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Twilight is not one thing
Most people think of twilight as the dim light after sunset, but astronomers split it into three stages, and each one matters for a different activity. Civil twilight runs while the Sun is 0 to 6 degrees below the horizon. The sky is still bright, street lights come on, and you can read outdoors without help. Nautical twilight covers 6 to 12 degrees below the horizon. The horizon at sea is still visible, which is why sailors once used it to take star sights, but the sky is dark enough for the brighter stars. Astronomical twilight covers 12 to 18 degrees below the horizon. The sky looks dark to the eye, yet a faint glow remains, and only after it ends is the sky truly dark for deep-sky observing.
This calculator gives you all three stages, morning and evening, for any date, latitude, and longitude, plus sunrise, sunset, solar noon, the golden-hour windows, and the length of the day. Enter the date and your coordinates, and the tool returns a full table in your device’s time zone. If a stage does not occur, for example astronomical twilight in a London summer, the tool says so honestly instead of inventing a time.
How the stages are defined
Every twilight boundary is a Sun angle below the horizon, measured from the centre of the Sun’s disc. Civil twilight ends at 6 degrees down, nautical at 12, astronomical at 18. Those numbers are conventions, not laws of nature, but they are used worldwide, so a civil twilight time in Tokyo means the same kind of light as one in Paris. Near the equator it dives almost straight down, so twilight is short. At high latitudes it slides sideways along the horizon, so twilight stretches for hours, and in midsummer it may never finish.
Morning twilight runs in reverse order. Astronomical twilight ends first, in the early hours, then nautical, then civil, then sunrise. Evening twilight runs the other way: sunset, civil, nautical, astronomical, then true night. The calculator lists them in time order so you can read the whole day from top to bottom. Golden hour sits inside civil twilight and just beyond it: the morning golden hour runs from sunrise to about 6 degrees above the horizon, and the evening one from 6 degrees above down to sunset. Photographers plan around golden hour, astronomers plan around astronomical twilight, and both use the same table.
A worked example
Take London on 20 March. Civil twilight starts in the morning well before sunrise, nautical twilight earlier still, and astronomical twilight earlier again, in the small hours. Sunset in the evening is followed by civil twilight, then nautical, then astronomical, and true night begins late in the evening. The day is about 12 hours long, because the date is near the equinox, when day and night are nearly equal everywhere on Earth.
Compare that with London on 21 June. The Sun barely dips below the horizon at midnight, so astronomical twilight never ends and nautical twilight is short. The sky stays in a permanent glow all night, which is beautiful for evening walks and useless for galaxies. A photographer, by contrast, loves June in London, because the long twilight gives hours of soft light. The same sky, the same table, two opposite verdicts, which is why the calculator shows every stage instead of a single sunset time.
Using twilight times well
- Photographers: plan landscape shots for civil twilight, when the sky still holds colour and the land is readable.
- Stargazers: wait for the end of astronomical twilight before judging a dark-sky site or starting deep-sky imaging.
- Drivers and cyclists: civil twilight is when headlights become necessary, even though the sky still looks light.
- Wildlife watchers: many animals are most active during civil and nautical twilight, at dawn and dusk.
- Muslim observers: dawn twilight times relate to Fajr prayer, but religious authorities use their own definitions, so check local guidance rather than relying on this tool alone.
Frequently asked questions
Why is there no astronomical twilight on some summer nights? Because at high latitudes the Sun never sinks 18 degrees below the horizon in summer. The sky glows all night, and no calculator can honestly give an end time that does not exist.
Does twilight last longer in the tropics or at the poles? At the poles, by far. Near the equator the Sun drops fast and twilight lasts barely an hour. Near the poles it can last for weeks around the equinoxes, as the Sun hovers near the horizon.
Are these times exact? They are approximate to a few minutes. Hills, weather, and air clarity shift the visible light, and different sources round the Sun angle slightly differently. For observing, arrive early and let your eyes, not the clock, make the final call.
Use the calculator above before you plan a shoot, an observing session, or an evening outdoors: enter your date and coordinates, read the full twilight table, and pick the stage of light your plan actually needs.
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