Twilight explained: civil, nautical and astronomical

Sunset is a moment, but darkness is a process. Between the sun dropping below the horizon and the sky going properly black there is a stretch of usable light, and it has three standard definitions. Knowing which one a table is quoting is the difference between packing a torch and not needing one.

The three stages

• Civil twilight runs until the sun is 6 degrees below the horizon. There is enough light to see and to work outdoors without artificial lighting, and the brightest stars and planets start to appear. Legal lighting-up times for vehicles are usually tied to this stage.
• Nautical twilight runs from 6 to 12 degrees below. The horizon at sea is still faintly visible against the sky, which is what allowed navigators to take star sightings against it. On land it feels like night with a lingering glow.
• Astronomical twilight runs from 12 to 18 degrees below. To the eye it is night, but the sky is not yet fully dark for a telescope. Past 18 degrees, it is astronomical night.

Each stage happens twice a day, once before sunrise and once after sunset, in mirror order.

Why the length varies so much

Twilight lasts as long as the sun takes to travel those degrees below the horizon, and that depends on the angle at which it descends. Near the equator the sun drops almost vertically, so twilight is short, sometimes barely twenty minutes from sunset to full dark. At high latitudes the sun skims the horizon at a shallow angle and twilight stretches for hours.

Far enough north or south in summer, the sun never gets 18 degrees below the horizon at all, so astronomical night does not occur: this is the white nights effect, and it is why northern summer skies never go fully black. In midwinter at the same latitudes, the opposite happens and the day may consist of twilight without a proper sunrise.

The golden and blue hours are something else

Photographers use two other terms that overlap these definitions without matching them. The golden hour is the period around sunrise and sunset when the sun is low and the light is warm and directional. The blue hour is roughly civil twilight, when the sky holds an even blue light and artificial lights balance with it. Both are aesthetic descriptions rather than astronomical ones, which is why their published times vary between sources.

Which one you need

For planning outdoor work or a walk home, civil twilight is the useful number. For astronomy, astronomical twilight is the one that decides when observing can start. For photography, look up the golden and blue hour times, which the golden hour tool gives directly, and use the sunrise and sunset times for the underlying sunrise, sunset and twilight boundaries at your location.