The New International Encyclopædia/Armillary Sphere
AR′MILLARY SPHERE (Lat. armilla, a ring). An instrument intended to give a just conception of the constitution of the heavens, and of the motions of the heavenly bodies, as seen by an observer on the earth. It consists of a number of rings fixed together so as to represent the principal imaginary circles of the celestial sphere, and these are movable round the polar axis within a meridian and horizon, as in the ordinary celestial globe. It was by means of such rings furnished with sights that Hipparchus, Ptolemy, and other ancient astronomers made many of their observations, and we find even Tycho Brahe making most of his planetary observations with the help of such an instrument. The armillary sphere can now be used only for instruction in astronomy, and even in this respect it is altogether supplanted by the celestial globe.