Sunset tells you when the upper edge of the Sun disappears below the horizon. It does not tell you when the sky becomes fully dark. The atmosphere continues to scatter sunlight after sunset, and astronomers divide that fading light into civil, nautical and astronomical twilight.
Why twilight continues after sunset
Earth’s atmosphere extends far above the surface. When the Sun has dropped below your local horizon, sunlight can still strike higher layers of the atmosphere that remain geometrically exposed to the Sun. Molecules and aerosols scatter part of that light into your line of sight, so the sky remains bright enough to wash out faint stars and deep-sky objects.
As Earth rotates and the Sun moves farther below the horizon, progressively less of the atmosphere above you receives direct sunlight. The scattered glow weakens. This gradual change is why there is no single physical switch between “day” and “night.” The three standard twilight categories provide useful operational boundaries.
Civil twilight: the Sun is 0° to about -6°
Civil twilight is the brightest twilight stage. Outdoors, there is usually enough natural light to move around without artificial lighting under clear conditions. The brightest planets and stars can begin to appear, but the background sky remains much too bright for serious deep-sky observing.
For astronomy, civil twilight is useful preparation time. It is a good period for setting up equipment, levelling a tripod, checking cables, aligning a finder and allowing a telescope to begin reaching thermal equilibrium. Bright targets such as Venus, Jupiter or the Moon may already be observable, but contrast against the sky is still limited.
Nautical twilight: the Sun is about -6° to -12°
During nautical twilight, the sky darkens enough for many more stars to become visible while the horizon can remain distinguishable. The name comes from traditional navigation, when sailors used bright stars while retaining a visible horizon for altitude measurements.
For an observer, this stage can be productive for bright clusters, double stars and planets. Astrophotographers may use it for focusing and framing. Faint nebulae and galaxies, however, still compete with a brighter sky background than they will later in the night.
Astronomical twilight: the Sun is about -12° to -18°
Astronomical twilight is the final twilight stage. As the Sun descends from roughly 12 to 18 degrees below the horizon, the residual solar contribution to sky brightness becomes progressively less important. Once the Sun passes about -18°, astronomers conventionally describe the sky as being in astronomical night.
This boundary is especially useful when planning faint-object observing or long-exposure imaging. It provides a reproducible definition of the part of the night when sunlight scattered by the atmosphere should no longer dominate the natural sky background.
Astronomical night is not always a dark-sky night
The -18° boundary only describes the Sun. It says nothing about the Moon, local light pollution, clouds, haze or natural sky brightness. A full Moon can illuminate the sky strongly throughout astronomical night. A city can remain bright enough to hide the Milky Way even when the Sun is far below -18°. Thin cloud can amplify urban skyglow by reflecting artificial light back toward the ground.
This is why observing planners need more than sunset and twilight. A useful plan combines the astronomical-darkness window with lunar phase, Moon altitude, target altitude, weather and local horizon conditions.
Latitude can change the length of twilight dramatically
Twilight duration depends strongly on how steeply the Sun crosses the horizon. Near the equator, the Sun’s path often cuts the horizon at a relatively steep angle, so it moves through the twilight altitude bands quickly. At higher latitudes, especially around summer, the Sun can travel at a shallow angle below the horizon. Twilight can then last for hours.
At sufficiently high latitudes in summer, the Sun may never reach -18° at all. Astronomical night does not occur on those dates. The same location can have very long dark nights in winter. This seasonal variation explains why a fixed rule such as “wait two hours after sunset” is unreliable.
How the Night Observation Planner uses twilight
The Astronomy Times Night Observation Planner calculates approximate times for sunset and the end of civil, nautical and astronomical twilight from your latitude, longitude, UTC offset and selected date. It also reports the corresponding morning boundary and estimates the duration of the astronomical-darkness window.
The planner then adds lunar illumination and approximate planet visibility, because the practical question is rarely “when is astronomical twilight?” by itself. The useful question is closer to “when is the sky dark, and what will be high enough to observe during that window?”
What twilight calculations do not account for
Published twilight times assume a mathematical horizon. Mountains, buildings, trees and terrain can hide the Sun earlier in the evening or delay its visible appearance in the morning. Atmospheric refraction also changes the apparent position of objects close to the horizon. Temperature, pressure and weather affect refraction slightly.
For ordinary planning these effects are usually smaller than local obstructions and weather, but they matter if you need precise timing. An open sea horizon and a mountain valley can produce very different visual experiences even when the calculated astronomical coordinates are identical.
Practical observing strategy
If you are targeting planets or the Moon, you may not need to wait for astronomical darkness. Bright planets often look excellent during twilight because their glare is reduced against a brighter background, and atmospheric seeing can sometimes be steadier before the ground has cooled substantially.
If you are targeting galaxies, faint nebulae or low-surface-brightness objects, prioritise the part of the night after astronomical twilight and before moonlight or dawn brightens the sky. A target that culminates during this window is especially valuable because it will be high in the sky when conditions are darkest.
Sources and further checking
Astronomy Times uses the established solar-altitude definitions of civil, nautical and astronomical twilight and follows the calculation approach documented by NOAA’s Solar Calculator resources. For precision navigation or professional observing work, use authoritative ephemeris services and account for local atmospheric and horizon conditions.