Sunset marks the moment the upper edge of the sun disappears below the horizon. However, immediate darkness does not follow this event because Earth’s atmosphere continues to scatter sunlight. This scattering process creates a gradual transition from daylight to full darkness.
The Period After Sunset
After the sun dips below the horizon, a period known as twilight begins. During this time, the sun is no longer directly visible, but its light still illuminates the upper layers of the atmosphere. This phenomenon causes the sky to remain illuminated as the sun descends further.
Defining Levels of Darkness
The transition from sunset to complete darkness is categorized into three distinct phases of twilight, each defined by the sun’s angular position below the horizon.
Civil twilight is the first phase, occurring when the sun is between 0 and 6 degrees below the horizon. During this period, the horizon remains clearly visible, and the brightest stars or planets may become apparent. This phase lasts for 30 minutes.
Following civil twilight is nautical twilight, which occurs when the sun is between 6 and 12 degrees below the horizon. As the sun descends further, the horizon begins to become indistinct, making it challenging to discern the boundary between the sea and the sky. Fainter stars become visible during this phase, and artificial lighting may be necessary for outdoor activities. Nautical twilight also lasts 30 minutes.
The final phase is astronomical twilight, which takes place when the sun is between 12 and 18 degrees below the horizon. At this point, the sky is considered truly dark, and all but the very faintest celestial objects are visible without interference from scattered sunlight. Full darkness arrives once the sun descends more than 18 degrees below the horizon. This final phase also spans 30 minutes, leading to a total twilight period of roughly 90 minutes.
Why Darkness Duration Varies
The duration of twilight is not constant and depends on several factors. Twilight periods are shorter near the equator and progressively longer closer to the Earth’s poles. This is because the sun sets at a steeper angle at lower latitudes, causing it to descend below the 18-degree threshold more quickly.
The time of year also influences the length of twilight. Due to Earth’s axial tilt, the sun’s setting angle changes throughout the year. In extreme polar regions, during specific seasons, the sun may not descend far enough below the horizon for astronomical twilight to occur at all, resulting in prolonged periods of lingering light.
Practical Aspects of Nightfall
Understanding the phases of twilight provides practical insights into when certain light conditions can be expected. For most outdoor activities, light persists through civil twilight. Once nautical twilight begins, artificial illumination becomes necessary for safe navigation or tasks like driving.
True darkness, suitable for activities like stargazing or astrophotography, only arrives after astronomical twilight has concluded. The precise timing for full darkness is variable, depending on one’s specific geographic location and the current season. To determine the exact times for sunset and the various twilight phases, individuals can use local sunset and sunrise calculators or dedicated astronomical applications.