After the sun dips below the horizon, the world does not instantly plunge into darkness. Instead, a gradual transition occurs, known as twilight, during which the sky slowly darkens. The exact duration of this period can vary significantly, influenced by several factors that determine how long it takes for true night to arrive.
Defining Sunset and True Darkness
Sunset is the moment the sun’s upper edge disappears below the horizon. “True darkness” or “night” is when the sky becomes dark enough for the faintest stars and celestial objects to be visible without scattered sunlight.
The reason darkness does not fall immediately after sunset lies in the Earth’s atmosphere. Even when the sun is below the horizon, its light interacts with atmospheric particles. This phenomenon, called atmospheric scattering, redirects sunlight back towards the surface, illuminating the lower atmosphere. As the sun sets, light travels through more atmosphere, scattering away blue and allowing reds and oranges to dominate the twilight sky.
The Three Phases of Twilight
The transitional period between day and true night is divided into three distinct phases, each characterized by the sun’s position below the horizon and the resulting light conditions. These phases provide a structured way to understand the gradual descent into darkness.
Civil twilight is the first phase, beginning at sunset and lasting until the sun is 6 degrees below the horizon. During this period, there is enough natural light for most outdoor activities without artificial illumination. The brightest planets and stars may become visible, and the horizon remains clearly defined. Civil twilight can last around 20 to 30 minutes.
Following civil twilight is nautical twilight, which occurs when the sun is between 6 and 12 degrees below the horizon. This phase was historically important for sailors, as enough ambient light remains to distinguish the horizon for navigation. Terrestrial objects become difficult to discern without artificial light. Nautical twilight lasts approximately 30 to 45 minutes in mid-latitudes.
The final phase is astronomical twilight, which begins when the sun is 12 degrees below the horizon and ends when it reaches 18 degrees below. At this point, the sky is considered truly dark, with minimal scattered sunlight remaining. Faint celestial objects, such as nebulae and distant galaxies, become observable, marking the onset of astronomical night. This deepest twilight phase can last from about 30 minutes to an hour or more, depending on location and season.
How Location and Season Affect Darkness
The duration of twilight, and thus how quickly it gets truly dark, is not uniform across the globe; it significantly depends on both latitude and the time of year. These factors influence the angle at which the sun descends below the horizon.
Near the equator, the sun’s path drops almost perpendicularly to the horizon. This steep angle means the sun passes through twilight degrees rapidly, resulting in shorter twilight periods. For instance, civil twilight near the equator can last as little as 24 minutes. As a result, darkness descends quickly in equatorial regions.
Conversely, at higher latitudes, closer to the Earth’s poles, the sun’s path is shallower relative to the horizon. This causes the sun to spend more time traversing each degree of twilight, extending the transitional period. In polar regions, especially during summer, twilight can last for several hours or even weeks, leading to “white nights” where true darkness never fully arrives.
Seasonal changes also affect twilight duration due to the Earth’s axial tilt. Even at the same latitude, twilight periods are longer around the summer solstice and shorter near the equinoxes. This is because the sun’s apparent angle of descent changes throughout the year, with a shallower angle in summer leading to a prolonged twilight.