An armillary sphere is ancient Greek, Arabic, and medieval device used to show and observe the movements of astronomical bodies. The celestial sphere is modeled by a skeletal framework of rings or armillaries (from the Latin armilla meaning "ring"), each representing an important circle, such as the ecliptic or celestial equator. Some of the rings may be moveable to enable the sky's appearance to be shown at different times and latitudes.

 

Armillary spheres are also found in the form of equatorial sundials, in which the ring representing the celestial equator is graduated with hours.