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An armillary sphere is a model of the universe that is made up of rings or circles that astronomers use to study the night sky. The word "armillary" comes from the Latin word armilla, which means "bracelet, armband, metal ring". Here are some details about armillary spheres: Purpose Armillary spheres were used for teaching, demonstration, and observing the heavens. Design Armillary spheres are cut-open celestial globes with circles that represent the orbits of celestial bodies. The circles often include the horizon, meridian, equator, tropics, polar circles, and ecliptic. History Armillary spheres were invented independently in ancient Greece and China around the same time, possibly as early as the 4th century BC in China and the 3rd century BC in Greece. They were later used in the Islamic world and Medieval Europe. Variations Armillary spheres are also known as spherical astrolabes, armilla, or armil. Difference from celestial globes Armillary spheres differ from celestial globes, which are smooth spheres used to map constellations.