An armillary sphere is basically a skeletal celestial sphere with a model of the Earth or, later, of the Sun placed in the center. It is useful as a teaching tool and as an analog computer for solving various astronomical problems to a crude degree of accuracy. Armillary spheres were developed by the Greeks in antiquity for use as teaching tools. In larger and more precise forms they were also used as observational instruments, being preferred by Ptolemy. Armillary spheres became popular again in the late middle ages. With the advent of the Copernican model of a Sun centered Universe pairs of spheres contrasting the Copernican and Ptolomaic models became common teaching/demonstration tools. Such small teaching sphere remained popular through the nineteenth century.