Armillary sphere

Armillary sphere is a model of the celestial globe which is able to revolve on its axix. It is made of rings and hoops representing the equator, the tropics, and other celestial circles. The model derives its name from 'armilla', the astronomical instrument used by the ancient astronomers to show the recurrence of equinoxes and solstices. A hoop was fixed on it in the plane of equator, sometimes crossed by one in the plane of the meridian.