Dusk is the period of fading light that follows the sun’s disappearance below the horizon, marking the transition from day into night. Understanding what dusk is requires moving beyond common language to the precise definitions used in atmospheric science. This time casts the world in a soft, diminishing glow, setting the stage for the dramatic colors that paint the western sky as the sun’s illumination wanes.
Sunset, Dusk, and Twilight: Clarifying the Terms
In everyday conversation, sunset, dusk, and twilight are often used interchangeably, but they refer to distinct moments and periods of time. Scientifically, Sunset is the exact instant the upper edge of the Sun drops below the true horizon. This moment is affected by atmospheric refraction, which causes the sun to appear slightly higher than its geometric position.
Twilight is the entire period between sunset and the beginning of true night, when the sky is still illuminated by sunlight scattered in the upper atmosphere. Dusk, in its most technical sense, refers to the end of the final twilight phase when the sky becomes completely dark. Colloquially, however, dusk is commonly used to describe the whole period of fading light after sunset.
The Three Scientific Phases of Dusk
Twilight is scientifically categorized into three phases based on the Sun’s angular position below the horizon; true night begins once the Sun is 18 degrees below the horizon.
Civil Twilight
This phase begins at sunset and lasts until the sun is 6 degrees below the horizon. During this stage, there is sufficient light for most ordinary outdoor activities without artificial lighting, and the horizon remains clearly visible.
Nautical Twilight
This occurs when the Sun is between 6 and 12 degrees below the horizon. The term is historical, as this is the point at which the main stars become visible, allowing sailors to navigate using a faintly discernible horizon line.
Astronomical Twilight
This spans the time the sun is between 12 and 18 degrees below the horizon. The sky appears almost completely dark, though the faintest celestial objects may still be obscured by scattered sunlight.
The Science of Dusk’s Colors
The vivid colors of dusk, featuring yellows, oranges, and deep reds, are the result of Rayleigh scattering. During the day, shorter blue and violet wavelengths are scattered most effectively by atmospheric molecules, making the sky appear blue. At dusk, the sun’s low angle means its light must travel a much longer path through the atmosphere to reach the observer. This extended journey causes almost all the shorter blue and green wavelengths to be scattered away. Only the longer wavelengths, primarily red and orange, pass through relatively unimpeded, creating the warm, striking colors near the horizon during twilight.