, primarily a phenomenon called Rayleigh scattering. This process involves sunlight interacting with the tiny molecules of nitrogen and oxygen in the Earth’s atmosphere. Even when the sun is no longer visible from the surface, its rays still pass through the upper layers of the atmosphere above the horizon.
The atmosphere redirects this sunlight back toward the Earth’s surface. Shorter wavelengths of light, such as blue and violet, are scattered much more easily than longer wavelengths like red and orange. During the day, this scattering makes the sky appear blue, but at sunset, the light must travel a much longer path through the atmosphere. This extended path means most of the blue light is scattered away, leaving the longer red and orange wavelengths to dominate the sky immediately after the sun disappears.
The Three Official Stages of Twilight
The period of twilight is precisely divided into three sequential stages, each defined by the sun’s angular position below the horizon, or its solar depression angle. The first stage, which begins immediately after sunset, is called Civil Twilight.
Civil Twilight
Civil Twilight lasts until the sun reaches 6 degrees below the horizon. During this time, there is enough light for most outdoor activities to occur without artificial illumination. The horizon remains clearly visible, and the brightest stars and planets become discernible in the fading sky. Civil dusk marks the end of this phase.
Nautical Twilight
The next stage is Nautical Twilight, which occurs while the sun is between 6 and 12 degrees below the horizon. As the sky darkens, the horizon becomes less distinct, though it is usually still visible in clear conditions. This stage is named for the time when sailors would use a sea horizon and visible stars to navigate.
Astronomical Twilight
Astronomical Twilight begins when the sun is 12 degrees below the horizon and ends at 18 degrees below. During this time, the sky appears completely dark to the casual observer. However, the sky is not yet fully dark for professional observations, as a faint residual illumination can still interfere with viewing very dim celestial objects.
How Location Affects Twilight Length
The duration of these three twilight stages is not constant across the globe; instead, it is strongly dependent on the observer’s latitude and the time of year. This variability occurs because the angle at which the sun descends below the horizon changes significantly based on geographic location.
Near the Earth’s equator, the sun sets almost perpendicularly to the horizon, causing it to pass through the 18-degree twilight zone very quickly. In these tropical locations, the entire twilight period can last as little as 72 minutes, with each of the three stages taking about 24 minutes.
Conversely, at higher latitudes, such as in the temperate zones and near the poles, the sun sets at a much shallower, oblique angle. This means the sun takes much longer to drop the full 18 degrees below the horizon, stretching the duration of twilight. For instance, in London, the civil twilight period can last between 33 and 48 minutes, depending on the season. At latitudes above 48 degrees, near the summer solstice, the sun may not sink more than 18 degrees below the horizon at all, causing a phenomenon known as “white nights” where true darkness never occurs.
When True Darkness Begins
The moment “true darkness,” or night, officially begins is precisely defined by the end of astronomical twilight. This occurs when the geometric center of the sun drops to 18 degrees below the horizon. At this point, the faint solar illumination scattered in the upper atmosphere is finally negligible.
When the sun is below this 18-degree threshold, the sky is dark enough for the most sensitive astronomical observations. The sky’s illumination is minimal, allowing astronomers to effectively study diffuse light sources such as distant nebulae and galaxies.