The published time for sunset is often a point of confusion. Many people wonder if the stated time marks the beginning of the sun dipping below the horizon or the moment the entire sky finally becomes dark. This discrepancy arises because “sunset” has a strict, standardized astronomical definition. This definition differs significantly from the extended period of fading light observed afterward, known as twilight.
The Precise Definition of Sunset
The official time for sunset refers to a singular, calculated instant. Astronomically, sunset occurs when the upper limb (uppermost edge) of the Sun’s disk disappears below a theoretical, level horizon. This standardized definition is used universally by almanacs and weather services.
The calculation must account for atmospheric refraction, which bends light and makes the Sun appear higher than its true geometric position. Due to refraction, the Sun appears to touch the horizon even when it is actually about 50 arcminutes below it. The official sunset time is when the Sun has already geometrically set, but refraction still makes it momentarily visible.
Understanding the Phases of Twilight
The confusion about when the setting “ends” is actually about the duration of twilight, which immediately follows sunset. Twilight is the period of illumination caused by the scattering of sunlight in the upper atmosphere after the Sun is no longer visible. This duration is divided into three distinct phases based on how far the Sun’s center is below the horizon, measured in degrees.
The three phases of twilight are:
- Civil Twilight: Lasts while the Sun is between 0 and 6 degrees below the horizon. There is enough light for outdoor activities without artificial illumination, and the brightest stars become visible.
- Nautical Twilight: Occurs when the Sun is between 6 and 12 degrees below the horizon. Historically important for sailors, the horizon is no longer clearly discernible, but enough stars are visible for celestial navigation.
- Astronomical Twilight: Begins when the Sun is 12 degrees below the horizon and ends when it reaches 18 degrees below. Past 18 degrees is considered true night, providing the necessary condition for sensitive astronomical observations.
Twilight’s entire duration varies based on latitude and time of year.
Factors Influencing Observed Sunset Time
While the published time is a precise astronomical calculation, an individual observer’s experience can differ due to local conditions. One factor is the observer’s altitude, or height above sea level. Viewing the sunset from a mountain or tall building delays the observed time because the horizon is effectively lowered, allowing the observer to see over more of the Earth’s curvature.
Atmospheric conditions also alter the perceived sunset. Heavy cloud cover, dense fog, or haze can prematurely obscure the Sun, making the visual sunset occur earlier than the official time. Minor variations in the observer’s exact longitude and latitude can also cause small differences in the calculated time.