This armillary sphere is signed by Girolamo della Volpaia. The rings surround a large rock crystal globe representing the Earth. The user would orient the instrument in the north-south direction by means of two magnetic compasses (now missing). The polar axis would be tilted to match the altitude of the celestial pole in the place of observation. Two sights could be oriented relative to two graduated scales, one a zodiac scale, the other a calendar scale. By rotating the central part of the instrument around the polar axis, one could make the shadow of the sight aimed at the Sun overlap the second sight exactly. The resulting configuration showed the precise arrangement of the main celestial circles. By reading the degree of the celestial equator that intersected the meridian, one could thus determine the hour of observation.

The entire sphere rests on a heavy brass pedestal painted in black, with feet in the shape of animal paws. The supports of the horizon circle are also shaped and pierced. Famous since its earliest days, the sphere has often been cited and illustrated, particularly in the characteristic nineteenth-century iconography celebrating past scientific glories. Provenance: Medici collections.