Twilight explained: civil, nautical, and astronomical — and what each is for

After the sun sets, the sky doesn't simply switch off. It fades through three distinct stages of twilight, each defined precisely by how far the sun has dropped below the horizon. Knowing the difference turns “it's getting dark” into a useful schedule for photography, navigation, and stargazing.

The three phases

Twilight is measured by the sun's angle below the horizon:

• Civil twilight — sun from 0° to 6° below the horizon. The brightest stage. There's still enough natural light to do most outdoor activities without artificial lighting; the brightest stars and planets begin to appear. This overlaps with blue hour, the photographer's favourite.
• Nautical twilight — sun from 6° to 12° below. The horizon at sea is still faintly visible against the sky — historically the window when sailors could take star sightings against a discernible horizon, which is where the name comes from. The sky is deep blue to dark.
• Astronomical twilight — sun from 12° to 18° below. Almost fully dark; only the faintest residual glow remains, detectable mostly by sensitive instruments. Once the sun passes 18° below, it's officially night.

What each phase is good for

Civil twilight / blue hour is for cityscapes and architecture: artificial lights glow against a sky that still holds deep blue colour, and reflections in water are at their best. It's also the safe end of the day for hiking out.

Nautical twilight is the transition — good for moody seascapes and silhouettes, and the moment landscape photographers often pack up but probably shouldn't.

Astronomical twilight (and after) is for stargazing and astrophotography. The Milky Way, faint deep-sky objects, and meteor showers only reveal themselves once the sun is more than 18° down and the sky is truly dark. If you're planning a night-sky shoot, the end of astronomical twilight is your real start time.

Why the timings vary

How long each phase lasts depends on your latitude and the season. Near the equator the sun plunges almost straight down, so all three phases pass quickly. At high latitudes in summer the sun may never drop 18° below the horizon at all — the source of the “white nights” where astronomical twilight never ends and it never gets fully dark.

Plan around it

For a city shoot, target civil twilight (blue hour). For the stars, wait until astronomical twilight ends. Both are predictable to the minute for your exact location and date — look them up, and you'll always know precisely when the light you need begins and ends.