Civil, Nautical & Astronomical Twilight: The Six Moments Between Day and Night民用、航海与天文晨昏蒙影：昼夜之间的六个时刻

Ask ten photographers what "twilight" means and you will get ten answers — because twilight is not one thing. Astronomers divide the period between day and night into three phases, each defined by how far the sun has sunk below the horizon. Each phase has a different light level, a different color, and a different photographic use. Knowing the three types turns a vague "shoot at dusk" plan into a minute-accurate sequence.

Twilight is three windows, not one

The sun sinks below the horizon, and as it does, the sky passes through three defined states:

• Civil twilight: sun 0-6° below the horizon. The brightest phase; most outdoor activities can still proceed without artificial light.
• Nautical twilight: sun 6-12° below. The horizon is still usable for navigation at sea; on land, the deep blue "blue hour" lives in the first half of this phase.
• Astronomical twilight: sun 12-18° below. The last trace of solar light in the sky; when the sun passes 18°, true night begins.

Each boundary — 6°, 12°, 18° — is an altitude of the sun, not a clock time. The phases stretch and compress with latitude and season, which is exactly why they must be calculated, not memorized.

Civil twilight: the usable light

Civil twilight is the practical window. In the evening it runs from sunset until the sun is 6° down — roughly 25-35 minutes at mid-latitudes. During this phase:

• Sky still holds strong warm-to-rose color in the west and blue overhead.
• Landscape detail is still visible; handheld photography works with moderately high ISO.
• Window lights in cities are just turning on — the famous "dusk" city look starts here.

For most photographers, the golden-to-transition shots of the evening are civil twilight shots. This is also the phase where you can still see where you are walking — an underrated advantage for returning safely from a location.

Nautical twilight: the blue playground

Nautical twilight is where blue hour lives. In the first minutes of the phase, the sky reaches its richest, most saturated blue; as the sun sinks toward 12°, the blue deepens into indigo and then toward black. Photographic uses:

• City skylines with lit windows and a deep blue backdrop.
• Long exposures — water, clouds and traffic all blur beautifully at the 5-30 second exposures this phase supports.
• Transition to astro: the last blue fades exactly as the first stars become visible.

The horizon is still detectable during nautical twilight, which keeps compositions anchored — the sky and land read as separate shapes instead of merging into black.

Astronomical twilight: the astro door

Astronomical twilight is the photographer's signal for night work. When the sun is between 12° and 18° below:

• Stars become fully visible and the Milky Way begins to be photographable.
• There is still a faint glow in the sky that fades minute by minute; deep-sky photography prefers waiting until the sun passes 18°.
• Moonlight is now the only natural light competing with the stars — the moon phase becomes the dominant planning variable.

When the sun passes 18° below, astronomical night begins. For astrophotography planning, the "dark window" is the stretch of the night between astronomical twilight's end and its beginning the next morning — but only if the moon is down or new.

What each phase means for your settings

Use the twilight calculator on this site to get all six boundary times (start and end of each phase, morning and evening) for your exact coordinates, and build your session around the phase that matches your subject.

Because the light level drops by roughly a factor of ten in each phase, one base exposure never carries you through the whole sequence. Set your camera to bracketing (three frames, one stop apart) at the start of each phase, and re-meter whenever you cross a boundary — the sun dropping from 6° to 12° changes the scene by several stops even though it feels gradual to your eyes. This is also the best time for interval shooting: one frame every 30-60 seconds through the whole arc produces a seamless time-lapse of light, and the individual frames make a reliable contact sheet for choosing the exact minute you want.

Planning the twilight sequence

A complete evening, planned by phase:

1. Golden hour: warm directional light on the subject.
2. Civil twilight: transition frames; warm sky + blue ambient; city lights switching on.
3. Nautical twilight: main blue-hour series — skylines, long exposures, lights.
4. Astronomical twilight: stars over the last blue; test Milky Way frames.
5. Night: astro photography, or pack up knowing you shot every phase.

The sequence works in reverse for mornings, compressed into a shorter, faster-moving version of the same arc. Once you think in phases, "shoot at dusk" stops being a plan and becomes five separate shoots you can schedule like appointments.