The reverse of this disc has a double shadow square divided 0 to 12 to 0 to 12 to 0, subdivided to 3, to 1 and to 0.5, numbered by 3, marked 'Umb: Recta', 'Umbra Versa', 'Vm: Recta'. The upper half of the disc has a degree scale 0 to 90 to 0, divided to 10, subdivided to 5 and to 1, numbered by 10, marked 'Ad Solis & stellaru<m> altitudines dimetiendas. ROM?. ao. 1578'. The remainder of the disc has foliate decoration.

The nocturnal face has a zodiacal calendar, with the constellation names and symbols, each with a scale 0 to 30 divided to 10, subdivided to 1, numbered by 10; the months names and divided to 10 and to 1 day, numbered by 10 as appropriate to each month. The hour disc is divided to 1 hour, subdivided to 1/4, the hours 4 to 12 to 9 numbered by Roman numerals; each hour indicated by a point, the midnight point extended to the zodiac scale. Index arm; the are and central area of the hour disc with scrollwork and foliate decoration.

The base plate has a shaped and pierced handle and suspension ring. The whole assembly is held together by a threaded tube attached to the trigonus and a wing nut.