Twilight Times Calculator

In short The three twilights are angles, not moods. Civil twilight is the sun between the horizon and 6 degrees below it, nautical runs from 6 to 12 degrees below, and astronomical from 12 to 18. Each threshold has a practical reason behind it: civil twilight is roughly the point at which you need a light to work outdoors, nautical twilight lasts while the sea horizon is still visible against the sky for a sextant sight, and astronomical twilight ends when the sky stops contributing any light of its own to a telescope. Below 18 degrees it is as dark as that night is going to get. How long each lasts depends almost entirely on latitude, because what matters is the angle at which the sun descends. At the equator it drops nearly vertically and civil twilight is over in about 22 minutes; the further from the equator, the shallower the path and the longer the sun spends crossing each band. Past roughly 48.6 degrees of latitude the effect becomes absolute: for part of the summer the sun never gets 18 degrees below the horizon at all, so astronomical darkness does not occur on those nights and the sky stays faintly lit right through. That is not a failure of the calculation, and this tool reports it as the answer rather than leaving a blank. Morning and evening twilight are also not exact mirrors, because the sun declination shifts while the night passes.