Twilight Calculator
Civil / nautical / astronomical twilight times for any date and location.
Overview
The Twilight Calculator returns the times of civil, nautical, and astronomical twilight for any latitude, longitude, and date. Each twilight phase is defined by how far the Sun's centre is below the horizon — −6°, −12°, and −18° respectively — and the result is shown for both the morning and evening, in the location's local time.
Useful for astronomers waiting for proper darkness before observing, photographers shooting blue-hour and night exteriors, naval navigators using a sextant against the visible horizon, military and aviation operations that key off "Beginning Morning Nautical Twilight" (BMNT), and anyone curious why "darkness" means three different things.
How it works
The Sun's altitude is computed with the NOAA solar position algorithm, accurate to within a minute for any earthbound location and any date. Civil twilight (Sun centre between 0° and −6°) is when the brightest stars are visible but you can still read outside without artificial light. Nautical twilight (−6° to −12°) is when the horizon is no longer distinguishable at sea — important historically for sextant readings of stars against the horizon. Astronomical twilight (−12° to −18°) ends when the sky is fully dark and even faint stars are observable.
At high latitudes one or more twilight phases may not occur — for example in summer the Sun may stay above the −18° threshold all night, producing the famous "white nights" of Scandinavia and St Petersburg.
Examples
San Francisco (37.77°N, −122.42°W) on 2026-05-18
→ Astronomical dawn 04:13, nautical 04:51, civil 05:25, sunrise 05:58
→ Sunset 20:18, civil 20:51, nautical 21:25, astronomical 22:03
Stockholm (59.33°N, 18.06°E) on 2026-06-21 (summer solstice)
→ No astronomical twilight; Sun stays above −18° all night (white night)
Equator (0°N, 0°E) on equinox
→ Each twilight phase lasts about 24 minutes morning and evening
FAQ
What are the three twilight phases?
Civil twilight ends when the Sun is 6° below the horizon — bright enough to still read outside. Nautical twilight ends at −12°, when the sea horizon is no longer distinguishable. Astronomical twilight ends at −18°, when even faint stars are visible against a fully dark sky.
Why does it matter for navigation?
A sextant needs both a star and a horizon. The crossover from civil to nautical twilight historically gave navigators a window where both were visible — too late, and the horizon disappeared; too early, and the stars were washed out.
What is BMNT?
"Beginning Morning Nautical Twilight" — a military term for the start of nautical twilight before sunrise. It is the official "stand-to" hour in many armed forces.
Why do twilights last so long at high latitudes?
The Sun travels a shallow arc near the horizon at high latitudes, so it takes hours for it to drop from −6° to −18°. Near the poles, the transitions can last weeks rather than minutes.
What is the relationship to blue hour?
Blue hour overlaps with civil twilight (roughly Sun between −4° and −6°), when the sky takes on a deep blue cast that photographers prize.