Many believe darkness falls immediately after sunset. However, the transition from daylight to true night is a gradual process, influenced by the Earth’s atmosphere and its interaction with sunlight. The period following sunset, known as twilight, extends visibility due to atmospheric phenomena. This lingering light means darkness does not fall instantly at the moment of sunset.
Understanding Sunset: More Than Just a Moment
Astronomical sunset is formally defined as the precise moment when the upper edge of the sun’s disk disappears below the horizon. Despite this definition, light continues to be visible for some time afterward. Even after the sun has geometrically set, the atmosphere still plays a significant role in illuminating the sky.
The Science of Twilight: Why Darkness Lingers
The primary reason for lingering light after sunset is the scattering of sunlight in the Earth’s upper atmosphere. Even when the sun is no longer directly visible, its rays still illuminate the atmosphere above the observer. This light is then scattered by atmospheric particles and gases, redirecting it back towards the Earth’s surface, creating the diffuse illumination known as twilight.
Atmospheric refraction also contributes to the extended period of light. The Earth’s atmosphere bends light rays, making celestial objects, including the sun, appear higher in the sky than their actual geometric position. This bending of light allows the sun to remain visible for several minutes even after it has geometrically dipped below the horizon, delaying the onset of complete darkness. This ensures some illumination persists, even without direct sunlight.
Phases of Twilight: From Dusk to Night
The transition from sunset to full night occurs in distinct phases of twilight, each characterized by the sun’s angular position below the horizon and the resulting level of natural light.
Civil twilight is the first phase, beginning at sunset and lasting until the sun’s center is 6 degrees below the horizon. During this period, there is enough natural light for outdoor activities without artificial illumination. The horizon remains clearly defined, and only the brightest stars and planets become visible.
Following civil twilight is nautical twilight, which occurs when the sun’s center is between 6 and 12 degrees below the horizon. During this phase, the natural light diminishes significantly, making the horizon difficult to discern at sea, which was historically important for navigation using stars. Artificial lighting generally becomes necessary for most outdoor activities.
The final stage is astronomical twilight, defined as the period when the sun’s center is between 12 and 18 degrees below the horizon. At this point, the sky is dark enough for most astronomical observations because residual sunlight is minimal. True night begins once the sun descends more than 18 degrees below the horizon, as virtually all scattered sunlight ceases to illuminate the sky.
Factors Affecting Post-Sunset Visibility
Several external factors can influence the duration and perceived brightness of twilight, altering how quickly darkness sets in after sunset.
Cloud cover can either enhance or diminish the lingering light. Clouds can reflect scattered sunlight back towards the ground, making the sky appear brighter, especially if they are high-level clouds positioned to catch the sun’s rays from below the horizon. Conversely, thick, low-lying clouds can block residual light, leading to a quicker onset of perceived darkness.
Light pollution, originating from artificial lights in urban and suburban areas, creates an artificial glow in the night sky. This “skyglow” can significantly obscure the natural progression of darkness during twilight, making it harder to distinguish fainter celestial objects. In heavily light-polluted regions, the sky may never truly reach the darkness of astronomical night.
The season and latitude also play a substantial role in twilight duration. At higher latitudes, such as those closer to the poles, the sun sets at a shallower angle relative to the horizon, meaning it takes longer for the sun to drop through each twilight phase. This results in significantly longer periods of twilight, sometimes lasting for several hours or even all night during summer months. Conversely, near the equator, the sun sets almost perpendicularly to the horizon, leading to a much shorter twilight period, typically around 24 minutes for civil twilight.