How did we find the hour at night on 16th century?This nocturnal watch “horologium nocturnal” was the instrument used to measure the time through the observation of the Pole Star and the position of bright stars in the sky.This nocturnal is a reproduction of the one made in 1570 by Girolamo della Volpaia (currently at the Galileo Museum in Florence, former History of Science Museum). Because it was made before the Gregorian reform (1582) and its creator had not very accurate calculation boards, to use this instrument correctly one must add 38 days to observation date.