The instrument consists of two superposed disks of different diameters. Part of the circumference of the smaller disk is toothed and fitted with a short index. On the disks is mounted a long, finely decorated index. The front of the instrument served as a nocturnal for determining the time from the positions of the stars. The front also carries the zodiac signs, the initials of the names of the months, and the hour markings. On the back is the sundial, complete with a folding gnomon, for telling time during the day. The back also displays the date (1554).