This pocket sundial can be used anywhere in the world without any additional aid. It’s based on the simple principle that in the course of the solar year the angle of the sun in relation to the equatorial plane (the so-called declination) assumes specific daily values. In order to determine the time of day, the gnomon (shadow caster) of the sundial is adjusted to the calendar date. The angle of the punched mask to the inner edge of the chapter ring then corresponds to the declination of each day and the correct (astronomical) time can be read on the inner edge of the ring. In order to calculate CET one adds four minutes for every longitudinal degree west of the location of measurement.