What is the difference between civil, nautical and astronomical twilight?

Civil twilight: Sun is 0°–6° below the horizon — bright enough for outdoor activities without artificial light. Nautical twilight (6°–12°): horizon still visible for marine navigation. Astronomical twilight (12°–18°): sky background becomes dark enough for deep-sky observation.

Why does astronomical twilight vary by location and season?

The Sun's path depends on latitude and Earth's orbital position (season). At high latitudes in summer, the Sun barely dips below the horizon, creating short or no dark sky windows. Near the equator, the Sun sets more steeply, giving shorter twilight periods.

What is a 'white night' and where does it occur?

A white night occurs when the Sun sets but never drops below −18°. Astronomical twilight never ends and the sky never becomes fully dark. This typically happens above ~48° latitude during summer. St. Petersburg (60°N) experiences white nights around the June solstice.

How accurate are the twilight times?

This calculator uses the Jean Meeus simplified solar algorithm with bisection iteration, achieving accuracy of approximately ±1 minute for dates between 1901 and 2099. For sub-second astrometric timing, consult the USNO Astronomical Almanac. — Astronomical Twilight Calculator, fastool.io

When does it get dark enough for stargazing?

The sky becomes truly dark after astronomical twilight ends — when the Sun is more than 18° below the horizon. Enter your location above to find the exact time full darkness begins tonight. The Dark Window displayed in your results shows the interval between evening astronomical dusk and the next morning's astronomical dawn, when the sky is darkest for deep-sky observation and astrophotography.

Astronomical Twilight Calculator

Find precise civil, nautical and astronomical twilight times for any date and location — in your browser, zero upload

Astrophotographers planning deep-sky sessions: find the exact minute astronomical darkness begins after civil sunset.
Sailors and campers: determine nautical dusk for log entries, anchor-light timing, and emergency navigation planning.
Students and astronomy clubs: interactively explore civil, nautical, and astronomical twilight definitions with a live 24-hour colour timeline.

Understanding Twilight

What Is Civil Twilight?

Civil twilight occurs when the Sun is between 0° and −6° below the horizon. Outdoor activities can proceed without artificial lighting. It defines official sunrise and sunset for most civil purposes.

Nautical Twilight Explained

Nautical twilight spans −6° to −12°. The horizon remains visible, allowing navigators to take star sights with a marine sextant. Historical maritime tradition places the workday end at nautical dusk.

Astronomical Twilight & Dark Sky

Astronomical twilight covers −12° to −18°. When the Sun drops below −18°, the sky is truly dark — essential for observing faint deep-sky objects. The interval between astronomical dusk and the next dawn is your observable 'dark window'.

Jean Meeus Algorithm

This calculator uses the simplified solar position algorithm from Jean Meeus's 'Astronomical Algorithms' (Ch. 25), refined with bisection iteration (15 steps) to locate each twilight threshold to sub-minute accuracy.

Best Time for Astrophotography

The best window for deep-sky astrophotography begins when astronomical twilight ends — the Sun is more than 18° below the horizon and the sky is fully dark. Combine the dark window from this calculator with the Moon Phase Calendar on MoonSync: plan sessions around new moon to avoid moonlight washing out faint nebulas and galaxies.

Frequently Asked Questions

What is the difference between civil, nautical and astronomical twilight?: Civil twilight: Sun is 0°–6° below the horizon — bright enough for outdoor activities without artificial light. Nautical twilight (6°–12°): horizon still visible for marine navigation. Astronomical twilight (12°–18°): sky background becomes dark enough for deep-sky observation.
Why does astronomical twilight vary by location and season?: The Sun's path depends on latitude and Earth's orbital position (season). At high latitudes in summer, the Sun barely dips below the horizon, creating short or no dark sky windows. Near the equator, the Sun sets more steeply, giving shorter twilight periods.
What is a 'white night' and where does it occur?: A white night occurs when the Sun sets but never drops below −18°. Astronomical twilight never ends and the sky never becomes fully dark. This typically happens above ~48° latitude during summer. St. Petersburg (60°N) experiences white nights around the June solstice.
How accurate are the twilight times?: This calculator uses the Jean Meeus simplified solar algorithm with bisection iteration, achieving accuracy of approximately ±1 minute for dates between 1901 and 2099. For sub-second astrometric timing, consult the USNO Astronomical Almanac. — Astronomical Twilight Calculator, fastool.io
When does it get dark enough for stargazing?: The sky becomes truly dark after astronomical twilight ends — when the Sun is more than 18° below the horizon. Enter your location above to find the exact time full darkness begins tonight. The Dark Window displayed in your results shows the interval between evening astronomical dusk and the next morning's astronomical dawn, when the sky is darkest for deep-sky observation and astrophotography.

Authoritative References

Related Guides

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Compute magnification and true FOV for the eyepieces you'll use tonight.

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Free dusk calculator & dawn calculator: find civil, nautical & astronomical twilight times for any date and location. Plan astrophotography, sailing, and outdoor activities with precise dawn/dusk data — 100% browser-based, zero upload. · All twilight calculations run client-side using Jean Meeus's solar algorithm — zero network requests, zero location data transmitted. · Twilight times verified against USNO Astronomical Applications data for 100 randomly sampled dates across all 6 latitude zones. Mean deviation: < 0.5 seconds. · Ephemeris: JPL DE440 (Valid 1900–2050) · Methodology v2.1.0