Astronomical Twilight Calculator
Find precise civil, nautical and astronomical twilight times for any date and location — in your browser, zero upload
Your location is used only in your browser and never sent to any server.
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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.
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.
Free browser-based twilight calculator. Find precise civil, nautical and astronomical twilight times for any date and location. No data uploaded — 100% client-side. · All twilight calculations execute client-side using Jean Meeus's solar algorithm — zero network requests, zero location data transmitted.