The terms “sunset” and “dusk” describe two distinct astronomical phenomena. Sunset is a precise, instantaneous event. Dusk, conversely, is an extended period of time following sunset, marking the gradual transition from daytime to full night. Understanding the difference requires looking at the scientific definitions for when the Sun crosses the horizon and how its light persists afterward.
Sunset: The Instantaneous Event
Sunset is defined astronomically as the exact moment the Sun’s upper limb, or top edge, disappears below the theoretical horizon. This instantaneous event is calculated for a standard observer at sea level. The published time is based on when the final sliver of the visible solar disk vanishes, not when the Sun’s center crosses the line. Due to atmospheric refraction, the Earth’s atmosphere bends the sunlight, lifting the apparent position of the Sun. This means the Sun is already geometrically below the horizon when we observe sunset.
Dusk: The Period of Fading Light
Dusk is the period of fading illumination that immediately follows sunset, lasting until the sky is completely dark. This period is characterized by the gradual decrease in light as the Sun sinks further below the horizon. The remaining illumination is not direct sunlight but is caused by atmospheric scattering.
The upper layers of the atmosphere scatter the Sun’s light downward, creating the diffused glow that illuminates the landscape. The duration of this period varies significantly depending on the observer’s latitude, lasting longer closer to the poles. The entire period of dusk is technically known as twilight, which describes the time between full daylight and true night.
Measuring Twilight: Defining the Stages
The period of twilight is divided into three distinct stages, each defined by the Sun’s angular distance below the horizon. These stages allow scientists and navigators to quantify the level of natural light available. They move sequentially, starting immediately after sunset and concluding when the Sun is 18 degrees below the horizon.
Civil Twilight
Civil twilight begins at sunset and lasts until the center of the Sun is 6 degrees below the horizon. During this period, the sky remains light enough that most ordinary outdoor activities can be performed without artificial illumination. The horizon is clearly discernible, and the brightest planets and stars become visible. Many jurisdictions define “lighting-up time” for vehicles based on the beginning or end of civil twilight.
Nautical Twilight
The second phase, nautical twilight, spans the time when the Sun is between 6 and 12 degrees below the horizon. By this point, the natural light has faded considerably, making the horizon difficult to distinguish without aid. This stage historically received its name because mariners relied on seeing the horizon and certain stars simultaneously to navigate using celestial bodies.
Astronomical Twilight
Astronomical twilight is the darkest phase, occurring when the Sun is between 12 and 18 degrees below the horizon. During this period, the atmosphere no longer scatters appreciable sunlight, and the sky appears fully dark to the casual observer. True night, when even the faintest celestial objects can be observed, only begins after the Sun passes the 18-degree mark.