Armillary spheres have become an icon of science with their elegant rings and beautiful silhouettes. Eratosthenes may have invented armillary spheres around 255 bc. These instruments are made up of a set of graduated rings representing circles on the celestial sphere, such as the meridian, equator, ecliptic horizon, tropics, and colures. With the aid of an armillary sphere, one can track the path of the sun for any given day of the year or determine a star's coordinates. The earth is represented by the brass sphere located at the center of the armillary sphere.