Civil, nautical and astronomical twilight

Civil twilight extends to a solar-centre altitude of 6° below the horizon, nautical twilight to 12° below, and astronomical twilight to 18° below. Morning crossings are dawn; evening crossings are dusk. The city tables show the instant of each crossing.

If the sun never gets 18° below the horizon during the local day, there is no astronomical night for that date. This can happen in the United Kingdom in summer even though the sun still rises and sets.

Farther north, the sun may remain above the horizon all day or below it all day. A day without sunrise can still contain twilight. The absence of an event is a real result, not missing data.

Daylight duration is the actual elapsed time above the sunrise/sunset threshold. A clock-change day can contain 23 or 25 hours; subtracting two displayed clock times can therefore give the wrong answer. Our calculation integrates the local day using the location’s timezone rules.