About twilight and the calculator

Nautical twilight is defined as the time in the morning and evening when the centre of the Sun is 12° below the horizon. At high latitudes, the Sun will not be lower than 12° below the horizon for a period in summer, and will not rise above 12° below the horizon for a period in winter.

This calculator provides the times of nautical twilight at a user-defined position, for today and at fortnightly intervals during a selected calendar year. The times of sunrise and sunset are included, and all times are adjusted to reflect the UTC offset of the user's clock.

The calculator is intended for use with CCAMLR's Conservation Measure 25-02 (Minimisation of the incidental mortality of seabirds in the course of longline fishing). In fisheries where this conservation measure applies, longlines must be set at night only (i.e. during the hours of darkness between the times of nautical twilight) and, wherever possible, the setting of lines should be completed at least three hours before sunrise.