The word “sunset” is frequently used to describe the entire period when the day transitions into night, leading to a common misconception about its exact timing. Many people assume sunset is the moment the sun begins its visible arc toward the horizon. However, the term has a precise astronomical and scientific definition. It refers not to a process, but a single, measurable instant, which is why official times are published down to the minute. Understanding this specific definition corrects the popular idea that sunset is merely the beginning of the sun going down.
Defining the Astronomical Moment
Sunset is not the start of the sun’s descent, but the precise moment the sun’s visible disk finally disappears below the horizon. Specifically, astronomers define sunset as the instant the upper edge, or “upper limb,” of the sun becomes tangent with the horizon. This definition is used globally for consistency.
The official timing is focused on the last visible sliver to ensure a standardized measurement worldwide, providing a reliable point of reference for navigation and timekeeping. The entire solar disk takes approximately two to three minutes to pass below the horizon. This precise, repeatable criterion is necessary because using the sun’s center point would not align with the visual event of the light disappearing.
The Role of Atmospheric Refraction
The moment of sunset is complicated by the Earth’s atmosphere, which acts like a lens, bending sunlight. This phenomenon, known as atmospheric refraction, causes the sun to appear higher in the sky than its true geometric position. Light rays from the sun are curved downward as they pass through the denser air near the Earth’s surface.
Due to this bending, we continue to see the sun for a short time even after it has already dropped below the true horizon. The effect of refraction is greatest when the sun is near the horizon, lifting the apparent position. Consequently, at the very instant of official sunset, the sun’s geometric center is already about 50 arcminutes below the horizon. This atmospheric effect makes the day seem longer, delaying the visible sunset and advancing the visible sunrise by roughly two minutes each. Therefore, when you observe the sun touching the horizon, you are actually seeing a refracted image of the sun that has already, in a geometric sense, set.
The Different Phases of Twilight
Once the sun’s upper limb has disappeared and sunset has officially occurred, the transition to full night is not immediate; it progresses through three distinct phases of twilight. Twilight is the period of time when the sky remains illuminated by scattered sunlight even though the sun is below the horizon.
Civil Twilight
This phase begins at sunset and lasts until the sun reaches 6 degrees below the horizon. During civil twilight, there is enough ambient natural light for outdoor activities to be performed without artificial illumination. The horizon remains clearly visible, and only the brightest stars and planets are apparent.
Nautical Twilight
This stage continues until the sun is 12 degrees below the horizon. This phase is named for the time when mariners historically used the horizon and visible stars for celestial navigation. During nautical twilight, the horizon becomes indistinct, and most stars are visible, but the sky still holds a faint glow.
Astronomical Twilight
This final stage lasts until the sun is 18 degrees below the horizon. Once the sun drops 18 degrees below the horizon, the sky is considered truly dark. The faintest celestial objects, including the Milky Way, can be observed without the interference of scattered sunlight. The end of astronomical twilight marks the beginning of night.