An instrument consisting of an arrangement of rings, all of which are circles of the same sphere, used to show the relative positions of the celestial equator, ecliptic, and other circles of the clestial sphere. See Medici article.
A model of the universe, consisting of a skeleton globe with metal rings. The centre of the globe represents the Earth, and the rings represent the horizon, equator and tropics. It was used to teach a particular kind of astronomy called spherical astronomy.
An armillary sphere (variations known as a spherical astrolabe, armilla, or armil) is a model of the celestial sphere, invented by Eratosthenes in 255 BC. Its name comes from the Latin armilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center.