Calculate the three twilight phases for a latitude and date.
The three twilight phases correspond to fixed solar depression angles, and their duration depends on how steeply the sun descends. At high latitudes in summer the sun never reaches 18 degrees below the horizon, so astronomical darkness never arrives. This is why astronomers at northern latitudes lose observing entirely for weeks around the summer solstice.
Twilight
Twilight phases end at 6, 12 and 18 degrees of solar depression below the horizon
Twilight phases end at 6, 12 and 18 degrees of solar depression below the horizon The three twilight phases correspond to fixed solar depression angles, and their duration depends on how steeply the sun descends. At high latitudes in summer the sun never reaches 18 degrees below the horizon, so astronomical darkness never arrives.
This is why astronomers at northern latitudes lose observing entirely for weeks around the summer solstice.
This calculator takes 3 inputs: Latitude, Day of the year, Sunset time. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.