Twilight: civil, nautical and astronomical

Why darkness does not arrive at sunset

Once the Sun has set for you, air high overhead can still be in direct sunlight. These layers scatter light back towards the ground. As the Sun sinks, the illuminated layers are higher and thinner, and less scattered light reaches your sky. Twilight is therefore a continuous change described by the Sun’s depth below the horizon, divided into conventional stages for practical use.

Part of the process is visible directly. After sunset, face away from the Sun towards the eastern horizon. Earth’s shadow rises as a blue-grey band, with a pink band above it called the Belt of Venus. As the Sun descends, the shadow rises and eventually blends into the darkening sky. A clear horizon makes the effect easier to recognise.

Twilight limits use the geometric centre of the Sun relative to the mathematical horizon. The standard sunrise and sunset convention includes the solar radius and refraction, placing the centre around −0.833°. The −6°, −12° and −18° boundaries are geometric reference values, rather than separate refraction-corrected sightings.

The three stages

Civil twilight extends from sunset until the Sun’s centre is 6° below the horizon, with the sequence reversed in the morning. There is often enough natural light for familiar outdoor activities. Venus and the brightest stars can become visible, although actual conditions depend on clouds, terrain and the observer. The term also occurs in navigation and other practical definitions; a legal rule must be checked for the relevant jurisdiction rather than inferred from an astronomy app.

Nautical twilight, from −6° to −12°, takes its name from navigation at sea. The sea horizon can remain distinguishable while enough stars are visible for taking a sextant sight. On land, landscape detail fades and constellations become more prominent. Its early part can overlap the deep-blue light valued in photography.

Astronomical twilight, from −12° to −18°, often looks nearly dark to the unaided eye. Sensitive measurements and faint features such as the Milky Way or zodiacal light can still be affected by scattered sunlight. Below −18°, the Sun’s contribution is conventionally negligible and astronomical night begins. Moonlight and artificial light are separate sources of brightness.

Evening moves through these stages in order; morning reverses them. Astronomical dawn begins as the Sun rises through −18°.

Why the duration changes

The important factor is the angle at which the Sun crosses the horizon. Near the equator the crossing is relatively steep, so each range of solar altitude is passed quickly. At higher latitudes, a shallower path lengthens the transition. Latitude and season work together: a twilight interval that is brief on a tropical trip may be much longer during a northern summer evening.

New York, an Arizona desert viewpoint and Alaska illustrate different observing contexts without sharing one set of clock times. Your location, date and chosen darkness threshold determine the relevant interval. Even at one place it changes over the year, so a fixed rule such as “an hour after sunset” cannot define astronomical darkness reliably.

At middle latitudes, twilight is often shortest near the equinox seasons and longer closer to the solstices. The exact behaviour depends on latitude, and some altitude thresholds cease to be crossed at all.

When a summer night never becomes fully dark

Around the local summer solstice, the Sun may not descend far enough to reach −18°. Near 50° north, evening astronomical twilight can merge into morning twilight. Farther north, even the −12° nautical threshold may not be reached; at still higher latitudes, bright twilight or the midnight sun can persist throughout the night.

These are latitude effects, not properties of a language or national border. A trip to Alaska can therefore change the meaning of “night” much more than moving between nearby cities. Southern high latitudes have the corresponding effect around the December solstice. For faint-sky photography, a summer date and a dark-looking clock time alone do not establish that astronomical night occurs.

Tips for observing and photography

• The Moon and bright planets can often be observed before astronomical twilight ends. Mercury and Venus are usually easiest to plan in twilight because of their proximity to the Sun; Venus can also remain above the horizon into darkness under favourable geometry.
• For the Milky Way, faint star fields or auroral photographs, consider both astronomical twilight and moonlight. A bright Moon may contribute more sky brightness than the final part of twilight.
• Arrive while there is enough light to check the terrain and set up safely. Your eyes then have time to adapt: full dark adaptation can take twenty to thirty minutes.