Dark-sky timing
−18° solar-altitude conventionAstronomical Twilight
Estimate the morning and evening times when the Sun crosses 18° below the horizon.
Astronomical twilight is conventionally bounded by a solar center altitude of −18°. At high latitudes some dates have no such crossing.
High-latitude behavior
At some latitudes and seasons the Sun never crosses −18° on a given date, so no astronomical-night boundary exists.