How to use the twilight calculator
Enter a city and verify its country. Leave the date blank for that city’s current evening or select the evening you intend to observe. Read the full local dates as well as the clocks. The evening and following morning are one observing night, even when the UTC date differs. Use the weather planner next, rather than treating a darkness interval as a clear-sky forecast.
Sunset is not astronomical darkness
The twilight table separates the Sun’s −6°, −12° and −18° geometric crossings. These are civil, nautical and astronomical thresholds, not three forecasts of cloud. At sunset the sky can still hide faint targets. Moon and bright-planet sessions may not need the same darkness as a faint nebula; always match the target to the task.
A cross-midnight example
Illustration, not a forecast: if astronomical dusk falls at 00:10, the first useful dark interval belongs to the evening you selected but begins on the following calendar date. A displayed 00:10–03:00 is not an afternoon session. Use the attached dates and destination time zone, and check whether the site is still legally open then.
Polar light, long nights and clock changes
A missing crossing is not midnight and is not an error to replace with a guessed time. The tool lists actual dark intervals in a noon-to-noon span and distinguishes no astronomical darkness from continuous darkness. Daylight-saving transitions can make that span 23 or 25 hours; the UTC instants remain ordered even when a local clock repeats.
Accuracy and what is not included
Calculations use Astronomy Engine at zero observer elevation and a level horizon. Sunrise and sunset use its conventional upper-limb/refraction model; twilight uses the geometric Sun center. Terrain, observer height and unusual refraction can shift what you actually see. Times are rounded to minutes for planning, not supplied for navigation, emergency or safety decisions.
Frequently asked questions
Why is sunset earlier than the dark interval?
Sunset concerns the solar disk at the horizon. Faint-sky work often waits until scattered sunlight falls much further, which is why the −18° crossing is shown separately.
Why is astronomical dusk missing?
At some places and dates the Sun never crosses −18° in this span. Read the actual dark-interval result; do not substitute sunset or a guessed midnight.
Which date should I choose after midnight?
Choose the evening that starts your observing night. The next morning has its own displayed date; leaving the date blank selects the destination’s current evening.
Does it include my mountains or altitude?
No. It assumes a level horizon and zero observer elevation. Local terrain and refraction can change the apparent events.
Is this a weather forecast?
No. It is an astronomical calculation. Follow the cloud and night-planner links before making a trip.