The astrolabe is a multi-functional instrument which enables the user to perform such diverse tasks as timekeeping at day and night, surveying, determining latitude, and casting horoscopes.
Geoffrey Chaucer (about 1342-1400), better known for his Canterbury Tales, also wrote a treatise on the astrolabe which was widely disseminated. The type of astrolabe he described matches the features of this instrument, with its distinctive Y-shaped rete, a dog's head as a star-pointer for Sirius (known as the dog-star), and other star-pointers in the shape of birds. The frame around the circumference has a dragon's head and tail respectively at the ends.
Three of the saints mentioned in the calendrical list on the back are of particular English significance, and one of the latitude plates is marked for Oxford, while the others are laid out for Jerusalem, 'Babilonie', Rome, Montpellier, and Paris.