Astronomical compendium

A nocturnal is an instrument that enables the user to determine the time at night from the position of particular stars in relation to the Pole Star, which is observed through the central hole. The instrument was developed in the late Middle Ages.
The calendrical scale is used to set the instrument for the appropriate time of year. The hour-scale has small teeth for the half and full hours, which allow the user to read off the time in the dark by simple counting from the larger pointer at 12 o'clock. The meaning of the letters 'WF' on the handle is unknown.
The back has markings which can be used to determine the time of high tide at a particular port from the age of the moon (that is, how many days have passed since the last new moon), provided the direction of the place is known.