Dusk is a period of fading light that marks the transition from day to night following sunset. While the term “dusk” is used loosely in common language to describe this evening dimness, scientists, sailors, and astronomers define this time precisely by the sun’s position below the horizon. Understanding “what time does dusk start” requires embracing the specific, astronomical definitions of diminishing light, which provide clarity on how much light is available after the sun disappears.
Defining Dusk and Twilight
The period of light following sunset is scientifically known as twilight, which occurs when the sun is below the horizon but still illuminates the atmosphere. This illumination is caused by the scattering and refraction of sunlight in the upper atmosphere, casting a diffuse glow onto the surface below. Twilight encompasses the entire duration of this atmospheric lighting until true night begins. Sunset is defined as the moment the sun’s upper edge disappears below the horizontal plane.
The duration of twilight is measured by the solar elevation angle, which tracks the sun’s geometric center as it drops further below the horizon line. Because usable light changes dramatically as the sun sinks, twilight is formally divided into three distinct phases. Each phase is separated by a 6-degree increment of solar depression, which dictates the visibility of objects and the overall brightness of the sky.
The Three Scientific Stages
Civil Twilight
The first phase is Civil Twilight, which begins immediately at sunset and lasts until the sun’s center is 6 degrees below the horizon. This period is what most people consider “dusk,” as there is still enough natural light for most outdoor activities without artificial illumination. During Civil Twilight, the horizon remains clearly visible, and only the brightest stars and planets can be seen.
Nautical Twilight
Following this is Nautical Twilight, spanning the time the sun is between 6 and 12 degrees below the horizon. The name originates from celestial navigation, as this light level is dark enough for sailors to observe many stars while the horizon remains faintly discernible. The sky darkens noticeably during this stage, and artificial lighting becomes necessary for nearly all outdoor tasks.
Astronomical Twilight
The final and darkest phase is Astronomical Twilight, which lasts from 12 degrees until the sun reaches 18 degrees below the horizon. At 18 degrees of solar depression, the sky is considered truly dark, and scattered sunlight no longer interferes with observing the faintest celestial objects. For an observer without light pollution, the sky during Astronomical Twilight appears virtually indistinguishable from full night.
Why the Start Time Changes
The duration of twilight, and the exact time any stage begins or ends, changes daily based on geographic location and time of year. The most significant factor is latitude, which determines the angle at which the sun sets relative to the observer’s horizon. Near the equator, the sun’s path is nearly perpendicular to the horizon, causing it to plunge rapidly through the 18 degrees of twilight. This steep drop results in a short twilight period, with Civil Twilight lasting as little as 24 minutes at the equator.
Conversely, at higher latitudes closer to the poles, the sun’s path is at a shallow angle to the horizon. This shallow angle means the sun takes much longer to sink below the 6, 12, and 18-degree thresholds. The season also plays a role because the Earth’s axial tilt changes the sun’s declination throughout the year.
At high latitudes during the summer solstice, the sun may never sink more than 18 degrees below the horizon, meaning true night never occurs and twilight lasts from sunset to sunrise. Even at mid-latitudes, twilight is generally longer in summer than in winter, as the sun’s apparent path makes a shallower angle with the horizon. Local atmospheric conditions like cloud cover or air pollution can influence the perceived brightness, but the official calculated timing remains fixed by the sun’s geometric position.
Finding the Exact Timing
Because twilight timing depends on precise solar geometry relative to an observer’s location, the start time cannot be universally stated without calculation. Determining the exact beginning of Civil, Nautical, or Astronomical Twilight requires specialized tools that account for the observer’s latitude, longitude, and the specific date. Astronomical almanacs, meteorological websites, and dedicated mobile applications use complex algorithms to solve these solar geometry problems.
These specific definitions have many practical and legal applications. Jurisdictions use the end of Civil Twilight (Civil Dusk) to mandate when vehicle headlights must be turned on or when construction work must cease. Aviation regulations reference Civil Twilight for determining airport operational periods. Astronomical Dusk is paramount for astronomers and photographers who need the sky completely free of residual sunlight.