Excerpt:
The Armillary Sphere
A drawing of a demonstrational armillary sphere, from Libros del saber de astronomia del rey D. Alfonso X De Castilla.
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Armillary spheres can be divided into two main categories: the observational armillary, as used by Ptolemy and Tycho Brahe; and the demonstrational instrument. Both types consist of a number of rings (Latin: armillae) which are arranged so as to model the circles of the celestial sphere. Typically, armillary spheres used for observation were larger and possessed fewer rings than those which served as demonstrational instruments; this made them more accurate and easier to use. Often the rings of demonstrational armillaries, like those of the observational spheres, were divided, and some incorporated sights which could be used to orient the instrument appropriately.