Civil, nautical and astronomical twilight times
Twilight is the transition between full daylight and full night, and it is more useful than sunset alone for many kinds of observing. The Twilight Times tool above separates civil, nautical and astronomical dawn and dusk for your selected location and date. Those three stages are defined by the Sun’s center being approximately 6, 12 and 18 degrees below the geometric horizon. They describe how much scattered sunlight remains in the atmosphere after sunset or before sunrise. A person planning an evening walk may care mainly about civil twilight, a landscape photographer may work through civil and nautical twilight, while an astronomer looking for faint objects often waits until evening astronomical twilight has ended.
The distinction matters because “sunset” is not the same as “dark.” Immediately after sunset, the sky can still be bright enough to hide many stars. As the Sun moves lower, the background sky darkens progressively. The timing depends strongly on latitude and season. Near the equator, twilight is often relatively short because the Sun crosses the horizon at a steeper angle. At high latitudes, twilight can last much longer, and during parts of summer astronomical darkness may not occur at all. The tool therefore calculates each threshold for the location instead of applying a fixed number of minutes after sunset.
What civil twilight means
Civil twilight is the brightest twilight stage. In the evening it begins at sunset and ends when the Sun reaches about 6 degrees below the horizon. Outdoor scenes usually remain easy to navigate without special lighting, and the brightest planets and stars may begin to appear. This can be a good time to look for Venus or Jupiter when they are favorably placed. The Planets Visible Tonight tool can show whether a bright planet is actually above your horizon during that period. In the morning, civil dawn begins when the Sun rises past the same -6 degree threshold and ends at sunrise.
Nautical twilight and the changing horizon
Nautical twilight occupies the interval from roughly 6 to 12 degrees below the horizon. Historically, its name reflects the period when the horizon and brighter stars could both be useful for marine navigation. For modern skywatchers it is a useful middle stage: the sky is substantially darker than during civil twilight, yet residual sunlight still changes the background. More stars become visible, bright constellations are easier to recognize and some satellite passes can stand out clearly. If you are using Night Sky Tonight, comparing object altitude with nautical dusk helps explain why something that is technically above the horizon may still be difficult to see against a bright sky.
Astronomical twilight and true dark-sky planning
Astronomical twilight extends until the Sun is about 18 degrees below the horizon. After evening astronomical dusk, scattered sunlight is normally faint enough that the sky is treated as fully dark for most practical astronomy. That makes this value especially important for the Milky Way, dim comets, meteor observing and deep-sky photography. Use Best Time to Stargaze Tonight after checking twilight, because darkness is only one requirement: cloud cover, Moonlight, precipitation and local light pollution can still make an otherwise dark interval poor for observing.
The tool also shows the approximate dark interval between evening astronomical dusk and the next morning’s astronomical dawn. That interval can become very short or disappear at high latitudes in summer. Conversely, winter can produce long nights with extended astronomical darkness. If you are comparing several dates, the Sunrise & Sunset Calendar provides a month-wide view, while Day Length Calculator shows how quickly daylight is changing around the selected date.
Twilight for photography, Moon viewing and satellites
Photographers often use the changing color and contrast of twilight rather than waiting for complete darkness. The nearby Golden Hour Calculator focuses on the low-Sun and blue-hour windows around sunrise and sunset. Moon observers can also benefit from twilight planning: a thin crescent near evening twilight can be visually striking, whereas a bright Full Moon later in the night may dominate the sky. Check Moon Phase Tonight and Moonrise & Moonset to combine solar twilight with lunar timing.
For satellites, twilight creates the classic visibility geometry in which the observer is dark while the spacecraft remains sunlit. This is why many memorable ISS passes occur not long after sunset or before sunrise. Twilight alone cannot tell you whether a satellite will pass over your city, so use it together with ISS Visible Tonight or the appropriate tracker. The U.S. Naval Observatory provides authoritative background and data services for twilight and rise/set calculations. SpaceTracker.live connects those solar thresholds to the rest of your local sky-planning workflow.