How Soon Does It Get Dark After Sunset?

The transition from daylight to darkness after sunset is a gradual process, governed by the Sun’s position relative to the horizon and Earth’s atmosphere. Many people wonder how quickly the sky truly darkens once the sun dips below the horizon.

Understanding Sunset and Twilight

“Sunset” occurs when the upper edge of the Sun disappears below the horizon. This definition accounts for atmospheric refraction, which bends sunlight and makes the Sun appear higher than it actually is. After sunset, a period of indirect illumination known as “twilight” begins. This phase is characterized by diffuse sky radiation, where sunlight from the upper atmosphere continues to scatter, illuminating the lower atmosphere and Earth’s surface.

The Gradual Descent into Darkness

Twilight is divided into three phases, each marked by the Sun’s progressively lower position below the horizon. Each phase lasts for a typical duration, leading to a gradual reduction in ambient light.

The first phase, civil twilight, starts at sunset and continues until the Sun is 6 degrees below the horizon. During this time, there is enough natural light for most outdoor activities, and the brightest stars and planets may become visible. Civil twilight typically lasts 20 to 30 minutes.

Following civil twilight, nautical twilight begins when the Sun is between 6 and 12 degrees below the horizon. This phase was historically significant for sailors, as the horizon remains visible, allowing for navigation by prominent stars. Artificial lighting is generally required for detailed outdoor tasks. Nautical twilight usually lasts 20 to 25 minutes.

The final stage is astronomical twilight, which occurs when the Sun is between 12 and 18 degrees below the horizon. During this phase, the sky becomes dark enough for most celestial objects to be observed. Beyond 18 degrees below the horizon, true darkness arrives, as almost no indirect sunlight illuminates the sky. This phase also typically lasts 20 to 25 minutes. The entire process from sunset to complete darkness can take roughly an hour to an hour and a half.

Factors Affecting Twilight’s Length

The duration of twilight varies based on geographical location and the time of year. Latitude plays a significant role; twilight tends to last longer at higher latitudes, those closer to the Earth’s poles. This occurs because the Sun’s apparent path at these latitudes is at a shallower angle relative to the horizon. Consequently, it takes the Sun more time to descend through each 6-degree segment that defines the twilight phases. Conversely, near the equator, the Sun sets at a more direct, steeper angle, resulting in a quicker transition through the twilight stages and shorter twilight durations, sometimes as little as 21 to 24 minutes for civil twilight.

The time of year also influences twilight’s length. Due to the Earth’s axial tilt and its orbit around the Sun, the Sun’s angle relative to the horizon changes throughout the year. At mid-latitudes, civil twilight can last longer during summer and winter and be shorter in spring and fall. This seasonal variation is less pronounced near the equator, where twilight durations remain relatively consistent throughout the year.

The Science Behind Fading Light

The gradual fading of light after sunset is a consequence of atmospheric scattering, primarily a phenomenon called Rayleigh scattering. As sunlight enters Earth’s atmosphere, it interacts with tiny gas molecules and particles. Shorter wavelengths of light, such as blue and violet, are scattered more efficiently than longer wavelengths like red and orange.

Even after the Sun has set below the horizon, its light continues to travel through the upper layers of the atmosphere. This scattered light then illuminates the lower atmosphere and the ground, creating the various stages of twilight. As the Sun sinks further below the horizon, fewer sunlight rays reach the atmosphere at an angle that allows for scattering back towards the observer, causing the sky to progressively darken.