The time between the disappearance of the Sun below the horizon and the onset of complete darkness is a transitional period often loosely called “dusk.” This period is characterized by sunlight scattered by the Earth’s upper atmosphere, even after the direct solar disk is no longer visible. Astronomers and meteorologists precisely define this time, dividing it into three distinct phases based on the Sun’s position. These measured boundaries have broad applications, from maritime navigation to setting legal requirements for public lighting.
Defining Twilight and Dusk
The terms “twilight,” “dusk,” and “sunset” describe three related but separate astronomical events. Sunset is the instantaneous moment when the Sun’s upper edge drops below the horizon, marking the end of the day. Twilight is the entire duration following sunset when the atmosphere remains illuminated by diffused sunlight.
Dusk is the specific term used to denote the conclusion of the evening twilight period, marking the instant the sky transitions into night. Twilight is a span of time, while dusk is a single, precise moment marking the end of the final phase. The entire period of fading light is collectively known as evening twilight, which moves through three defined stages before true darkness.
The Three Official Phases of Twilight
The atmosphere’s diminishing illumination after sunset is categorized into three sequential phases: Civil, Nautical, and Astronomical twilight. Civil twilight is the brightest phase and begins immediately after sunset. During this time, there is enough residual light for terrestrial objects to be clearly distinguished, and outdoor activities can generally proceed without artificial lighting. Only the brightest stars and planets are visible.
Following the civil phase, the sky enters nautical twilight. The horizon remains dimly discernible, but the overall illumination is significantly reduced. This stage is named for its historical utility to mariners, who used the distinct horizon line for celestial navigation. As the Sun sinks further, the sky transitions into astronomical twilight, the darkest phase.
Astronomical twilight is the final stage before true night, reached when solar illumination is so faint it no longer interferes with the observation of the faintest celestial objects. The end of astronomical twilight marks astronomical dusk and the official beginning of night.
The Mechanism of Measurement
The precise timing of the three twilight phases is determined by the angular position of the Sun’s center relative to the horizon. The horizon is the zero-degree reference point, and the Sun’s depth below this line is measured in degrees of depression. The entire twilight period spans the time when the Sun is between 0 and 18 degrees below the horizon.
Civil twilight is defined as the time the Sun is between 0 and 6 degrees below the horizon. Civil dusk occurs when the Sun reaches 6 degrees of depression. Nautical twilight then follows, defined by the Sun’s position between 6 and 12 degrees below the horizon, ending at nautical dusk when the Sun is at 12 degrees of depression.
The final phase, astronomical twilight, spans the time the Sun is between 12 and 18 degrees below the horizon. The period ends, and true night begins, at astronomical dusk, the moment the Sun reaches 18 degrees below the horizon.
Practical Applications of Dusk Time
The precise definitions of the twilight phases are utilized by various industries and government agencies to establish standards for safety and operations. Civil dusk (Sun 6 degrees below the horizon) is frequently used as the legal benchmark for public lighting regulations. Many jurisdictions mandate that vehicle headlights and streetlights must be switched on at or near this time to ensure public safety.
In the aviation industry, flight regulations often classify “night” hours for logging airtime as beginning after the end of civil twilight. Nautical twilight is still used in celestial navigation as the optimal period to measure stars against a visible horizon. For astronomers, the end of astronomical twilight is the definitive moment when the sky is dark enough for sensitive and detailed observations of distant galaxies and nebulae, eliminating solar interference.