The transition from sunset to complete darkness is gradual, with the sky darkening as the sun dips below the horizon. This intermediary phase, often experienced as dusk, involves a series of distinct stages, each characterized by varying levels of natural light. Understanding this transition explains why the time it takes for night to fall can differ depending on location and time of year.
The Concept of Twilight
Twilight refers to the period when the sun is below the horizon, but its light is still visible in the sky due to scattering and reflection by the Earth’s atmosphere. This phenomenon occurs because sunlight strikes particles and gases in the upper atmosphere. These atmospheric components then scatter the light, diffusing it, creating the characteristic glow of twilight. As the sun descends further, less sunlight reaches the upper atmosphere, leading to a continuous decrease in sky brightness until complete darkness.
Understanding the Stages of Twilight
The transition from sunset to night is divided into three stages, each defined by the sun’s angular position below the horizon. These stages mark a progressive dimming of the sky, influencing visibility and the appearance of celestial objects.
Civil Twilight
Civil twilight is the first stage after sunset, occurring when the sun is between 0 and 6 degrees below the horizon. During this period, there is typically enough natural light for most outdoor activities without artificial illumination. Only the brightest planets and stars become visible in the sky. This phase typically lasts 24 to 30 minutes.
Nautical Twilight
Following civil twilight is nautical twilight, which occurs when the sun is between 6 and 12 degrees below the horizon. During this stage, the horizon often becomes indistinct, making it challenging to differentiate between the sea and the sky. Brighter stars used for navigation become more prominent, allowing mariners to take reliable sightings.
Astronomical Twilight
The final stage is astronomical twilight, defined as the period when the sun is between 12 and 18 degrees below the horizon. Once the sun drops more than 18 degrees below the horizon, complete darkness, or astronomical night, begins. During astronomical twilight, even the faintest celestial objects become observable, as scattered sunlight fades. Nautical and astronomical twilight periods typically last 40 to 50 minutes.
Factors Influencing Twilight’s Length
The duration of twilight is not constant and is influenced by several geographical and seasonal factors. These variables alter the angle at which the sun appears to set, directly affecting how quickly it descends through the different twilight stages.
Latitude
Latitude plays a role in determining twilight’s length. Near the equator, the sun sets almost perpendicularly to the horizon, causing it to pass through the twilight stages relatively quickly. This results in shorter twilight periods, with civil twilight lasting as little as 24 minutes. In contrast, at higher latitudes closer to the poles, the sun sets at a much shallower angle, meaning it takes longer for it to dip below the specific angular thresholds for each twilight phase. Consequently, twilight can last for hours, or even weeks in extreme polar regions.
Seasonal Changes
Seasonal changes also affect twilight duration due to the Earth’s axial tilt. In summer, especially at higher latitudes, the sun’s path causes it to descend at a more gradual angle relative to the horizon, leading to extended twilight periods. Conversely, during winter, the sun’s path is steeper, resulting in shorter twilight durations. The shortest twilight periods occur around the equinoxes, when the sun’s apparent daily path is most directly perpendicular to the horizon.
Atmospheric Conditions
Atmospheric conditions, such as the presence of clouds, haze, or pollution, can impact the perceived brightness during twilight. While these conditions do not change the astronomical duration of the twilight phases, they can reduce the amount of sunlight scattered, making the sky appear darker than it would under clear conditions. For instance, heavy cloud cover can obscure the faint light of twilight, making the transition to night seem more abrupt.