Dawn and dusk are the familiar transitional periods that bridge the gap between day and night. This change in light is a gradual process, known scientifically as twilight, which occurs when the Sun has dipped below the horizon but still illuminates the atmosphere. The scattering and refraction of sunlight by the Earth’s upper atmosphere prevent the planet from plunging immediately into total darkness after sunset. Twilight is divided into three distinct phases that define the changing sky brightness before sunrise and after sunset.
The Defining Factor: Solar Angle
The precise definition of twilight is determined by the geometric position of the Sun’s center relative to the horizon, measured in degrees. The intensity of light depends entirely on this solar elevation angle, which defines how far the Sun is beneath the horizon line. Once the Sun disappears, its light continues to travel through the upper atmosphere, scattering off air molecules and dust particles to illuminate the sky below. As the Sun sinks deeper, the path its light travels lengthens, causing increased scattering that removes shorter, bluer wavelengths, contributing to the reddish and orange hues seen near the horizon. Scientists use specific angular thresholds, typically in increments of six degrees, to mark the boundaries between the three distinct twilight phases, standardizing the measurement of ambient light.
The Three Stages of Twilight
The brightest phase is Civil Twilight, occurring when the Sun is between 0 degrees and 6 degrees below the horizon. During this period, there is still enough light for terrestrial objects to be clearly distinguished. Outdoor activities can generally be conducted without the aid of artificial illumination, and only the brightest stars and planets, such as Venus, are visible.
The next stage is Nautical Twilight, which begins once the Sun drops to 6 degrees below the horizon and lasts until it reaches 12 degrees below. This phase is named for its historical significance to mariners, as the horizon remains faintly discernible against the sky, allowing sailors to take reliable star sightings for navigation. While the outlines of ground objects may still be visible, artificial light is necessary for detailed outdoor operations.
Finally, the darkest phase is Astronomical Twilight, which occurs when the Sun is positioned between 12 degrees and 18 degrees below the horizon. As the Sun sinks further, the sky illumination becomes so faint that it is virtually indistinguishable from full night to the casual observer. Once the center of the Sun drops below 18 degrees, the sky is considered fully dark, and the delicate light from faint celestial objects like nebulae and distant galaxies becomes visible for astronomical observation.
How Location and Season Affect Timing
The duration of twilight is not fixed and varies significantly based on both geographic latitude and the time of year. Near the equator, where the Sun’s path is nearly perpendicular to the horizon, the period of transition is short, sometimes lasting only about 20 to 25 minutes between sunset and full darkness. The rapid descent minimizes the time available for its light to scatter in the atmosphere.
Moving toward the poles, the Sun’s path dips below the horizon at a much shallower angle, dramatically increasing the duration of each twilight stage. In high-latitude regions during summer, the Sun may not sink far enough below the horizon to reach the 18-degree threshold required for astronomical darkness. This leads to the phenomenon known as “White Nights,” where the sky remains in a perpetual state of civil or nautical twilight from sunset to sunrise, eliminating true nighttime for weeks.
This seasonal variation is most pronounced near the solstices, as the Earth’s axial tilt causes the solar angle to change most slowly during the summer and winter extremes. Northern locations experience their longest twilight periods in the summer, while the same locations see the shortest and most rapid transitions during the winter months.
Practical Applications of Twilight Definitions
The precise definitions of twilight have been adopted across various industries to establish clear, standardized operating conditions for safety and legal compliance. In aviation, the Federal Aviation Administration (FAA) uses the end of evening civil twilight and the beginning of morning civil twilight to legally define the span of “night” for purposes of logging flight time and requiring specific aircraft lighting. Night flying experience, such as carrying passengers, is regulated by the civil twilight period.
These definitions also find utility in legal settings, where civil twilight is frequently used to set the boundaries for activities that depend on natural light. Many jurisdictions use civil twilight to define when vehicle headlights must be activated or when certain outdoor activities, such as hunting, are legally permitted. Photographers also seek the predictable ambient light levels during civil twilight, often referring to this time as the “blue hour” because of the soft, saturated light quality.