The suspension ring of the instrument is mounted on a simple throne. The instrument is incomplete, lacking all but one of the plates, the rete and the pointers.
The back of the instrument displays, along its limb, a division into degrees from 0? to 90? and 90? to 0?, repeated twice. This is followed by divisions from V to XXX twelve times, the names of the signs of the zodiac in Latin, the letters a-g, repeated, the days of the month and a list of Saints, each of which corresponds to one day of the year. The central part of the instrument is divided into two parts.
The upper part bears the curved hour lines, departing from the centre, numbered from 1 to 11 (the number 12 is on the diameter line), and a disc bearing concentric circles relating to the 'Aureus numerus', 'Litera Dominicalis', 'Quadragesima', 'Paschas', 'Pentecoste', 'Adventus'. The centre of the disc contains the instructions for the use of the table for the 'Computus secundum descriptionem Dionisii' and for the 'Usus antecedentium'.
The lower part bears the half shadow-square, with the indications 'VMBRA RECTA' and 'VMBRA VERSA', repeated twice. The centre of the half-square contains an engraving depicting an extraordinary pastoral landscape, while embellishments and faces are engraved on the outside. The centre of the half square also contains the signature of the author: 'Iohannes Praetorius ioachimicus Altorfij faciebat Anno 1591'.
See M. Miniati, Museo di Storia della scienza: Catalago (Florence, 1991), p. 34.