The common experience of watching the sun dip below the horizon is, for the scientific community, a precisely calculated astronomical event. The official definition of “sunset” is a moment determined with geometric accuracy, distinct from the period when the sky actually becomes dark. This specific timing is essential for fields like navigation and meteorology, requiring precise calculation and accounting for atmospheric effects. The scientific criteria establish a clear, repeatable standard for when the day officially ends.
The Technical Criteria for Defining Sunset
Sunset is officially defined as the instant the upper limb, or the very top edge of the Sun’s disk, becomes tangent to the ideal horizon. This ideal horizon is a perfectly level surface at sea level, assuming standard atmospheric conditions without local obstructions. This definition is adopted by astronomical and meteorological agencies to ensure consistency in published times.
A complexity in this calculation is atmospheric refraction, the bending of light rays as they pass through Earth’s atmosphere. The atmosphere acts like a lens, lifting the apparent position of the sun, making it appear higher than its true geometric position. This phenomenon allows the sun to remain visible even when it has physically dropped below the horizon line.
To account for this optical illusion, official computations factor in a standard refraction value. At the exact moment of official sunset, the true geometric center of the Sun is already approximately 50 arcminutes below the ideal horizon. This 50 arcminutes is derived from adding the Sun’s average apparent radius (about 16 arcminutes) and the average atmospheric refraction at the horizon (about 34 arcminutes). This calculation ensures the upper edge of the visible disk is just touching the horizon at the calculated time.
The Science Behind Sunset Colors
The spectacular colors observed during sunset result from how sunlight interacts with gases and particles in the atmosphere. During the day, the sky appears blue due to Rayleigh scattering, where short-wavelength light (violet and blue) is scattered most efficiently by small nitrogen and oxygen molecules. This scattered blue light reaches our eyes from all directions.
As the sun nears the horizon, its light must travel a much longer path through the atmosphere to reach an observer. This extended journey causes nearly all of the short-wavelength blue and violet light to be scattered away from the direct beam. Only the longer-wavelength light (yellow, orange, and red) penetrates this greater atmospheric distance and reaches our eyes directly.
The intensity and specific hue of the colors are modified by the presence of larger atmospheric particles, such as dust, smoke, and aerosols. This is explained by Mie scattering, which involves particles larger than the wavelength of light and scatters all colors relatively equally. When combined with Rayleigh scattering, these larger particles can introduce a white or hazy component. This often intensifies the reds and oranges or creates softer, more pastel hues, sometimes pushing the sunset colors toward the deep red end of the spectrum.
The Defined Stages of Twilight
The moment the sun officially sets does not signify an immediate transition to complete darkness, but rather the beginning of twilight, a three-stage period of diminishing light. Each stage is mathematically defined by the angular position of the Sun’s center below the ideal horizon. This systematic definition provides a framework for understanding the duration of residual daylight.
Civil Twilight
The first stage is Civil Twilight, which starts at sunset and ends when the Sun’s center is 6 degrees below the horizon. During this period, there is enough light for outdoor activities, and the horizon remains clearly discernible. The brightest stars and planets become visible, but details on the ground are easily made out without artificial illumination.
Nautical Twilight
Following this is Nautical Twilight, which lasts until the Sun is 12 degrees below the horizon. The term originates from its historical importance for mariners, as the light level is dim enough to allow for reliable star sightings for navigation, while the sea horizon is still faintly visible. By the end of this stage, outlines of terrestrial objects are difficult to see.
Astronomical Twilight
The final stage is Astronomical Twilight, which concludes when the Sun’s center reaches 18 degrees below the horizon. At this depth, the illumination from scattered sunlight is so faint that it is less than the light pollution from natural sources like starlight and airglow. When the sun drops past this 18-degree mark, the sky is considered truly dark, allowing for optimal astronomical observation.