When the sun dips below the horizon, the shift from day to night is a gradual process of diminishing light known as twilight. This period is often casually referred to as dusk, though the terms have distinct astronomical meanings describing the transition from full daylight to complete darkness. The entire span of evening light attenuation is technically the evening twilight, which is subdivided into three scientifically defined phases based on the sun’s precise angle below the horizon.
Defining Dusk and Its Relation to Sunset
Sunset is the precise moment when the upper edge of the sun’s disc disappears below the horizon, marking the end of daytime. Light persists afterward because the sun still illuminates the upper atmosphere, scattering light back toward the ground. Dusk, in its most technically accurate sense, refers to the end of the evening twilight period, just before true night begins. While often used interchangeably with the entire twilight phase, dusk specifically denotes the final, darkest moment before total darkness.
The Three Phases of Twilight
Astronomers divide the evening twilight into three phases, each defined by the angular distance of the sun’s center below the horizon.
Civil Twilight
The first phase, Civil Twilight, begins immediately after sunset and continues until the sun is 6 degrees below the horizon. During this time, there is enough natural light for outdoor activities without artificial illumination, and only the brightest planets or stars are visible.
Nautical Twilight
Following Civil Twilight is Nautical Twilight, which occurs while the sun is between 6 and 12 degrees below the horizon. During this phase, the horizon line begins to become indistinguishable from the sky, which historically complicated navigation for sailors. Most stars are visible, and artificial lighting is required for outdoor work.
Astronomical Twilight
The final phase is Astronomical Twilight, which lasts from 12 degrees until the sun reaches 18 degrees below the horizon. At this point, the sky is dark enough that the remaining scattered sunlight is imperceptible to the naked eye. Once the sun’s center drops below 18 degrees, the sky transitions into true night, marking the end of the evening twilight and the technical moment of astronomical dusk.
The Astronomy of Fading Light
The sky does not instantly turn black at sunset due to atmospheric scattering, specifically Rayleigh scattering. Even below the horizon, the sun’s light travels through the atmosphere at high altitudes, where gas molecules scatter light back toward the Earth’s surface.
As the sun sinks lower, the sunlight must travel through an increasingly larger portion of the atmosphere, causing more shorter-wavelength blue light to be scattered away. This increased atmospheric path length allows longer-wavelength red and orange light to penetrate more directly, creating the characteristic colors of twilight. The gradual darkening is a direct result of the Earth’s curvature, which progressively blocks the direct path of sunlight. As the sun descends further, the angle of illumination decreases, eventually reducing the scattered light below the threshold of human perception and ending twilight.