An 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.