Sunset marks when the Sun appears to vanish below the horizon. It does not plunge the world into immediate darkness. Instead, a period of gradual light reduction, known as twilight, follows. The Earth’s atmosphere continues to scatter sunlight even after direct rays are no longer visible, creating a lingering glow that slowly diminishes.
The Phases of Twilight
Twilight is defined by the Sun’s position below the horizon, divided into three distinct phases, each progressively darker. Sunlight continues to be scattered by the upper atmosphere, illuminating the lower atmosphere even when the direct light source is obscured.
Civil twilight is the first phase, beginning at sunset and lasting until the Sun’s center is 6 degrees below the horizon. 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 discernible. This phase commonly lasts between 20 to 30 minutes.
Nautical twilight follows civil twilight, extending from when the Sun is 6 degrees to 12 degrees below the horizon. The sky becomes noticeably darker, and the horizon starts to become indistinct. Artificial lighting is needed for clear visibility, and more stars become apparent. This phase gets its name because sailors historically relied on the visibility of stars and a discernible horizon for navigation at sea.
Astronomical twilight represents the darkest phase, occurring when the Sun is between 12 and 18 degrees below the horizon. The sky is nearly completely dark, especially in areas with minimal light pollution. The horizon is no longer visible, and the faintest celestial objects, such as nebulae and distant galaxies, can be observed. Once the Sun dips beyond 18 degrees below the horizon, true night begins.
What Affects Twilight’s Length
The duration of twilight varies based on geographical and seasonal factors. Latitude is a primary influence, determining the angle at which the Sun descends below the horizon. Closer to the equator, the Sun’s path is nearly perpendicular, leading to shorter twilight periods. Conversely, at higher latitudes closer to the poles, the Sun’s path is at a shallower angle, resulting in extended twilight durations.
The time of year also plays a role, influenced by the Earth’s axial tilt and its orbit around the Sun. Twilight tends to be longer around the summer and winter solstices due to the sun’s more oblique angle of descent. In contrast, around the spring and autumn equinoxes, its path is more direct, leading to shorter twilight periods. In some very high-latitude regions during summer, the Sun may not dip below certain twilight thresholds, leading to continuous twilight or “white nights.”
Atmospheric conditions can also influence the perceived brightness of twilight. Factors such as cloud cover, humidity levels, and atmospheric dust can scatter or absorb sunlight differently. This affects how much light reaches the ground and how clearly objects are seen, making twilight appear brighter or darker.
Experiencing the Dimming Light
The gradual dimming of light during twilight impacts everyday activities. This period represents a natural transition from full daylight to complete darkness, offering a unique visual transformation of the sky. It highlights the continuous interplay between Earth’s atmosphere and the Sun’s position, guiding our perception of day and night.