Armillary Sphere

Armillary Sphere, ancient astronomical instrument that shows the principal divisions of the heavens and the motion of celestial bodies. It consists of a number of graduated brass rings representing the chief celestial circles, such as the meridian, equator, ecliptic horizon, tropics, and colures (circles that intersect each other at right angles at the Poles). The instrument was probably invented about 255 bc by the Greek astronomer Eratosthenes. Armillary spheres were used until the 17th century.