This large disc bears numerous markings and degree scales along its circumference.
Two semicircular sectors, a pointer with a scale, and a ruler with semicircle and moveable index accompany the instrument. These accessories could be pivoted about the centre of the instrument, depending on the types of observations or calculations being carried out.
The instrument was intended for nautical use and, although it is unsigned, has been attributed to Charles Whitwell. It belonged to the group of instruments left by Robert Dudley to the Grand Duke of Tuscany in 1649, and was referred to as a 'nautical astrolabe' in early Medicean inventories.